prism experiment class 12 values

NCERT Solutions for Class 6, 7, 8, 9, 10, 11 and 12

To determine angle of minimum deviation for a given prism by plotting a graph between angle of incidence and the angle of deviation

December 1, 2016 by Sastry CBSE

Physics Lab Manual NCERT Solutions Class 12 Physics Sample Papers

Aim To determine angle of minimum deviation for a given prism by plotting a graph between angle of incidence and the angle of deviation.

Apparatus Drawing board, a white sheet of paper, prism, drawing pins, pencil, half-metre scale, office pins, graph paper and a protractor.

• Fix a white sheet of paper on the drawing board with the help of drawing pins or tape.
• Draw a straight line XX’ parallel to the length of the paper nearly in the middle of the paper.
• Mark points Q 1 , Q 2 , Q 3 ,… on the straight line XX’ at suitable distances of about 5 cm.
• Draw normals N 1 Q 1 , N 2 Q 2 , N 3 Q 3 ,… on points Q 1 , Q 2 , Q 3 ,…  as shown in diagram.
• Draw straight lines R 1 Q 1 , R 2 Q 2 , R 3 Q 3 ,… making angles of 35°, 40°, … 60° (write value of the angles on the paper) respectively with the normals.
• Mark one comer of the prism as A and take it as the edge of the prism for all the observations.
• Put it prism with its refracting face AB in the line XX’ and point Q 1 in the middle of AB.
• Mark the boundary of the prism.
• Fix two or more office pin P 1 and P 2 vertically on the line R 1 Q 1 . The distance between the pins should be 10 mm or more.
• Look the images of point P 1 and P 2 through face AC.
• Close your left eye and bring open right eye in line with the two images.
• Fix two office pins P 3 and P 4 vertically, and 10 cm apart such that the open right eye sees pins P 4 and P 3 and images of P 2 and P 1 in one straight line.
• Remove pins P 3 and P 4 and encircle their pricks on the paper.
• Repeat steps 7 to 13 with points Q 2 , Q 3 ,… for i = 40°,…, 60°. To measure D in different cases
• Draw straight lines through points P 4 and P 3  (pin pricks) to obtain emergent rays S 1 T 1 ,S 2 T 2 ,S 3 T 3 ,……
• Produce T 1 S 1 , T 2 S 2 , T 3 S 3 , … inward in the boundary of the prism to meet produced incident rays R 1 Q 1 , R 2 Q 2 , R 3 Q 3 ,… at points F 1 , F 2 , F 3 ,…
• Measure angles K 1 F 1 S 1 ,K 2 F 2 S 2 ,K 3 F 3 S 3 ,…….. These give angle of deviation D 1 ,D 2 ,D 3 ,….
• Write values of these angles on the paper. To measure A
• Measure angle BAC in the boundary of the prism. This gives angle A.
• Record your observations.

• i-D graph indicates that as the angle of incidence (i) increases, the angle of deviation (D) first decreases, attains a minimum value (D m  ) and then starts increasing for further increase in angle of incidence.
• Angle of minimum deviation, D m  = ……..
• Refractive index of the material of the prism, n = ……….

Precautions

• The angle of incidence should lie between 35°-60°.
• The pins should be fixed vertical.
• The distance between the two pins should not be less than 10 mm
• Arrow heads should be marked to represent the incident and emergent rays.
• The same angle of prism should be used for all the observations.

Sources of error

• Pin pricks may be thick.
• Measurement of angles may be wrong.

Question. 1.Define a prism. Answer. Read Art. 9.04.

Question. 2.Define edge of the prism. Answer. Read Art. 9.04.

3.Question. .Define angle of prism. Answer. Read Art. 9.04.

Question. 4.Define angle of deviation. Answer. The angle through which a ray of light turns away from its original path on passing through a prism, is called angle of deviation.

Question. 5.On what factors does the angle of deviation depend? Answer. The angle of deviation depends upon the following factors :

• The angle of incidence.
• The refracting angle of the prism.
• The material of the prism. (Refractive Index)
• The colour of the light used i.e., wavelength of light.

