Page 152 - NCERT Science Class 10 English Medium

Activity 9.10
Activity 9.109.10
Activity 9.10
9.10
Activity
Activity
n Fix a sheet of white paper on a drawing board using drawing pins.
n Place a rectangular glass slab over the sheet in the middle.
n Draw the outline of the slab with a pencil. Let us name the outline
as ABCD.
n Take four identical pins.
n Fix two pins, say E and F, vertically such that the line joining the
pins is inclined to the edge AB.
n Look for the images of the pins E and F through the opposite edge.
Fix two other pins, say G and H, such that these pins and the
images of E and F lie on a straight line.
n Remove the pins and the slab.
n Join the positions of tip of the pins E and F and produce the line
up to AB. Let EF meet AB at O. Similarly, join the positions of tip
of the pins G and H and produce it up to the edge CD. Let HG
meet CD at O′.
n Join O and O′. Also produce EF up to P, as shown by a dotted line
in Fig. 9.10.

In this Activity, you will note, the light ray has changed its direction
at points O and O′. Note that both the points O and O′ lie on surfaces
separating two transparent media. Draw a perpendicular NN’ to AB at O
and another perpendicular MM′ to CD at O′. The light ray at point O has
entered from a rarer medium to a denser medium, that is, from air to
glass. Note that the light ray has bent towards
the normal. At O′, the light ray has entered
from glass to air, that is, from a denser
medium to a rarer medium. The light here
has bent away from the normal. Compare the
angle of incidence with the angle of refraction
at both refracting surfaces AB and CD.
In Fig. 9.10, a ray EO is obliquely
incident on surface AB, called incident ray.
OO′ is the refracted ray and O′ H is the
emergent ray. You may observe that the
emergent ray is parallel to the direction of
the incident ray. Why does it happen so? The
extent of bending of the ray of light at the
opposite parallel faces AB (air-glass interface)
and CD (glass-air interface) of the rectangular
glass slab is equal and opposite. This is why
the ray emerges parallel to the incident ray.
However, the light ray is shifted sideward Figure 9.109.10
Figure
Figure 9.10
9.10
Figure
Figure 9.10
slightly. What happens when a light ray is Refraction of light through a rectangular glass slab
incident normally to the interface of two
media? Try and find out.
Now you are familiar with the refraction of light. Refraction is due to
change in the speed of light as it enters from one transparent medium to
another. Experiments show that refraction of light occurs according to
certain laws.

Light – Reflection and Refraction 147

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