How Light Travels Through A Prism?

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

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: 35^o, 45°, 50^o and 55^o.

RESULT

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

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.

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:

Angle of minimum deviation depends upon the

1. Angle of prism

2. Angle of incidence

3. Nature of material

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.

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.

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

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.

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

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.

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

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

The angle of the emergence, is calculated as:

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

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.

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