Question. 6.What is the relation between different angles involved in refraction through a prism? Answer. Read Art. 9.05.

Question. 7.Define angle of minimum deviation. Answer. The least value of the angle of deviation is known as the angle of minimum deviation. [Art. 9.06 (a)]

Question. 8.How is angle of minimum deviation related with refractive index of prism material? Answer. Read Art. 9.06 (b).

Question. 9.What is the speciality about minimum deviation? Answer. Under minimum deviation condition, light ray travels inside prism parallel to the base of the prism and the angle of incidence = angle of emergence.

Question. 10.Does the angle of minimum deviation depend upon the colour of light used? Answer. Yes, it is different for different colours or wavelengths.

Question. 11.What word helps in remembering the name of seven colours into which white light is splitted? Answer. The word is VIBGYOR.

Question. 12.What name is given to the phenomenon of splitting white light into seven colours? Answer. The phenomenon is called dispersion of light.

Question. 13.What does the graph between i and D indicate? Answer. There is only one unique value of angle of incidence for which the deviation is minimum.

Question. 14.Why there are two values of angle of incidence for one value of angle of deviation? Answer. If the emergent ray be reversed, then angle e becomes i and angle i becomes e. The reversed ray will have same deviation as before.

Question. 15.Hence, i and e are two different values for same angle of deviation. Which colour will travel (i) fastest (ii) slowest in prism? Answer. (i) Red colour (ii) Violet.

Question. 16.Does a beam of white light give a spectrum on passing a hollow prism? Answer. No. Because dispersion does not occurs in air.

Question. 17.Which colour deviate (i) Most (ii) Least. Answer. (i) Violet (ii) Red.

Question. 18.Will all colour of light travel with same speed inside a prism? Answer. No. Red colour travel fastest and violet colour travel smallest.

Question. 21.Can X-rays be dispersed? Answer. Yes.

Question. 22.What is angular deviation? Answer. The angle between the emergent rays of any two colours is called angular dispersion of those colours.

Question. 23.On what factors angular deviation depends? Answer. (i) Angle of prism (ii) refractive index.

Question. 24.What is dispersive power of prism? Answer. It is define as the ratio of the angular dispersion for these two colours to the mean deviation produced by the prism.

Question. 25.What is factor on which dispersive power depends? Answer. It depends upon refractive index of the material of prism.

Slab A slab is a piece of transparent material with rectangular faces. All faces are transparent and opposite faces are parallel. The dimension (side) along which the light travels inside the slab is called its thickness.

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• To Determine Angle of Minimum Deviation for a Given Prism

To Determine Minimum Deviation for Given Prism By Plotting Graph Between Angle of Incidence and Angle of Deviation

This article covers the procedure to determine the angle of minimum deviation for a prism. It contains the aim, Apparatus, theory, procedure, and observation, for determining the same.

To determine the minimum angle of deviation for a given prism, we need to plot a graph of the angle of incidence versus the angle of deviation. So, what does the angle of deviation mean?

Well! The angle of deviation of a prism is the angle that is obtained by finding the difference between the angle of incidence and the angle of refraction created by the light ray traveling from one medium to another that has a varying refractive index.

What is a Prism?

While looking at the sky, you might have wondered why there are rainbows during the rainy season? Did you ever see the work of a prism and wondered why it seems to create rainbows? Well, in this lesson you'll learn about prisms, how and why they work, and also the applications that you find in everyday phenomena.A prism is a thick transparent material just like glass or plastic, which has two flat surfaces that form an acute angle (less than 90°). 

White light contains seven colors of the rainbow. when it is passed through a prism, the colors of the rainbow emerge from the prism much like in the figure here. 

(Image will be Uploaded Soon)

We'll learn more about why a prism spreads white light out into the colors of the rainbow with an aim to determine angle of minimum deviation for a given prism.

