CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens

BASIC BUILDING CONCEPTS
Spherical Mirror: A spherical mirror is a part of hollow sphere with one side having silver/mercury coating, further coated with paint to protect it from damage.
According to the position of silvered surface, spherical mirrors are of two types:
(a) Concave mirror: Silvered at outer surface so that reflection takes place from inner (concave) surface.
(b) Convex mirror: Silvered at inner surface so that reflection takes place from outer (convex) surface.
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 1

IMPORTANT TERMS RELATED TO SPHERICAL MIRROR:
(а) Pole: Pole is the central point ‘P’ of the reflecting spherical surface.
(b) Aperture: Aperture is the width (LL’ or MM’) of the reflecting surface from which reflection takes place actually.
(c) Centre of curvature: Centre of curvature is the geometrical centre of the hollow sphere ‘C’ from which the mirror is formed.
(d) Radius of curvature: Radius of curvature is the radius ‘R’ of the hollow sphere whose part is spherical mirror or distance between the pole and center of curvature.
(e) Principal axis: Principal axis is the straight line which joins the pole and centre of curvature.
(f) Principal focus: Principal focus is the point ‘F’ on the principal axis where a parallel beam of light parallel to the principal axis actually meets (in case of concave mirror) or appears to come from (in case of convex mirror) after reflection is called principal focus.
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 2
(g) Focal length: Focal length is the distance ‘f’ between the pole and principal focus of the spherical mirror.

You can also download Class 10 NCERT Science Solutions to help you to revise complete syllabus and score more marks in your examinations.

Mirror formula: The relation between the object distance (u), the image distance (v) and the focal length (f)of a mirror is called mirror formula and is given by
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 3
This expression is valid nd remains the same for various positions of the object with concave as well as convex spherical mirrors

SOME MORE FACTS ABOUT CONCAVE MIRROR:

  1.  It reflects the sunlight leading to brighter dazzling patch of light.
  2. The sharpest and brightest spot of light on the screen is the focus of a concave mirror.
  3.  Using the relation R = 2f, we can calculate the radius of curvature of the mirror.
  4. The nature, size and position of the image formed by a concave mirror depends on the position of object in front of the mirror.
  5. A virtual, erect and enlarged image of an object is formed behind the concave mirror while the real and inverted image is formed in front of the mirror.
  6.  A concave mirror is called a converging mirror because it converges the parallel beam of light rays after reflection at a point.
  7. If a light emitting source is kept at the focus of a concave mirror, it spreads a powerful parallel beam of light to a larger distance.
  8.  If a piece of carbon paper is kept at the focus of a concave mirror, it begins to burn with smoke initially and after that it catches fire and burns quickly.
  9.  It is used to concentrate the heat at a point, so it can be used in solar heating devices.

AIM
To determine the focal length of a concave mirror by obtaining the image of a distant object.

MATERIALS REQUIRED
A concave mirror, a metre scale, a mirror holder.

THEORY

  1. The nature of the image formed by a concave mirror, depends upon the position of the object in front of it.
  2. When a parallel beam of light coming from a distant object, such as tree or pole is incident on the reflecting surface of a mirror, then after reflection, the rays converge at a point and this point is called principal focus of the concave mirror as shown in the figure.
  3. If a screen is placed at the point of focus, a sharp, real and inverted image of the distant object is obtained.
  4.  The distance between the pole and principal focus of a spherical mirror is called focal length of the mirror. It is equal to half the radius of curvature of the mirror.

CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 4
PROCEDURE

  1.  Select a distant object such as a tree or pole or the sun.
  2.  Mount the concave mirror in a mirror holder.
  3.  Adjust the concave mirror in such a way that the rays of light coming from the tree fall on its reflecting surface.
  4.  Obtain a well defined and sharp image on a white wall of the laboratory by moving the mirror backward or forward.
  5. Measure the distance between the wall and the concave mirror with the help of a meter scale as shown in figure (a) or figure (b).
  6. This will give you approximate focal length of the mirror.
  7. Repeat the experiment by selecting the different distant objects at different distances.

CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 5
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 6

OBSERVATION AND CALCULATION
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 7

RESULT
The approximate focal length of the given concave mirror is………. cm, as determined.
According to the sign conventions, the focal length of a concave mirror is negative.  Therefore,
f = -………cm

PRECAUTIONS

  1.  The distant object should be clearly visible.
  2. The image on the wall should be well defined and sharp.
    (Student must pay special attention to get the sharp image. Obtaining a blurred image will give you an incorrect measurement of the focal length of a concave mirror).
  3.  While measuring the distance, the meter scale should be kept parallel to the ground.
  4.  The meter scale must be correctly positioned between the wall and center of the concave mirror. Important note: This experiment can be performed outside the laboratory by taking the image on the screen (white paper) or white painted board or outer wall of the laboratory.

INTERACTIVE SESSION

Question 1:
What are you doing?
Answer:
I am finding the focal length of a concave mirror by obtaining the image of a distant object.

Question 2:
What is spherical mirror?
Answer:
It is a part of a hollow sphere having a highly polished reflecting surface inside or outside.

Question 3:
Which surface in your mirror is reflecting surface, inner or outer?
Answer:
Outer surface is silvered and regular reflection takes place from the inner surface of the mirror.

Question 4:
Is the image formed by a concave mirror real or virtual?
Answer:
Image formed by the concave mirror depends on the position of the object. When the object is kept between pole and its focus, a virtual image is obtained, whereas a real image is obtained for all other positions of the object.

Question 5:
What is the difference between a real and virtual image?
Answer:
A real image is obtained on the screen, whereas a virtual image cannot be obtained on the screen. Also, when the rays of light after reflection actually meet, a real image is formed. If the rays of light appear to meet after reflection, a virtual image is formed.

Question 6:
What is focal length of a concave mirror?
Answer:
The distance between the pole and the focus of a mirror is called focal length of the concave mirror.

Question 7:
What is radius of curvature?
Answer:
The radius of the hollow sphere, from which a mirror is formed, is called its radius of curvature.

Question 8:
Is there any relationship between radius of curvature and focal length of the mirror?
Answer:
Yes, the radius of curvature is twice the focal length.

Question 9:How will you get more accurate value of focal length?
Answer:
By obtaining the sharp image of the Sun.

Question 10:
State the laws of reflection of light.
Answer:

  1. The angle of incidence is equal to the angle of reflection.
  2. The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.

Question 11:
Suppose two spherical mirrors with reflecting surface towards your face are lying on the table. How will you distinguish between them just by looking?
Answer:
The mirror which forms the virtual, erect and magnified image of our face is concave. If it produces diminished image then it is convex mirror.

Question 12:
What are the uses of a concave mirror?
Answer:
It is used as a reflector in ophthalmoscope, telescope, headlights of motor vehicles, shaving or make up mirror and also reflector in searchlight and an electric room heater.

NCERT LAB MANUAL QUESTIONS

Question 1:
How will you distinguish between a concave and a convex mirror?
Answer:
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 8

Question 2:
To determine the focal length of a concave mirror, a student focusses a classroom window, a distant tree and the sun on the screen with the help of a concave mirror. In which case will the student get more accurate value of focal length?
Answer:
In case of the sun student will get more accurate value of focal length.

Question 3:
What will be the nature of the image formed by a concave mirror for a distant object?
Answer:
The image formed by a concave mirror for a distant object is real and inverted.

Question 4:
In reflector type solar cookers, special concave (parabolic) mirrors are used. In such cookers, what should be the preferable position of food vessel for cooking?
Answer:
The preferable position of food vessel should be at focus of the concave mirror.

Question 5:
What type of mirror is used in a torch? Give reasons.
Answer:
Concave spherical or parabolic mirror is used because when the bulb (source) is kept at the focus of a mirror, parallel beam of light is obtained which travels a large distance.

Question 6:
What type of mirror is used as shaving mirror or in vanity boxes?
Answer:
Concave mirror because when the object is placed between its focus and pole, the magnified, erect and virtual image of the object will be formed.

