CBSE Class 10 Science Practical Skills – Refraction Through Prism

CBSE Class 10 Science Practical Skills – Refraction Through Prism

BASIC BUILDING CONCEPTS
Prism
Prism is a homogeneous, transparent, refracting material (such as glass) enclosed by two inclined plane refracting surfaces at some fixed angle called refracting angle or angle of the prism.
It has two triangular bases and three rectangular lateral surfaces which are inclined to each other as shown in the given figure. It has six vertices and nine edges. Since base of this prism is in triangular shape, it is called a triangular prism.

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In the given figure of prism:

1. Triangular bases are PQR and TUS.
2. Three rectangular lateral surfaces are PQUT, PTSR, QRSU.
3. Six vertices are P, Q, R, S, T and U.
4. Nine edges are RS, PT, QU, UT, ST, US, QR, QP and RP.

Some Important Facts About The Triangular Prism:

1. It is a polyhedron, with two parallel faces called bases. The other faces are always parallelograms. It is named by the shape of its base.
2. An oblique ray of light suffers two refractions on passing through a triangular prism and hence, deviates through a certain angle from its original path.
3. The angle between the incident ray and emergent ray is called the angle of deviation.
4. Angle of deviation depends upon the
   • angle of prism,
   • angle of incidence, and
   • nature of material of the prism.
5. When angle of incidence increases, angle of deviation decreases, till it becomes minimum at a particular angle of incidence. The minimum value of the angle of deviation for a triangular prism is called the angle of minimum deviation.
6. The refracted ray becomes parallel to the base of the prism under the minimum deviation position.
   
7. When a beam of white light is incident on one of refracting surfaces of the prism, it splits into a band of seven colours. This phenomenon exhibited by the prism is called dispersion of light. The band of coloured components of a light beam is called its spectrum.
8. Glass prism shows different refractive indices for different colour components of white light due to varying speed of different colours. Therefore, different colours emerge out through the prism along different directions and becomes distinct.
9. The order of colours from the base of the prism is Violet, Indigo, Blue, Green, Yellow, Orange and Red. It can be learnt by the word VTBGYOR.
10. Violet colour deviates through the maximum angle and red colour deviates through the minimum angle.
11. The combination of two inverted prisms placed together shows the recombination spectrum of white light.
    

AIM
To trace the path of the rays of light through a glass prism.

MATERIALS REQUIRED
A drawing board, few drawing pins or cello-tape, a glass prism, a few sharp pointed alpins, three sheets of white paper, a measuring scale, a pencil and a protractor.

THEORY

1. Let ABC be the principal section of a glass prism of refractive index ‘n’ The angle ‘A’ is the refracting angle of the prism.
2. When a ray of light PQ from rarer medium (such as air) is incident on a refracting surface AB, it gets refracted and bends towards the normal to the plane of face AB. Here, ray of light is entering from air to glass and refracted along QR.
3. At the second surface AC, the refracted light ray QR has entered from glass to air, i.e. denser to rarer medium and emerges out, along RS as it has bent away from the normal i.e. bends towards the base of prism shown in the adjacent figure.
   
4. In the figure:
   PQ – Incident ray
   QR – Refracted ray
   RS – Emergent ray
   ∠A – Angle of prism
   ∠i – Angle of incidence
   ∠r – Angle of refraction
   ∠e – Angle of emergence
   ∠δ – Angle of deviation
   Thus, the incident ray PQ suffers two refractions in passing through the prism.

PROCEDURE

1. Take a sheet of white paper and fix it on the drawing board with the help of a drawing pins or cello- tape.
2. Place a glass prism along its triangular base at middle on the paper. Mark the boundary ABC of the glass prism and then remove the prism.
3. Draw a thin dotted line XY along one of the refracting surfaces (say AB) as shown in the figure given below.
4. Draw a normal NFN₁, i.e. perpendicular to line AB at F. Also, draw a line EF making an angle 30° to the normal. EF will act as the incident ray.
5. Place the glass prism back to its original position ABC.
6. Fix two alpins P₁ and Q₁ on line EF vertically by gently pressing their heads with thumb into the drawing board about 6 cm apart from each other.
7. Look the images of P₁ and Q₁ from the other refracting face AC. The images of pins P₁ and Q₁ appear to be at I₁ and I₂.
8. Fix two more alpins P₂ and Q₂ vertically such that the feet of the pins P₂ and Q₂ appear to be on the same straight line as the feet of the images I₁ and I₂ respectively. Confirm it by moving the head slightly to either side while looking them. All the pins P₂, Q₂ and image of P₁ and Q₁ would appear to move together if they are on the same straight line.
9. Remove the prism and all alpins. Mark the position of feet P₂ and Q₂ and also P₁ and Q₁ [like Θ],
10. Join the pin pricks P₂ and Q₂ and produce P₂Q₂ so that it meets the refracting side AC of the prism boundary at G. The line HG represents the path of the emergent ray.
11. Join F and G. The line FG represents the path of the refracted ray.
12. Repeat the experiment on different paper sheets by taking different values of angle of incidence, viz. 35°, 45°, 50° and 55°.
    