Angle of Deviation in Prism

Theory of the angle of deviation in prism: A prism is a wedge-shaped object that is made from a refracting medium bounded by two plane faces inclined to each other at a certain angle. The two plane faces are the refracting faces and the angle between these faces is the angle of a prism (or the refracting angle).

To Determine the Angle of Minimum Deviation

To determine the angle of minimum deviation, let’s perform the angle of deviation of the prism experiment.

Aim of the Experiment:

To Determine the minimum angle of deviation of a prism.

To Find the angle of minimum deviation plotting a graph between angle of deviation and angle of incidence for a given prism.

Drawing board, meter scale, protractor, Pins(drawing and office), glass prism.

Materials or Apparatus Required:

A white sheet of paper

Drawing board

Drawing pins

A half-meter scale

Office pins

Graph paper

 Protractor

Theory of the experiment:

The formula for the refractive index in the prism is given by:

\[n=\frac{sin(A+DM)/2}{sin(A/2)}\]

Dm = angle of minimum deviation of a prism

A = angle of a prismn = refractive index

Diagram for the refraction of the light-ray through the prism at various angles

(ImagewillbeUploadedSoon)

Steps to Perform the Angle of Deviation of Prism Experiment:

Affix a white sheet of paper on the drawing board with the aid of drawing pins or tape.

Now, draw a straight line XX’ lying parallel to the length of the paper approximately in the middle of the paper.

Mark points Q1, Q2, Q3,… on the straight line XX’ at equal distances of around 5 cm.

Draw normals, viz: N1Q1, N2Q2, N3Q3,… on points Q1, Q2, Q3,…  as shown in the above diagram.Draw straight lines R1Q1, R2Q2, R3Q3,… making angles of 40°, 45°, … 70°, respectively with the normals, after noting down the value of the angles on the paper. 

Draw straight lines R1Q1, R2Q2, R3Q3,… making angles of 40°, 45°, … 70°, respectively with the normals, after noting down the value of the angles on the paper.

 Mark one corner of the prism as A and consider it as the edge of the prism for all your observations.

Put the prism with its refracting face AB in line XX’ and point Q1 in the middle of AB.

Now, draw the boundary of the prism.

Fix two or more office pins, viz: P1 and P2 vertically on the line R1Q1. The distance between the pins should be accurately 10 mm. Look at the images of points P1 and P2 via a face AC.

Now, close your left eye and bring the opened right eye in line with the two images.

Fix two office pins P3 and P4 vertically, and 15 cm apart such that the open right eye sees pins P4 and P3 and images of P2 and P1 in one straight line.

Remove pins P3 and P4 and encircle their pricks (sharp points) on the paper.

Now, repeat steps 7 to 12 with points Q2, Q3,… for i = 50°,…, 80°.

The materials refractive in index of the given prism is denoted by 

n=\[\frac{sin(A+DM)/2}{sin(A/2)}\]

In which A is the angle of the prism and Dm is the angle of minimum deviation

(Image will be uploaded soon)

Procedure: - 

With the help of drawing tape, pins fix a white sheet of paper on the drawing board.

Draw a straight line XX' nearly in the middle of the paper parallel to the length of the paper.

On the straight line XX' mark points Q1, Q2, Q3, at suitable distances of 5cm.

On point Q1, Q2, Q3, draw normals and N1 1 and 2 Q2 and Q3 as represented in the diagram.

Draw straight lines are R1Q1, R2Q2,R3Q3.., construct angles of 35°,40°...60°   and with the normal strike the value of the angles on the paper .

On one corner of the prism,mark A, and take it as the edge of the prism for all the observations. 

Point Q1 in the middle of AB, and put the prism with its refracting face in line XX'.

The boundary of the prism should be marked.

 Put two or more office pins vertically on the line R1Q1 as P1 and P2. There should be a distance of 10 mm or more between the pins.

Through face AC observe the images of points P1 and P2.

Bring an open right eye in line with the two images by keeping your left eye close.

  Fix two office pins at 10 cm apart as P3 and P4 so that the open right eye sees images of P2 and P1 in one and P4 and P3  in one straight line .