PRACTICAL BASED QUESTIONS
Multiple Choice Questions/VSA

Question 1:
In the set-up shown below, a clear image of a distant object is obtained on the screen. The focal length of the concave mirror is
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 9
(a)11.4 cm
(b)9.4 cm
(c)9.8 cm
(d)9.9 cm

Question 2:
Four students A, B, C and D carried out measure-ment of focal length of a concave mirror as shown in the following four diagrams.
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 10
The best result will be obtained by student
(a) A
(b) B
(c) C
(d) D

Question 3:
A student determines the focal length of a device X, by focussing the image of a far off object on the screen positioned as shown in the figure below.
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 11
The device X is a
(a) convex lens
(b) concave lens
(c) convex mirror
(d) concave mirror

Question 4:
To find the focal length of a concave mirror Rahul focuses a distant object with his mirror. The chosen object should be
(a) a tree
(b) a building
(c) a window
(d) the sun

Question 5:
While doing an experiment, a student found that if the object moves from infinity towards pole of a mirror, the image also moves from pole to infinity. The mirror must be
(a) parabolic
(b) concave
(c) convex
(d) all of these

Question 6:
A student selected the sun as a distant object to find the focal length of a concave mirror. His teacher advised him to use a screen of wood or hard cardboard. It is because
(a) white paper is not available in the lab.
(b) white paper is costlier than other papers
(c) white paper may start burning when sun rays converge on it.
(d) white paper scatters the light rays falling on it.

Question 7:
The image of a distant object is obtained on a screen by using a concave mirror. The focal length of the mirror can be determined by measuring the distance between
(a) the object and the mirror
(b) the object and the screen
(c) the mirror and the screen
(d) the mirror and the screen as well as that between the object and the screen

Question 8:
Out of the following, the best way to do the experiment on finding the focal length of a concave mirror by obtaining the image of a distant object, is to
(a) hold the mirror in hand and keep the screen in a stand kept behind the mirror
(b) hold the mirror in a stand and hold the screen in hand, with the screen in front of the mirror.
(c) keep both the mirror and screen in suitable stands with the screen put in front of the mirror.
(d) keep both the mirror and the screen in suitable stands with the screen put behind the mirror.

Question 9:
A student obtains a blurred image of an object on a screen by using a concave mirror. In order to obtain a sharp image of the same object on the screen, he will have to shift the mirror .
(a) to a position very far away from the screen
(b) little away from the screen
(c) towards the screen
(d) either towards or away from the screen depending upon the position of the object

Question 10:
A student focused the image of a distant object using a device X on a white screen S as shown in the figure. If the distance of the screen from the device is 30 cm, select the correct statement about the device X.
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 12
(a) The device X is a concave mirror of focal length 15 cm.
(b) The device X is a concave mirror of focal length 30 cm
(c) The device X is a concave mirror of radius of curvature 30 cm.
(d) The device X is a convex mirror of focal length 30 cm

Question 11:
A student obtained a sharp image of grills of a window on a white screen using a concave mirror. His teacher remarked that for getting better value of focal length of the mirror he should focus a distant object (preferably the sun) on the screen. What should the student do for this purpose?
(a) Move the mirror and the screen towards the object
(b) Move the mirror and the screen away from the object
(c) Move the screen slightly away from the mirror
(d) Move the mirror slightly towards the screen

Question 12:
A student has to do the experiment, on finding the focal length of a given concave mirror by using a distant object. Out of the following ‘set-ups’ (A, B, C, D) available to her –
(A) a screen, a mirror holder and a scale
(B) a mirror holder, a screen holder and a scale
(C) a screen holder and a scale
(D) a mirror holder and a screen holder the ‘set-up’ that is likely to give her the best result, is the ‘set-up’ labelled as
(a) A
(b) B
(c) C
(d) D

Question 13:
A student recalls the rules to draw ray diagrams with the help of a concave mirror.
(i) The ray of light parallel to the principal axis will converge at the focus after reflection.
(ii) The ray of light passing through the centre of curvature will retrace its path after reflec-tion.
(iii) The ray of light falling at the pole gets reflected at a different angle on the other side of principal axis.
The correct statement(s) is/are
(a) (i) only
(b) (i) and (ii) only
(c) (ii) and (iii) only
(d) (i) and (iii) only

Question 14:
Which of the following pictures depicts the correct image of a distant object by using a concave mirror?
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 13

Question 15:
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 14
In the above set-up, the focal length of the concave mirror is 4.0 cm. The marks on the scale on which the screen be placed to obtain a sharp image will be
(a) at 8 cm
(b) at 2 cm
(c) at 6 cm
(d) at 1 cm

Short Answer Questions

Question 1:
State the condition when a concave mirror forms an image

  1.  larger than the actual object.
  2.  smaller than the actual object.