OBSERVATIONS

1. The ray of light entering from air to glass at the first refracting surface bends towards the normal after refraction. At the second refracting surface, the ray of light entering from glass to air bends away from the normal.
2. Thus, the ray of light suffers two refractions on passing through a prism and hence deviates through a certain angle from its original path.

RESULT
EFGH represents the path of light through the prism as shown in the figure drawn.

PRECAUTIONS

1. The refracting faces of the glass prism should be smooth, transparent and without any air bubble or broken edge. It must be a triangular.
2. Use a sharp pencil to draw boundary of the prism and rays of light.
3. The alpins should have sharp tip and should be fixed exactly vertical to the plane of the paper.
4. The distance between the alpins should be atleast 6 cm. It provides the greater accuracy in finding the direction of incident ray and refracted ray.
5. Always adjust the feet and not the heads of alpins to lie along the same straight line.
6. The pin pricks should always be dotted and circled.
7. The angle of incidence should be in between 30° and 60°.
8. Proper arrows should be drawn to indicate direction of the incident ray, the refracted ray and the emergent ray.

SOURCES OF ERROR

1. The feet of alpins may not lie on the same line.
2. The alpins may not be exactly vertical.
3. The pin pricks may not be marked properly.
4. The lines which indicate the path of ray of light may not be drawn with accuracy.

ADDITIONAL ACTIVITY
To measure the angle of deviation.

1. The student will follow the following procedure in continuation with the previous one (steps 1 to 12).
2. Draw a normal N₂GN₃ to the refracting surface AC at point G.
3. Mark the angle of incidence (∠i), angle of refraction (∠r) and angle of emergence (∠e) as shown.
4. Now, produce EF to M and GH to O which will meet at point J.
5. Measure ∠GJM as the angle of deviation ∠δ.
6. Repeat the experiment for different values of angle of incidence, viz. 35°, 40°, 45°, 50° and 55° and measure corresponding angle of deviation each time. Record these observations in an observation table.
7. Plot a graph between the angle of incidence (i) on x-axis and angle of deviation (b) on y-axis.

OBSERVATION TABLE

RESULT

1. As the angle of incidence increases gradually, the angle of deviation decreases, it becomes minimum and further increases with the increase in the angle of incidence.
2. From the graph, the angle of minimum deviation, b = ………..°

INTERACTIVE SESSION

Question 1.
What is a triangular prism?
Answer:
It is a piece of homogeneous, transparent refracting material enclosed by three rectangular refracting faces and two triangular bases.

Question 2.
What is meant by the angle of prism?
Answer:
It is the angle of inclination between the two rectangular refracting faces of the prism.

Question 3.
What do you mean by the refracting edge of the prism?
Answer:
It is the edge where two rectangular refracting faces of the prism meet.

Question 4.
What is the angle of refraction?
Answer:
The angle between the refracted ray and the normal at the first refracting face of the prism is called the angle of refraction (∠r).

Question 5.
What is the angle of emergence?
Answer:
The angle between the emergent ray and normal at the second refracting face of the prism is called the angle of emergence.

Question 6.
If the emergent ray makes an angle of 40° with the second refracting surface of the prism, what is the angle of emergence?
Answer:
∠e = 90° – 40° = 50°

Question 7.
How many faces are there in a prism?
Answer:
Three rectangular faces and two triangular bases.

Question 8.
Which property of light is used by the prism to form a spectrum?
Answer:
Refraction of light.

Question 9.
Who was the first scientist to use a glass prism to obtain the spectrum of sunlight?
Answer:
Sir Isaac Newton

Question 10.
Which exciting phenomenon is shown by the inclined refracting surfaces of glass prism?
Answer:
The inclined refracting surfaces show the phenomenon of dispersion of light, i.e. they split the incident white light into its components.

