CLASS 10 / EXPERIMENT 5 / REFRACTION THROUGH GLASS SLAB

EXPERIMENT – 5

To study refraction of light in rectangular glass slab

THEORY

• To understand the refraction of light through a glass slab consider the figure given below which shows the refraction of light through a rectangular glass slab.
• Here in this figure AO is the light ray travelling in air and incident on glass slab at point O.
• Now on entering the glass medium this ray bends towards the normal NN’ that is light ray AO gets refracted on entering the glass medium.
• After getting refracted this ray now travels through the glass slab and at point B it comes out of the glass slab as shown in the figure.
• Since ray OB goes from glass medium to air it again gets refracted and bends away from normal N₁N'₁ and goes in direction BC.
• Here AO is the incident ray and BC is the emergent ray and they both are parallel to each other and OB is the refracted ray.
• Emergent ray is parallel to incident ray because the extent of bending of the ray of light at the opposite parallel faces which are PQ (air-glass interface) and SR (glass-air interface) of the rectangular glass slab is equal and opposite.
• In the figure i is the angle of incidence, r is the angle of refraction and e is the angle of emergence.
• Angle of incidence and angle of emergence are equal as emergent ray and incident ray are parallel to each other.
• When a light ray is incident normally to the interface of two media then there is no bending of light ray and it goes straight through the medium.

APPARATUS:

A drawing board, rectangular glass slab, office pins, sheet of white paper, a protractor and sharply pointed pencil.

PROCEDURE:

1. Fix a sheet of white paper on a drawing board with drawing pins. Place the given glass slab nearly in the middle of the sheet.
2. Mark the boundary of the glass slab with a sharp pencil and label it as PQRS after removing the slab from its position.
3. On the line PQ mark a point E and draw a normal N1EN2 at it. Draw a line AE making angle AEN1 with the normal.The angle should neither too small nor too large (say about 40 degree).
4. Now place the glass slab again on its boundary PQRS and fix two pins A and B vertically about 10 cm apart on the line AE (say points A and B).
5. Look through the glass slab along the plane of the paper from the side SR and move your head until the images of the two pins A and B are seen clearly. Closing your one eye ,adjust the position of your head in such a way that the images of the pins A and B lie in the same straight line.
6. Fix two other pins C and D vertically in such a way that the images of the pins A and B and pins C and D, all these four, lie in the same straight line. Ensure that the feet of the pins ( not their heads ) lie in the same straight line.
7. Remove the slab and also the pins from the board and encircle the pin-pricks on the paper,with a sharp pencil.
8. Join the points D and C and produce the line DC towards the slab so that it meets the boundary line RS at the point F. Join the points e and F. Thus for the incident ray represented by line AE, the refracted ray and the emergant ray are represented by EF and FD respectively.
9. On the line RS draw a normal N1'FN2'  at point F. Now, with a protractor, measure angle AEN1, angle FEN2 and angle DFN2' labelled as angle i, angle r and angle e respectively. 
10. Now place the glass slab at some other position on the sheet of paper fixed on the board and repeat all the above steps again taking another angle of incidence.
11. Measure the angle of incidence i.e angle of refraction, angle of emergence, again.

 

 RESULT:

1) Within the experimental error, ∠i = ∠e. It implies incident ray and emergent ray are parallel to each other

2) ∠i > ∠r. it means a ray of light while passing from air to glass bends towards normal.