Question 2:
Why is a concave mirror of large focal length often used as a shaving mirror?

Question 3:
What happens to the ray of light after reflection from a concave mirror, if it

  1. is parallel to the principal axis?
  2.  passes through the center of curvature?

Question 4:
A student takes a mirror which is depressed at the centre and mounts it on a mirror stand. An erect and enlarged image of his face is formed. He places the mirror on a stand along a metre scale at 10 cm mark. In front of this mirror, he
mounts a white screen and moves it back and forth along the metre scale till a highly sharp, well-defined image of a distant building is formed on the screen at 25.5 cm mark.

  1.  Name the mirror and find its focal length.
  2. Why does the student get sharp image of the distant building at 25.5 cm mark?

Question 5:

  1. A ray of fight passing through the center
    of curvature of a concave mirror is incident on its reflecting surface. What is the angle of incidence and angle of reflection of this ray?
  2. The radius of curvature of concave mirror is 42 cm. What is its focal length?

ANSWER KEY
Multiple Choice Questions/VSA –

1. (c)
2. (a)
3. (d)
4. (d)
5. (b)
6. (c)
7. (c)
8. (c)
9. (d)
10. (b)
11. (d)
12. (a)
13. (b)
14. (c)
15. (d)

Short Answer Questions

1.

  1.  When object is placed anywhere between
    pole and centre of curvature.
  2.  When object is placed anywhere between infinity and centre of curvature.

2. While shaving, we keep our face between the pole and focus of a concave mirror to obtain an erect and enlarged image of the face. This enlarged image of the face helps us in having a better and a closer shave.

3.

  1. After reflection, the ray of light passes
    through the focus of a concave mirror in this case.
  2.  In this case, the ray of light gets reflected back along the same path, i.e. through centre of curvature.

4.

  1. The mirror is a concave mirror. The focal
    length is the difference between mirror and screen, so, f = 25.5 cm-10 cm = 15.5 cm
  2. The rays of light coming from a distant object are parallel to principal axis and meets at the focus of concave mirror where the screen is placed. Thus, a sharp image of the distant building is formed.

5.

  1. The ray of light passing through the centre
    of curvature of a concave mirror strikes the mirror normally and gets reflected back along the same path; hence the angle of incidence and the angle of reflection will be equal to zero.
  2.  f = R/2 = 42/2 = 21 cm

 

Experiment  11(b)



BASIC BUILDING CONCEPTS
Lens: A homogeneous transparent material or medium bounded by two surfaces of different or same radii of curvature is called lens.
The curved surface of the lens may be either convex or concave and the other surface may be either plane or spherical.

TYPES OF LENS:
(а) Double convex lens: If both the refracting surfaces of the lens are convex, then the lens is said to be double convex lens or simply convex lens. It is thicker at the middle and thinner at the edges.
(б) Double concave lens: If both the refracting surfaces of the lens are concave, then the lens is said to be double concave lens or simply concave lens. It is thinner at the middle and thicker at the edges.

