Chapter 6: Activity 3 (73)

Keep a glass slab on a blank paper and draw its outline PQRS as shown in the figure. Draw an inclined straight line on the side of PQ so that it intersects PQ at N. Pierce two pins vertically at two points A and B along the line. Look at the pins A and B from the opposite side of the slab and pierce pins C and D vertically so that the images of A and B are in line with C and D. Now remove the chip and the pins and draw a straight line going through points C and D so that it intersects SR at M. Join points M and N. Observe the incident ray AN and emergent ray MD.
$Refraction of light through a glass slab$ Refraction of light through a glass slab The first refraction occurs when light ray enters the glass from air at N on the side PQ. The second refraction occurs when light enters air through glass at point M on the side SR. For the first refraction the angle of incidence is i while for the second it is i₁. The angle of refraction at N is r. Note that i₁ = r. In the second refraction, the angle of refraction is e which is equal to i. On both parallel sides PQ and RS of the glass slab, the change in direction of light ray is equal but in opposite directions. Thus, the light ray MD emerging from the glass slab is parallel to the incident ray AN on the side PQ of the slab. But the emergent ray is somewhat displaced with respect to the incident ray. 3 Marks

Keep a glass slab on a blank paper and draw its outline PQRS as shown in the figure.
Draw an inclined straight line on the side of PQ so that it intersects PQ at N. Pierce two pins vertically at two points A and B along the line.
Look at the pins A and B from the opposite side of the slab and pierce pins C and D vertically so that the images of A and B are in line with C and D.
Now remove the chip and the pins and draw a straight line going through points C and D so that it intersects SR at M.
Join points M and N. Observe the incident ray AN and emergent ray MD.

**Refraction of light through a glass slab**

The first refraction occurs when light ray enters the glass from air at N on the side PQ. The second refraction occurs when light enters air through glass at point M on the side SR. For the first refraction the angle of incidence is i while for the second it is i₁. The angle of refraction at N is r.
Note that i₁ = r. In the second refraction, the angle of refraction is e which is equal to i. On both parallel sides PQ and RS of the glass slab, the change in direction of light ray is equal but in opposite directions.
Thus, the light ray MD emerging from the glass slab is parallel to the incident ray AN on the side PQ of the slab. But the emergent ray is somewhat displaced with respect to the incident ray.
3 Marks

Watch Video of the above activity (8:23):