On the paper encircle the pricks after removing the pins P3 and P4.

Repetition of Steps 7 to 13 with points Q2,Q3...for i=40°,....,60°.

To Measure Various Values of D, Follow these Four Steps:

Draw straight lines via points P4 and P3 (pinpricks) to get emitting light rays, i.e., S1T1, S2T2, S3T3,……

Produce T1S1, T2S2, T3S3, … in an inward direction in the boundary of the prism to meet produced incident rays, viz: R1Q1, R2Q2, R3Q3,… at points F1, F2, F3,…, respectively.

Measure angles K1F1S1, K2F2S2, and K3F3S3,……... These give an angle of deviation; these are D1, D2, D3,…

.Jot down values of these angles on the paper.

Now, to measure the angle of the prism, measure the angle BAC, and you get the value of A.Finally, record your observations.

 For Measuring D in Different Cases

Through pinpricks points P4 P3 draw a straight line to obtain emergent races S1T1,S2T2,S3T3…

To meet produced incident rays R1Q1,R2Q2,R3Q3…,at points F1,F2,F3,.... produce T1S1,T2S2,T3S3,... inward in the boundary of the prism.

By measuring the angles K1F1S1, K2F2S2, K3F3S3,... gives angle deviation D1,D2 D3….

 Write the values on the paper

For measuring a

 In the boundary of the prism measure angle BAC that gives angle A .

Record the Observations

Angle of Deviation in Prism Experiment Observations

The first six observations of the Experiment:Angle of prism ‘A’ =.....

Our calculations gave the following graph:

Observation: 

Type of prism = equilateral glass prism.

The angle of prism = 60'    

Table for variation of the angle of deviation with the angle of incidence

FAQs on To Determine Angle of Minimum Deviation for a Given Prism

1.What is a prism shape?

A prism shape is a 3D shape that contains a constant cross section.  Both the ends are connected by rectangular sides and have the same 2D shape .The two ends are the same shape and size, meaning if you were to cut the shape in half, both pieces would look the same. A 2D shape with flat edges is a prism and some of the examples are rectangular prism, square prism, octagon prism and triangular prism.

2.What is prism and antiprism?

A regular prism consists of two regular N-sided polygons joined together by N squares. A regular antiprism consists of two regular N-sided polygons joined together by 2N equilateral triangles. The Square Prism and the Triangular Antiprism are the Cube and the Octahedron, respectively. The remaining regular prisms and antiprisms are closely related to Archimedean solids in the sense that they are convex polyhedra whose faces are regular polygons of two or more types that meet in the same pattern around each vertex.

3.Is a pyramid a prism?

A pyramid is a three-dimensional  structure Which has triangular  sites  joined at a vertex known as Apex and has one base  and its polygon in shape .  On the other hand, a 3D structure having two bases and rectangular sides is  known as a prism.Prism and Pyramid both are three-dimensional polyhedron shaped structures and the major difference lies in their base.

4.What are the different types of prisms?

A prism has polished surfaces that help in the refraction of light placed on one side of the prism and can be seen from the other side. Also, the cross-section in a prism is the same on all its sides. The shape of its base determines the type of prism. Few examples are hexagonal prisms, triangular prisms, pentagonal prisms and so on. In geometry and Optics prism is of utmost importance. It plays a vital role in the studies related to the splitting of the light, reflection and refraction.

5.What Precautions to be Taken While Performing the Angle of Minimum Deviation in the Prism Experiment?

A subject like Physics demands great practice on the theoretical parts and applying the knowledge practically requires the following precautions to be taken care of:The angle of incidence should always lie between 35° to 60°.We should fix the pins vertically. We should make sure that the distance between the two pins should not be less than 10 mm.Arrowheads must be marked to represent the incident and emerging rays.A similar angle of the prism should be used for all the observations.Always use thin pinpricks.Measurement of angles may get wrong, so be careful and accurate while performing the experiment.

6.What is the Angle Between Two Surfaces of a Prism? What is the Angle of Refraction of a Prism?