IMPORTANT TERMS ASSOCIATED WITH LENS:
(а) Optical centre: The central point ‘O’ on the principal axis of the lens, through which incident ray of light passes undeviated with negligible lateral displacement is called optical centre of the lens.
(b) Center of curvature: The center of the sphere, whose part is a spherical surface of the lens is called center of curvature. Since the lens has two spherical surfaces so two different center of curvatures C1 and C2 lie on either side of the surface.
(c) Radius of curvature: The radius of the sphere from which the spherical surface of lens is made, is called radius of curvature of lens. So each surface of lens have separate radii of curvature R1 and R2 respectively.
(d) Principal axis of lens: A line joining the centre of curvatures and C2 of the two spherical surfaces is the principal axis of the lens.
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 15
For the refraction by spherical lens, we assume
(i) Lens to be a thin lens having small thickness.
(ii) Aperture of the thin lens is small, much less than its radius of curvature.
(e) Principal focus:
(i) Convex lens: When a parallel beam of light is incident on one of the spherical surface of a convex lens, the rays after refraction through it, meet at a fixed point on the principal axis. This point is called the principal focus of the convex lens.
Same effect can also be obtained from the other surface also. So the convex lens has two focus Fx and F2 one on each side of the lens.
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 16
(ii)Concave lens: When a parallel beam of light is incident on either of the spherical surface of a concave lens, after refraction through it, they appear to come from a fixed point on the principal axis. This point is called the principal focus of a concave lens.
Fig. Principal Focus of a Concave Lens
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 17
(f) Focal length: The distance of the principal focus from the optical centre of the spherical lens is called the focal length T of the lens.
If the lens (both spherical surfaces) is surrounded by the same medium having equal refractive indices, then the two principal focal lengths of the lens will be equal, i.e., OFx = OF, = f ,
The focal length of the lens depends upon the following factors:
(i) Radius of curvature of both the refracting surfaces and
(ii) Refractive index of the material of the lens.
Lens Formula: The relation between object distance (u), image distance (v) and focal length (f) of lens is
given by
CBSE Class 10 Science Practical Skills – Refraction Through Glass Slab 19
This expression is valid for all the various positions of the object with a spherical lens including the virtual
image formed by concave lens or a convex lens.

SOME MORE FACTS ABOUT CONVEX LENS

  1.  It converges the incident parallel beam of light on the other side of the lens. So it is called convergent lens also.
  2.  A ray of light passing through the first principal focus (in a convex lens), or appearing to meet at it (in a concave lens) emerges parallel to the principal axis after refraction.
  3. The image formed by the convex lens may be real or virtual, inverted or erect, diminished, same size or magnified, i.e., enlarged depending on the various positions of the object.
  4.  Focal length for convex lens is positive while it is negative for concave lens.
  5. The linear magnification produced by a convex lens can be less than one, equal to one or greater than one and it has no unit.
  6. In case of convex lens, magnification is positive for virtual image and negative for real image. In case of concave lens magnification is always positive because virtual image is always formed.
  7.  The lens behaves as a plane glass plate only when it is kept in a medium whose refractive index is equal to that of lens.
  8. The degree of convergence or divergence of light rays incident on any refracting surface of the lens is called the power of a lens.
  9. A lens of large focal length bends the light rays less after refraction through a small angle by focussing them away from the optical center.
  10. . Magnification and sharpness of the image formed by a lens can be increased by combining a number of lenses. Such a lens system is commonly used in various optical instruments such as cameras, telescope, microscope etc.

AIM
To determine the focal length of a convex lens by obtaining the image of a distant object.

MATERIALS REQUIRED
A convex lens, lens holder, white screen such as wall or white painted board, meter scale, distant object.

THEORY

    1. The rays coming from the distant object such as tree/sun/electric bulb or tall building travelling a large distance, can be considered as a parallel beam of light.

CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 19

  1. These rays after refraction through the convex lens converge at a point called focus.
  2.  The separation between the lens and the screen placed at the focus gives the approximate focal length of a convex lens.
  3. The image obtained on the screen kept at principal focus is real, inverted and highly diminished i.e. much smaller in size than the object.

PROCEDURE

  1. Select a far off object such as tree/windows of other building/any other far away object in the laboratory/ Sun.
  2. Mount the lens in a lens holder.
  3. Keep the lens in a vertical position throughout the experiment.
  4. Place the lens towards a selected distant object.
  5. Keep a screen on the other side of the lens.
  6. Move the lens forward or backward to get a sharp, real and inverted image on the screen.
  7.  Measure the distance between lens and screen at this position of the lens.
  8.  Repeat this procedure by changing the distant object.
  9. The distance between the screen (when sharp image is obtained) and convex lens give approximate focal length of the convex lens.

OBSERVATION AND CALCULATION
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 20

RESULT
The approximate focal length of a convex lens = …………cm.