Question 11.
How does the ray of light get refracted through a transparent prism?
Answer:
The ray of light suffers two refractions on passing through a prism and deviates towards the base through a certain angle from its original path.

NCERT LAB MANUAL QUESTIONS

Question 1.
Define angle of deviation.
Answer:
The angle formed between the incident ray produced in the forward direction and the emergent ray produced in the backward direction is called the angle of deviation.

Question 2.
List the factors on which the angle of deviation through a prism depends.
Answer:
The angle of deviation through a prism depends on

• angle of prism,
• nature of material of the prism and
• angle of incidence.

Question 3.
Why does a ray of light bend towards the base when it passes through a glass prism?
Answer:
When the ray of light goes from optically rarer medium (air) to optically denser medium (glass) at the first surface, it bends towards the normal. At the second refractive surface, the light ray enters from denser medium (glass) to rarer medium (air). Hence, it bends towards the base of the prism i.e. it bends away from the normal.

Question 4.
Why does the white light split into different colours when passes through a glass prism?
Answer:
In a material medium, the light rays of different colours travel with different speed hence, the material shows different refractive indices for different colours of light. Therefore, different colours of light bend through different angles with respect to the incident ray. So, they emerge along different paths.

Question 5.
Why does the white light not split into different colours when it passes through a glass slab?
Answer:
A glass slab whose faces are parallel can be considered as comprising two prisms of same refractive angle. The second identical prism can be considered in an inverted position with respect to the first prism. When a beam of white light passes through one prism, it gets dispersed, but on passing through the second inverted prism, the spectrum recombines to form a beam of white light again. Therefore, white light does not split into different colours when it passes through a glass slab.

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

Question 1.
In an experiment on tracing the path of the rays of light passing through a glass prism, the correct setting of protractor (D) for measuring the angle of incidence ∠i in diagram is

(a) (i)
(b) (ii)
(c) (iii)
(d) (iv)

Question 2.
To trace the path of the ray of light through a glass prism, four students A, B, C and D did the following:
A: Kept the eyes at certain moderate distance from the glass prism while placing both the image pins.
B: Kept the eyes close to the glass prism while placing both the image pins.
C: Kept the eyes close to the glass prism while placing first image pin and far from the prism while placing the second image pin.
D: Kept the eyes at 45° angle while placing both the image pins on a line which makes an angle of 30° with the normal.
The correct procedure is that of the student
(a) A
(b) B
(c) C
(d) D

Question 3.
A student showed the following traces of the path of a ray of lighjt passing through a glass prism:

The trace most likely to have been done correctly by her, is
(a) (i)
(b) (ii)
(c) (iii)
(d) (iv)

Question 4.
Which of the following is the best set-up for tracing the path of ray of light through a glass prism?

(a) (i) only
(b) (ii) only
(c) (iii) only
(d) (iv) only

Question 5.
Four students A, B, C and D perform experiment on tracing the path of light ray through a glass prism. The correct position of the pins used to describe incident ray is shown on paper by four of them, respectively as

The correct result is obtained by
(a) A and B both
(b) B and C both
(c) A and D both
(d) A only

Question 6.
For the refraction of a ray of light through a glass prism, the path of a ray of light is shown below. [AI2013]

The angle of incidence, the angle of emergence and the angle of deviation respectively have been represented by
(a) Y, N, Z
(b) X, Z, M
(c) X, N, Z
(d) X, M, Z

Question 7.
While performing experiment to trace the ray of light through a triangular glass prism, a student is asked to draw the boundary of a prism on a paper. This is done to
(a) see whether the given prism is triangular or not.
(b) ensure that the prism is always kept within the boundary during experiment.
(c) check whether the edges of the prism are broken or not.
(d) make the diagram more clear.

Question 8.
A student traces the path of a ray of light passing through a triangular glass prism. For measuring the angle of emergence, the correct setting of the protractor (D) corresponds to diagram

(a) (A)
(b) (B)
(c) (C)
(d) (D)

Question 9.
In an experiment to trace the path of a ray of light through a glass prism for different values of angle of incidence, a student would find that the emergent ray [Delhi 2013]
(a) is parallel to the incident ray.
(b) perpendicular to the incident ray.
(c) is parallel to the refracted ray.
(d) bend at an angle to the direction of incident ray.