We know that the angle between two surfaces of a prism is known as refracting angle or the angle of prism. However, in a prism, a ray of light goes through two refractions and the result is deviation. The Ray of light passing through the prism is parallel to the base of the prism. The angle of refraction inside the transparent material of the prism is equal to half of the angle of the prism. A prism bears a very small refracting ∠A. A ray is an incident on the face of the prism in such a way that the angle of incidence ‘i’ is very small.

Angle of Minimum Deviation

To determine angle of minimum deviation for a given prism by plotting a graph between angle of incidence and angle of deviation.

Our aim in this experiment is to determine the angle of minimum deviation for a given prism by plotting a graph between the angle of incidence and the angle of deviation.

First, let’s understand the term “Deviation”,

Deviation of light can be defined as divergence in the path of light because of a change in the medium of propagation of light.

In this experiment, we are studying the deviation of light passing through a prism.

When a ray of light passes through a prism, it undergoes refraction twice from two adjacent surfaces. The final ray coming out of the prism deviates with respect to the ray incident on the prism. The angle between the two refracting surfaces is called the angle of the Prism (A).

The angle between the emergent ray and the incident ray is known as the angle of deviation.

Let us consider a case of minimum deviation to understand our experiment better.

A ray (TS) is incident on surface AB of the Prism at an angle of incidence i. The dotted line 

N’PN represents the normal to surface AB. After refraction from point P, it deviates along PO at an angle of refraction r. 

The ray again incidents at angle r 1 on surface AC and emerges (ray QR) at an angle e to the surface. The angle formed between ray TS (produced forward) and ray QR(produced backwards) is the angle of deviation ?.

From geometry, we can write as:

                        r 1  + r = A    (A is the angle of the prism)

                        ? = (i – r) + ( e –  r 1 )

                      ? = (i  +  e )  – (r +  r 1 )   = i  +  e – A

At the position of the prism in minimum deviation ? m , the light ray passes through the prism symmetrically, i.e. parallel to the base. So, at this position:

                                     i =   e,  which implies r = r 1  

The advantage of putting the prism in a minimum deviation position is that the image formed is the brightest in this position. This way, we can easily find the minimum deviation angle for a prism.

Which property of light does not change with a change in the medium of propagation?

Answer: Frequency depends upon the source and has nothing to do with the medium of propagation of light.

Which law relates the angle of incidence and the angle of refraction?

Answer: Snell’s Law

Define the angle of the Prism.

Answer: the angle between the two refracting surfaces of the prism is called the angle of the prism (A).

What is the relation between the angle of incidence and the angle of emergence when light suffers a minimum angle of deviation while passing through a prism?

Answer: In the case of the minimum angle of deviation, the angle of incidence is equal to the angle of emergence as the light passes parallel to the base of the prism.

What do you understand by the term angle of deviation?

Answer: Angle of deviation refers to the angle between the incident ray and refracted ray when they are extended to meet at a common point. It depicts the degree of divergence of light from its original path after refraction.

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Theory & Procedure, Spectrometer (Prism) | Physics Class 12 - NEET PDF Download

Our Objective:

(i) To study the angle of deviation (d) with angle of incidence (i) and to find the angle of minimum deviation (D) from i-d curve.

(ii) To find the refractive index of the material of the prism using A and D.

The Theory:

A prism is an optical element. It has polished flat surfaces that refract light. The traditional geometric shape of a prism has a triangular base and two rectangular sides. It is called triangular prism.

A prism can be made from materials like glass, plastic and fluorite. It can be used to split light into its components.

How a Prism Works

When light travels from one medium to another medium, it is refracted and enters the new medium at a different angle. The degree of bending of the light's path depends on the angle that the incident beam of light makes with the surface of the prism, and on the ratio between the refractive indices of the two media. This is called Snell's law.

where, n is the refractive index of the material of the prism. i is the angle of incidence. r is the angle of refraction.