PRECAUTIONS

  1. The lens holder along with lens should be kept vertical throughout the experiment.
  2.  Screen should be neat and clean to get clear image. It must be white.
  3.  While measuring the distance, metre scale should be kept horizontal and it must be parallel to the ground.
  4.  Distance should be measured only when well defined sharp image of the distant object is obtained.
  5. Lens and screen should be at same level.
  6. The lens used in the experiment must be clean.

Important Note: The procedure to find the approximate focal length of a convex lens does’ not apply to the concave lens because concave lens always forms a virtual and erect image for any position of the object. The virtual image cannot be taken on the screen.

INTERACTIVE SESSION

Question 1:
What are you doing?
Answer:
I am finding the focal length of a convex lens by forming the image of a far off object on a screen.

Question 2:
What type of image will you get?
Answer:
Real, inverted and diminished i.e. much smaller in size to that of the object.

Question 3:
Which other lens produces a diminished image?
Answer:
Concave lens produces a diminished image of an object held between optical centre and infinity.

Question 4:
What do you mean by infinity?
Answer:
When the object is at sufficiently large distance from the lens/mirror as compared to the focal length of the lens/mirror, the object is said to be at infinity.

Question 5:
Which optical phenomena is represented by this experiment?
Answer:
Refraction.

Question 6:
What is refraction?
Answer:
When the light rays travelling obliquely from one medium to another, a change in the direction of propagation in the second medium is observed. This phenomena is known as refraction of light.

Question 7:
It means light does not travel in the same direction in all media.
Answer:
Yes, it is true.

Question 8:
Suppose a lemon is kept in water in a bowl and viewed from outside. Will it appear larger or smaller than its actual size and why?
Answer:
It appears larger than its actual size because of refraction.

Question 9:
In what terms will you express the extent of the change in direction that takes place in a given pair of media?
Answer:
It is expressed in terms of refractive index.

Question 10:
Which important physical quantity can be linked with refractive index?
Answer:
Speed of light in different media.

Question 11:
How?
Answer:
The refractive index of second medium with respect to first medium is given by the ratio of the speed of light in media first to the speed of light in media second.

Question 12:
What is the relationship between angle of incidence and angle of refraction?
Answer:
The ratio of the sine of the incident angle to the sine of refracted angle is a constant and is equal to the refractive index of second medium with respect to first medium i.e.
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 21

Question 13:
What will be the position of the object to get the enlarged image on the same side of the lens as the object?
Answer:
Between focus F and optical centre O.

Question 14:
How the image is formed by the lens?
Answer:
Lens forms the image by refracting light. .

Question 15:
On what factor, the ability of a lens to converge the light rays depends?
Answer:
It depends on its focal length.

NCERT LAB MANUAL QUESTIONS

Question 1:
How will you distinguish between a convex and concave lens?
Answer:

  1.  Image formed by the convex lens may be real or virtual depending upon the position of object while concave lens always forms a virtual image wherever the object may be.
  2. Convex lens converges the parallel beam of light incident on it after refraction while concave lens diverges the same parallel beam of light which incident on it.
  3. The image formed by the convex lens may be equal in size, smaller or larger than the object while concave lens always forms diminished/smaller image of the object.

Question 2:
To determine the focal length of a convex lens, a student focusses a classroom window, a distant tree and the sun on the screen. In which case will the student is closer to accurate value of focal length?
Answer:
Sun

Question 3:
What is the nature of an image formed by a thin convex lens for a distant object? What change do you expect if the lens were rather thick?
Answer:
The image formed by a thin convex lens for a distant object will be real and inverted. A decrease in focal length will be observed when the lens becomes thicker.

Question 4:
You are provided with two convex lenses of same aperture and different thickness. Which one of them will be of shorter focal length?
Answer:
Thicker convex lens. .

Question 5:
If we cover one half of the convex lens while focussing a distant object, in what way will it affect the image formed?
Ans. A full size of image will still be obtained but only the intensity or brightness of image will reduce.

Question 6:
Can this method be used to find the approximate focal length of a concave lens?
Answer:
No, because concave lens always forms a virtual image wherever the object may be.

