THE HUMAN EYE AND THE COLOURFUL WORLD
ATMOSPHERIC REFRACTION
If an object is observed through hot air above a fire or radiator, it shows random wavering or flickering.
The hotter air just above
the fire is lighter (less dense) than the cooler air above
it. So its refractive index is slightly less than that of the cooler
air.
Here, the physical conditions of
the refracting medium (air) are not stationary. So, the apparent
position of the object fluctuates (wavering).
It is an effect of atmospheric refraction of light.
Twinkling of stars
It is due to atmospheric
refraction of starlight.
When starlight enters the
atmosphere, it undergoes continuous refraction. It occurs in a medium of
gradually changing refractive index.
The atmosphere bends starlight
towards the normal. So, the apparent position of star is slightly different
from its actual position. The star
appears slightly higher (above) than its actual position when viewed
near the horizon.
Apparent position of the star is
not stationary. It keeps on changing slightly due to varying physical
conditions of the atmosphere.
The stars are very distant. So,
they approximate point-sized sources of light. As the ray path 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 (sometimes
appears brighter, some other time, fainter). It is the twinkling effect.
The planets do not twinkle
because they are very close to the earth, and thus seen as extended sources.
If a planet is considered as a collection of many point-sized sources of light,
the net variation of light entering the eye will be zero. So it nullifies
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. Actual sunrise means the actual crossing of
the horizon by the Sun.
The time difference between actual sunset and the apparent sunset is about 2 minutes. The apparent flattening of the Sun’s disc at sunrise and sunset is also due to the same phenomenon.