The refractive index of many materials varies with the wavelength of the light used. This phenomenon is called dispersion. This causes light of different colors to be refracted differently and to leave the prism at different angles, creating an effect similar to a rainbow. This can be used to separate a beam of white light into its constituent spectrum of colors.

The relation between Refractive Index (n), Angle of Prism (A) and Angle of Minimum Deviation (D)

Consider the following triangular prism.

The angle A between the two refracting surfaces ABFE and ACDE is called the angle of prism.

A ray of light suffers two refractions on passing through a prism. If KL be a monochromatic light falling on the side AB, it is refracted and travels along LM. It once again suffers refraction at M and emerges out along MN. The angle through which the emergent ray deviates from the direction of incident ray is called angle of deviation 'd'.

As the angle of incidence is increased, angle of deviation 'd' decreases and reaches minimum value. If the angle of incidence is further increased, the angle of deviation is increased.

A graph is drawn between angle of incidence (i) and angle of deviation (d) by taking angle of incidence (i) along X-axis and angle of deviation (d) along Y-axis. It should be a curved graph.

The angle of minimum deviation is obtained from the graph. Let D be the angle of minimum deviation, then the refractive index (n) of the material of the prism is calculated using the formula,

Learning outcomes:

• Students understand the working of a prism.
• Students will be better able to do the experiment in a real laboratory by understanding the procedure.

Materials required:

• Glass prism
• Drawing board

Real Lab Procedure:

• A paper is fixed on the drawing board placed on the table.
• Place the given glass prism on the center of the paper.
• Using the pencil, mark the outline ABC of the prism on the paper.
• Remove the prism, and using the scale and pencil, normal N 1 O is drawn to the face AB at the point L.
• Using the protractor, measure an angle 30° from the normal.
• Another line KL is drawn at L making the angle 30° (angle of incidence i) with the normal N 1 O.
• Two pins R 1 and R 2 are fixed on this line.
• The prism is replaced on the outline ABC.
• Viewing the pins from the face AC of the prism, two other pins R 3 and R 4 are fixed so that R 1 , R 2 , R 3 and R 4 are in a line.
• Remove the pins.
• A line NM is drawn to meet on the face AC through the marks of R 3 and R 4 .
• The line LM is joined.
• The line KL is extended to get the LQ and NM is extended to get the line MP. These two lines meet at P.
• Using the protractor, measure the angle QPM. This is the angle of deviation d.
• Repeat the experiment for different values of angle of incidence (i) and the corresponding angle of deviations are measured.
• Draw a graph with angle of incidence (i) along the X-axis and angle of deviation (d) along the Y – axis.
• The angle of deviation corresponding to the lowest bend of the curve is the angle of minimum deviation (D).
• Angle of the prism A is directly measured from the outline of the prism using the protractor.

Simulator Procedure (as performed through the Online Labs)

• Select the prism from the drop down list.
• Change the angle of incidence using the slider.
• For each angle of incidence (i), you will get corresponding angle of deviation (d), angle of refraction on the side AB (r1) angle of refraction on the side AC(r2) and angle of emergence (e).
• Click on the ‘Show/Hide protractor’ button to measure the angle using protractor.
• Click on the centre cross mark on the protractor to drag the protractor.
• Click on the rotate icon on the protractor to rotate the protractor.
• Click on the close icon on the protractor to close the protractor.
• You can use the embedded worksheet to enter the values.
• To see the graph, click on the graph icon in the worksheet.
• From the graph you will get minimum angle of deviation D.

• You can verify your result by clicking on the ‘Show result’ button.
• To redo the experiment, click the ‘Reset’ button.

Observations:

Calculations:

• Angle of prism (A) = 60°.
• From graph, angle of minimum deviation D = ------------°

• A graph showing the variation of angle of deviation with the angle of incidence is plotted.
• Angle of minimum deviation, D = --------°
• Refractive index of material of the prism, n = ------------

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• Physics Article
• Tracing The Path Of The Rays Of Light Through A Glass Prism

Tracing the Path of the Rays of Light Through a Glass Prism

A prism is a transparent optical object with two flat surfaces that have an angle between them. When the light enters the prism, there is a bending of light as there is a change in the speed of light. The bending of the light is dependent on the angle of incidence, normal, and refractive indices. There are four different types of angles involved in this experiment, and they are the angle of incidence, angle of emergence, angle of prism and angle of deviation. Below is an experiment to trace the path of a light ray through a glass prism.