Question 7:
Which type of lens is used by the watch makers while repairing fine parts of a wrist watch?
Answer:
Convex lens because when the fine parts of a wrist watch are kept between its optical center and focus, magnified, virtual and erect images are formed.

PRACTICAL BASED QUESTIONS
Multiple Choice Questions/VSA (1 Mark)

Question 1:
Parallel rays, from a distant tree, incident on the device X, form its distinct image on a screen as shown. The diagram, correctly showing the image of the tree on the screen, is diagram
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 22
(a) A and the device X is a convex lens
(b) A and the device X is a concave mirror
(c) B and the device X is a convex lens
(d) B and the device X is a concave mirror

Question 2:
In an experiment to determine the focal length of a convex lens, a student obtained a sharp and inverted image of a distant tree on the screen behind the lens. She then removed the screen and looked through the lens in the direction of object. She will see
(a) an inverted image of the tree at the focus of the lens.
(b) no image as the screen has been removed.
(c) a blurred image on the wall of the laboratory
(d) an erect image of tree on the lens.

Question 3:
While performing an experiment on determining
the focal length of a convex lens, a student obtains  a sharp and inverted image of the laboratory ; window grill on the screen and measures the distance d between the screen and the lens. She then repeats the experiment and takes a distant tree as the object in the second case. In order to  get a sharp image on the screen, she will now need to move the screen
(a) slightly nearer to the lens
(b) slightly farther away from the lens
(c) very close to the lens
(d) very far away from the lens

Question 4:
If you are to determine the focal length of a convex lens, you should have
(a) a convex lens and a screen.
(b) a convex lens and a lens holder.
(c) a lens holder, a screen holder and a scale.
(d) a convex lens, a screen, holders for them and a scale.

Question 5:
After performing the experiment to determine focal length of a convex lens by focussing a distant object, a teacher asked Asha to draw a j ray diagram of her experiment and show where did she place the screen for getting sharp image. The figure drawn by her is given below
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 23
The point at which she had placed the screen during her experiment was
(a) at A
(b) at B
(c) at C
(d) at D

Question 6:
If the upper half portion of the lens is covered with black paper, the effect on image is
(a) upper half of the image will be absent
(b) the brightness of the image will be reduced
(c) size of the image depends on covered area of lens
(d)) there will be no effect

Question 7:
Which of the following can be used to find the focal length of a convex lens?
(a) Object at a distance of 10 cm for a lens of focal length 30 cm
(b) Light from a laboratory window
(c) Light from the sun
(d) Light from a tree next to the window

Question 8:
A sharp image of a distant object is obtained on a screen by using a convex lens. In order to determine the focal length of the lens, you need to measure the distance between the
(a) lens and the object
(b) lens and the screen
(c) object and the screen
(d) lens and the screen and also object and the screen

Question 9:
A student obtained a sharp image of the grill
of a window on a screen, using a convex lens. For getting better results, the teacher suggested focussing of a distant tree instead of the grill. In which direction should the lens be moved for this purpose?
(a) Away from the screen
(b) Very far away from the screen
(c) Behind the screen {d) Towards the screen

Question 10:
For the experiment on finding the focal length of
a convex lens by obtaining the image of a distant object, a laboratory assistant keeps the following apparatus on the table. [Foreign 2008]
A. A stand with a metal needle fixed in it
B. A stand with the given convex lens fitted in it
C. A thick cardboard white screen
D. A metre scale
A student can do the experiment by using the
apparatus listed as
(a) A, B, C, D
(b) B, C, D, A
(c) C, D, A, B
(d) D, A, B, C

Question 11:
Three students measured the focal length of a convex lens using parallel rays from a distant object. All of them measured the distance as given
Student A: distance between lens and screen
Student B: distance between object and screen
Student C: distance between object and lens Which student will get the correct focal length of lens?
(a) Student A only
(b) Student B and C both
(c) Student A and B both
(d) Student C only

Question 12:
Out of the following objects which one would you prefer to determine the focal length of a given convex lens by focusing its image on a screen?
(a) A burning candle placed on the far end of a laboratory table
(b) Grills of the laboratory window
(c)A tall tree visible from the laboratory window
(d)Sun rays entering the laboratory through its window.

