Page 173 - NCERT Science Class 10 English Medium

10.5 ATMOSPHERIC REFRACTION

You might have observed the apparent random wavering or
flickering of objects seen through a turbulent stream of hot air
rising above a fire or a radiator. The air just above the fire becomes
hotter than the air further up. The hotter air is lighter (less dense)
than the cooler air above it, and has a refractive index slightly
less than that of the cooler air. Since the physical conditions of
the refracting medium (air) are not stationary, the apparent
position of the object, as seen through the hot air, fluctuates.
This wavering is thus an effect of atmospheric refraction (refraction
of light by the earth’s atmosphere) on a small scale in our local
Figure
Figure 10.9
Figure 10.910.9
Figure 10.9
Figure 10.9
Apparent star position environment. The twinkling of stars is a similar phenomenon on
a much larger scale. Let us see how we can explain it.
due to atmospheric
refraction
Twinkling of stars
The twinkling of a star is due to atmospheric refraction of starlight. The
starlight, on entering the earth’s atmosphere, undergoes refraction
continuously before it reaches the earth. The atmospheric refraction
occurs in a medium of gradually changing refractive index. Since the
atmosphere bends starlight towards the normal, the apparent position
of the star is slightly different from its actual position. The star appears
slightly higher (above) than its actual position when viewed near the
horizon (Fig. 10.9). Further, this apparent position of the star is not
stationary, but keeps on changing slightly, since the physical conditions
of the earth’s atmosphere are not stationary, as was the case in the
previous paragraph. Since the stars are very distant, they approximate
point-sized sources of light. As the path of rays of light coming from
the star goes on varying slightly, the apparent position of the star
fluctuates and the amount of starlight entering the eye flickers – the
star sometimes appears brighter, and at some other time, fainter, which
is the twinkling effect.
Why don’t the planets twinkle? The planets are much
closer to the earth, and are thus seen as extended sources.
If we consider a planet as a collection of a large number
of point-sized sources of light, the total variation in the
amount of light entering our eye from all the individual
point-sized sources will average out to zero, thereby
nullifying the twinkling effect.

Advance sunrise and delayed sunset
The Sun is visible to us about 2 minutes before the actual
sunrise, and about 2 minutes after the actual sunset
because of atmospheric refraction. By actual sunrise, we
mean the actual crossing of the horizon by the Sun. Fig.
10.10 shows the actual and apparent positions of the
Figure
Figure 10.10
10.10
Figure 10.1010.10 Sun with respect to the horizon. The time difference
Figure
Figure 10.10
Atmospheric refraction between actual sunset and the apparent sunset is about
effects at sunrise and 2 minutes. The apparent flattening of the Sun’s disc at
sunset
sunrise and sunset is also due to the same phenomenon.
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