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

What Is a prism?

A prism is defined as a polyhedron with a triangular base and three rectangular lateral surfaces. It is used as an optical object to study the behaviour of white light when it is passed through it. The light bends at various angles like an angle of incidence, angle of reflection, angle of refraction, and angle of deviation.

Read More: Angle of Incidence

What Is the prism formula?

Following is the formula of the angle of prism:

where, µ is the refractive index. A is the angle of the prism. δ m is the minimum deviation.

What Is the angle of deviation?

The angle of deviation is defined as the angle between the incident ray and the emerging ray.

Materials Required

Following are the list of materials required for this experiment:

• A white sheet
• 4-6 all pins
• Drawing board

Experimental Setup

• Fix a white sheet on a drawing board using drawing pins.
• Place the triangular prism resting on its triangular base. Using a pencil, draw the outline of the prism.
• Draw NEN normal to the face of the prism AB. Make an angle between 30 ° and 60 ° with the normal.
• On the line PE, fix two pins at a distance of 5cm from each other and mark these as P and Q.
• Look for the images of the pins at P and Q through the other face of the prism AC.
• Fix two pins at R and S such that they appear as a straight line as that of the P and Q when it is viewed from the AC face of the prism.
• Remove the pins and the prism.
• At point F, make the points R and S meet by extending them.
• PQE is the incident ray which is extended till it meets face AC. SRF is the emergent ray which is extended backward to meet at point G.
• Now mark the angle of incidence ∠i, angle of refraction ∠r and the angle of emergence ∠e and the angle of deviation ∠D as shown in the experimental setup.
• Repeat the experiment for more angles between 30 ° and 60 °.

Observations

• At surface AB, the light ray enters and bends towards the normal on refraction.
• At surface AC, the light ray bends away from the normal as it travels from one medium (glass) to the other (air).
• The angle of deviation is observed. Here, the emergent ray bends at an angle from the direction of the incident ray.
• The incident ray bends towards the normal when it enters the prism and while leaving the prism it bends away from the normal.
• With the increase in the angle of incidence, the angle of deviation decreases. After attaining the minimum value, it increases with an increase in the angle of incidence.

Precautions

• For drawing the boundary of the prism, a sharp pencil should be used.
• Soft board and pointed pins should be used.
• The distance between the pins should be 5cm or more.
• The pins should be fixed vertically and should be encircled when they are removed from the board.
• The angle of incidence should be between 30 ° and 60 ° .
• The arrows drawn for incident ray, reflected ray and emergent ray should be proper.
• For viewing the col-linearity of all four pins and images, the head should be slightly tilted on either side. While doing this it can appear as if all are moving together.

Read More: Refractive Index

Q1. What is refractive index? Ans: Refractive index is defined as the ratio between the sine of the angle of incident ray i in a vacuum to the sine of the angle of refraction r in a given medium. The mathematical representation of the refractive index is given as:

• n is the refractive index
• i is the angle of the incidence
• r is the angle of the reflection

Q2. What is the unit of the refractive index? Ans: Refractive index has no unit. This is because it is a ratio between the speed of light in vacuum and in a medium.

Q3. Name the atmospheric refraction that causes the splitting of white light. Ans: Rainbow.

Q4. What is the dispersion of light? Ans: The dispersion of light is defined as the phenomenon of splitting white light into its seven constituent colours when it is made to pass through a transparent medium.

Q5. List the factors on which the angle of deviation through a prism depends. Ans: Following are the factors on which the angle of deviation through a prism depends:

• ∠A is called the angle of a prism
• Angle of incidence
• On the optical density of the material used in a prism.