Question 10.
For tracing the path of a ray of light through a glass prism, the ideal distance between two pins is [Foreign 2013]
(a) 2-3 cm
(b) 3-5 cm
(c) 6-8 cm
(d) 10 – 15 cm

Question 11.
The following figure depicts the bending of a ray of light on passing through a prism. Identify the angle of deviation from the given choices.

(a) ∠ABG
(b) ∠CEB
(c) ∠FEC
(d) ∠DCI

Question 12.
For the refraction of a ray of light through a glass prism, the path of ray of light is shown.

(a) PQ
(b) LM
(c) RS
(d) PM

Question 13.
Sumitra was given two glass prisms. One was made of ordinary glass and other one was of flint glass (Given that refractive index of flint glass is more than that of ordinary glass). She traced the path of rays through both and after observing the diagram and concluded that bending of light for the given angle of incidence was
(a) more in the prism made of flint glass.
(b) more in the prism made of ordinary glass.
(c) same in both the glass prisms.
(d) independent of the nature of glass of which the prism was made.

Question 14.
While performing the experiment on tracing the path of a ray of light through a glass prism as shown in the given diagram, four students interpreted the result as given below. At the minimum deviation position of the prism, which one of the four interpretations is correct?

(a) θ₁ = θ₃
(b) θ₁ = θ₂
(c) θ₁ = θ₄
(d) θ₁ > θ₃

Question 15.
In an experiment on tracing the path of ray of light passing through a glass prism, three students A, B, C performed the experiment for three different sets of angles of incidence. The set taken by them, were respectively as (15 , 25 , 45 ), (30 , 45 , 60 ) and (65 , 75 , 85 ). The student/students who is/are likely to get best results is/are
(a) A only
(b) B only
(c) C only
(d) all of them

Question 16.
While performing the experiment to trace the path of a ray of light passing through a glass prism, four students marked the incident ray and the emergent ray in their diagrams in the manner shown below:

The correct figure is given in
(a) I
(b) II
(c) III
(d) IV

ANSWER KEY

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

Short Answer Questions (2 Marks)

Question 1.
Draw a ray diagram to show refraction of light through a triangular glass prism and show angle of incidence, angle of deviation, emergent ray and emergent angle in it.
Answer:

Here,
PQ = incident ray
RS = emergent ray
∠i = angle of incidence
∠r = angle of refraction
∠e = angle of emergence
∠8 = angle of deviation

Question 2.
In the experiment to trace the path of a ray of light through a triangular glass prism,

1. a student is asked to draw the boundary of prism on the paper, why?
2. if the angle of incidence is 30°, what can be the measure of angle of emergence at the minimum deviation condition of the prism?

Answer:

1. It ensures that prism is always kept within this boundary during the experiment.
2. At the minimum deviation condition,
   ∴ angle of emergence = angle of incidence
   ∠e = ∠i = 30°

Question 3.
In the experiment to trace the path of a ray of light through a triangular glass prism,

1. if the emergent ray makes an angle of 35° with the second face of prism, then what is the angle of emergence?
2. can the angle of deviation be zero and why?
3. Answer:
4. The angle of emergence ∠e, = 90° – 35° = 55°
5. No, because at the second refracting surface of the prism, ray of light moves from denser to rarer medium; it bends away from the normal, i.e. bends towards the base of the prism.

Question 4.
When we place a glass prism in the path of a narrow beam of white light, a spectrum is obtained. What happens when a second identical prism is placed in an inverted position with respect to the first prism? Draw a labelled diagram to illustrate it.
Answer:
Second prism recombines the spectrum obtained by the first prism. So, from the other side of the second prism, a beam of white light is again obtained as shown in figure given below:

Question 5.
A very thin narrow beam of white light is made incident on one face of rectangular glass slab and one face of the combination of two prisms, one up and one down in contact .Comment on the nature of the behaviour of the emergent beam in the above two cases.
Answer:
Case 1: The emergent beam refracted through a rectangular glass slab emerges parallel to the incident ray and is shifted sideward slightly. No dispersion occurs.
Case 2: The emergent beam from the second inverted identical prism is again white light and emerges parallel to the incident beam and shifted sideward slightly. Overall no dispersion occurs due to the coipbination of two prisms placed inverted to each other.
Thus emergent ray in both the cases emerges from the opposite parallel faces and is parallel to the incident ray.

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