Question 12:
Given below are few steps (not in proper sequence) followed in the determination of focal length of a given convex lens by obtaining a sharp image of a distant object:
(i) Measure the distance between the lens and screen.
(ii) Adjust the position of the lens to form a sharp image.
(iii) Select a suitable distant object.
(iv) Hold the lens between the object and the screen with its faces parallel to the screen.

Question 14:
A student focussed the image of a distant object using a device ‘X’ on a white screen ‘S’ as shown in the figure. If the distance of the screen from the device is 40 cm, select the correct statement about the device.
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 24
(a) The device X is a convex lens of focal length 20 cm.
(b) The device X is a concave mirror of focal length 40 cm.
(c) The device X is a concave mirror of radius of curvature 40 cm.
(d) The device X is a convex lens of focal length 40 cm.

Short Answer Quetions

Question 1:
A student focused the image of a candle flame on a white screen by placing the flame at various distances from a convex lens. He noted his observations as given below:
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 25
(a) What is the focal length of a given convex lens?
(b) Which set of observations is incorrect and why?

Question 2:
With the help of a diagram, differentiate between a converging and diverging lens. State one use of a convex lens.

Question 3:
A teacher sets up a stand carrying a convex lens of focal length 15 cm at 20.5 cm on the optical bench. She asks the students to suggest the position of the screen on the optical bench so that a distinct image of a distant tree is obtained on it. What should be the correct position of screen as suggested by the students and why?

Question 4:
A point-sized image of a distant object is formed at the principal focus of a convex lens. If the object starts moving towards the lens, how will the position of the image change? Illustrate it with suitable diagrams.

Question 5:
Explain how a converging lens acts as a magnifying glass. For this purpose, why a lens having a short focal length is chosen instead of lens having a long focal length?

ANSWER KEY
Multiple Choice Questions/VSA

1. (a)
2. (a)
3. (a)
4. (d)
5. (a)
6. (b)
7. (c)
8. (b)
9. (d)
10. (b)
11. (a)
12. (d)
13. (b)
14. (d)

Short Answer Questions

1.

  1. When the position of the object is at 2F, the
    image formed at 2F on the end other side of the lens is real, inverted and of same size as that of the object. Therefore,
    object distance = image distance
    2f = 30 cm
    f = 15 cm
    or
  2. For the last observation, we find that the object lies between the optical center and focus of the lens, so virtual image is formed and does not fall on the screen. Hence last observation is incorrect.

2. Converging lens: When a parallel beam of light is incident on one of the spherical surfaces of a convex lens, the rays after refraction through it meet at the principal focus on the principal axis. Since the convex lens converges the parallel beam of light, so it is called converging lens.
CBSE Class 10 Science Practical Skills – Refraction Through Glass Slab 20

Diverging lens: When a parallel beam of light is incident on one of the spherical surfaces of a concave lens, the rays after refraction through it appear to come from the principal focus on the principal axis. Since the concave lens diverges the parallel beam of light, so it is called diverging lens.
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 27

One use of a convex lens: It is used as magnifying glass.

3. Distant object can be considered to be at infinity. The rays coming from the distant tree are parallel to each other and therefore, its image is formed at the principal focus of the convex lens. Hence, the screen should be placed at 20.5 + 15 = 35.5 cm mark on the optical bench.
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 28

4. When an object starts moving from infinity towards the lens and reaches its focus, the image starts moving from its principal focus on the other side of the lens towards infinity. When an object lies in between focus and optical center, the image is formed on the same side of the lens
Object lies between the focus and optical center of the lens

5. To use a converging lens or convex lens as a magnifying glass, the object should be placed within its focus to get a magnified and erect image of the object as shown in the figure below:
CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens 29
A convex lens of short focal length has a greater magnifying power in comparison to a convex lens of large focal length. Hence, the object will appear much larger when seen through a convex lens having short focal length.

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One thought on “CBSE Class 10 Science Practical Skills – Focal Length of Concave Mirror and Convex Lens

  1. I want to ask a question – if the distance between center of concave mirror is twice the focal length their sum being unity determine focal length and center of curvature of mirror ( use standard values)

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