Q6. What happens to the incident ray when it enters the prism? Ans: When an incident ray enters the prism, it bends towards the normal as it changes its path and gets deviated.

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Glass Prism Experiment

Experiment: To draw a graph between the angle of incidence (i) and angle of deviation (δ) for a glass prism and to determine the refractive index of the glass of the prism using this graph.

When light travels from one medium to another in which its speed is different, the direction of travel of the light is changed, when light travels from the medium of lesser speed to the medium of greater speed, the light is bent away from the normal. If light travels from a medium of greater speed to one of lesser speed, the light is bent towards the normal. The ratio of the sine of the angle of incidence in vacuum (i) to the sine of the angle of refraction (r) in a substance is equal to the ratio of speed of light (v 1 ) in the vacuum to the speed of light in (v 2 ) in the substance.

sin i / sin r = v 1 /v 2 = n

where the constant n is called the refractive index of the substance.

If a ray MN of light is incident on one surface of a prism ABC, the ray is bent at both the first and the second surface. The emergent ray RS is not parallel to the incident ray but is deviated by an amount that depends upon the refracting angle A of the prism the refractive index n of its material and also on the angle of incidence (i) at the first surface. As the angle of incidence is decreased from a large value, the angle of deviation decreases at first and then increases and is minimum when the ray passes through the prism symmetrically. The angle of deviation, δ m , is then called the angle of minimum deviation . For this angle of minimum deviation δ m , there is a simple relation between the refracting angle A, the angle of minimum deviation δ m , and the refractive index n. The relation is

n = sin i(A+δ m )/sin (A/2)

Material Required

Drawing board, white paper, prism, pins, pencil, scale, protractor, drawing pins

How To Perform Experiment

1. Fix a sheet of a white paper on the drawing board.

2. Draw a line AB representing a face of the given prism. At a point N on this line, draw normal KN and a line MN at angle z representing an incident ray. Do not keep i less than 30° as the ray may get totally reflected inside the prism.

3. Place the prism on the sheet so that its one face coincides with the line AB. Refracting edge A of the prism should be vertical.

4. Fix two pins P 1 and P 2 on the line MN. Looking into the prism from the opposite refracting surface AC, position your one-eye such that feet of P 1 and P 2 appear to be one behind the other. Now fix two pins P 3 and P 4 in line with P 1 and P 2 as viewed through the prism.

5.  Remove the pins and mark their positions. Put a scale along side AC, remove the prism and then draw a long line representing surface AC. Draw line joining P 3 and P 4 . Extend lines P 2 P 1 and P 4 P 3 so that they intersect at F. Measure the angle of incidence i (angle MNK), angle of deviation D (angle RFG) and angle of prism (angle BAG).

6. Repeat the experiment for at least five different angles of incidence between 30° and 60° at intervals of 5°

Common prisms are usually quite small with sides of 2.5 cm or 3 cm. So drawing the boundary of the prism and then measuring angle A does not lead to accurate value of A. Therefore, it is suggested that you draw a long line for faces AB and AC with a ruler and place the prism touching the ruler.

Class 12 Physics MCQ – Ray Optics – Refraction through a Prism

This set of Class 12 Physics Chapter 9 Multiple Choice Questions & Answers (MCQs) focuses on “Ray Optics – Refraction through a Prism”.

Sanfoundry Global Education & Learning Series – Physics – Class 12 .

To practice all chapters and topics of class 12 Physics, here is complete set of 1000+ Multiple Choice Questions and Answers .

• Practice Class 12 - Chemistry MCQs
• Check Class 12 - Physics Books
• Practice Class 12 - Biology MCQs
• Practice Class 12 - Mathematics MCQs
• Practice Class 11 - Physics MCQs

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Class 12 Physics (India)

Course: class 12 physics (india)   >   unit 9.

• Dispersion of light - definition and characteristics
• Prism & dispersion of light
• Newton's prism experiment
• Minimum deviation in prism
• Deriving prism formula
• Geometry of prism

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