How Light Travels Through A Prism? Class 10th Science Lab Manual CBSE Solution

Class 10th Science Lab Manual CBSE Solution

Lab Experiment 13
Question 1.

AIM

Tracing the path of the rays of light through a glass prism.


Answer:

MATERIALS REQUIRED


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


THEORY


Prism: It is a piece of a transparent medium bounded by three rectangular surfaces forming a triangle. One surface (may be transparent or opaque) is called base. Other two surfaces are transparent and are called refracting surfaces. The line along which the refracting surface meet is called the edge of the prism. The angle between the two refracting surfaces is called the angle of the prism.


(i) When a ray of light from the air is incident on the face of a triangular prism, it gets refracted and bends towards the normal to the plane of the face.


(ii) The refracted ray travels inside the prism until it strikes its other face.


(iii) At the second surface AC, the ray enters from glass to air, i.e. denser to rarer medium but bends away from the normal, i.e. bends away from the normal towards the base of the prism .



PQ - Refracted ray


∠β- Angle of prism


∠θ1 - Angle of incidence


∠θ2 - Angle of refraction



∠θ4 - Angle of emergence


∠θ3 - Angle of deviation


PROCEDURE


1. Fix a white sheet of paper on the drawing board using pins and cello-tape.


2. Draw a thin dotted line XY at the middle of the white sheet using a meter scale and a pencil.


3. Place a glass prism with one of its refracting surface AB along the line XY along.


4. Mark the boundary ABC of the glass prism holding it firmly with your hand and then remove the prism.


5. Draw a normal N1, perpendicular to line AB at P. Also, draw a line making an angle of incidence with the normal. This line will act as the incident ray. 


6. Place the glass prism back to its original position ABC.


7. Place two pins P1 and P2 on the line EF vertically by gently pressing their heads with thumb into the drawing board about 5 -7cm apart from each other.


8. Look the images of P1 and Q1 from the other refracting face AC. The images of pins P1 and Q1 appear to be at P2 and Q2.


9. Fix two more pins P3 and Q3 vertically such that the feet of the pins P3 and Q3 appear to be on the same straight line as the feet of the images P2 and Q2, respectively.


10. All the pins P3, Q3 and image of P1 and Q1 will appear to move together if they are on the same straight line.


11. Remove the prism and all pins. Mark the position of feet P2 and Q2 and also P1 and Q1 on the sheet of paper.


12. Draw a straight line joining the pin marks 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.


13. Draw a straight line joining F and G. The line FG represents the path of the refracted ray.


14. Repeat the experiment on different values of angle of incidence, viz: 35o, 45°, 50o and 55o.




RESULT



When a ray of light is incident on the first surface of a prism, it bends towards normal, on reaching other surfaces it is again refracted and bends away from the normal. In this process, it bends from its original path by an angle called angle of deviation.


PRECAUTIONS


1. The refracting faces of the glass prism should be smooth, transparent and without any air bubble or broken edge. It must be triangular.


2. Use a sharp pencil to draw the boundary of the prism and rays of light.


3. The distance between the pins should be at least 5-7 cm. It provides the greater accuracy in finding the direction of the incident ray and refracted ray.


World||Refraction and Dispersion of Light Through A Prism



Viva Questions
Question 1.

What is a prism?


Answer:

A prism is a homogenous, transparent, refracting material enclosed by two inclined plane refracting surfaces at some fixed angle called refracting angle or angle of the prism.



Question 2.

What happens to oblique ray when it enters the prism?


Answer:

When an oblique ray of light suffers two refractions on passing through a prism and hence deviates through a certain angle from its original path as shown in the picture below:



Question 3.

What are the factors angle of deviation depends upon?


Answer:

Angle of minimum deviation depends upon the

1. Angle of prism


2. Angle of incidence


3. Nature of material



Question 4.

What is the angle of minimum deviation?


Answer:

When the angle of incidence increases, the angle of deviation decreases, till it reaches a minimum value at a particular angle of incidence and then starts to increase with the angle incidence. Thus we get, the minimum value of the angle of deviation.



Question 5.

What will happen when white light passes through the prism?


Answer:

When a beam of white light is incident on one of the refracting surface of the prism, it splits onto a band of seven colours. This phenomenon exhibited by the prism is called dispersion of light. The band of the coloured component of the light is called as a spectrum.



Question 6.

Give the order of colour when the light gets dispersed by a prism?


Answer:

The order of clours from the base of the prism is Violet, Indigo,Blue,Green,Yellow,Orange and then red.



Question 7.

Why do different colour emerges through a prism in a different direction?


Answer:

The different colour present in the white light has a different wavelength, frequency, and speed which make them emerge out through the prism in a different direction.



Question 8.

Which colour will deviate maximum and minimum?


Answer:

The Violet colour will deviate through a maximum angle, and the red colour will deviate through a minimum angle.



Question 9.

What will When we pass light through the combination of two inverted prisms placed together?


Answer:

The light when passes through the first prism get dispersed, the second inverted prism gather all the dispersed light together to form a beam of light.



Question 10.

What is meant by an angle of the prism?


Answer:

It is the angle of inclination between the two rectangular refracting faces of the prism.



Question 11.

What is the angle of emergence?


Answer:

The angle between the emergent ray and the normal at the second refracting face of the prism is called as the angle of emergence.



Question 12.

If the emergent ray makes an angle of 40 with the second refracting surface of the prism ,then find the angle of emergence.


Answer:

The angle of the emergence, is calculated as:



Question 13.

In the experiment to trace the path of a ray of light through a triangular prism,

1. A student is asked to draw the boundary of the prism , why?

2. If the angle of incidence is 30, what can be the measure of the angle of incidence at minimum deviation ?


Answer:

1. It ensures that prism is always kept within this boundary during the experiment.


2. At the minimum deviation condition,


The angle of emergence= angle of incidence




Question 14.

In the experiment to trace the path of a ray light a triangular glass prism.

1. If the emergent ray makes an angle 35 with the second face of the prism , then what is the angle of emergence.

2. Can the angle of deviation be zero and why?


Answer:

1. The angle of emergence 


2. No, because at the second refracting surface of the prism , a ray of light moves from denser to a rarer medium, it bends away from the normal, I.e., bends towards the base of the prism.



Question 15.

What is the property of the image formed by the prism?


Answer:

The image formed by the prism is real, laterally inverted.


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