Light – Reflection and Refraction / Wonder of Light Part – II
Refraction of light rays caused due to variation in density of atmospheric air is called atmospheric refraction.
Examples of Refraction in Natural Phenomenon
Small – Scale Effects –
(a) Mirage – on a very hot summer day in the afternoon, the apparent random wavering of the hot air rising above the heated roads is observed.
(b) The apparent random wavering or flickering of objects is observed through a turbulent stream of the hot air rising above a fire (e.g. holi fire) or a radiator.
Reason – The air just above the heated road, fire or radiator becomes hotter than the air further up. The hotter air is lighter (less denser, rarer) than the cooler air (denser). Thus, the refractive index of hot air is less than the cool air. Due to change in physical conditions of the refractive medium (air), the apparent or exact position of object fluctuates.
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Large-Scale Effects –
(a) Twinkling of stars (Stellar Scintillation) –
- Stars are luminous heavenly bodies and point sized sources of light for us. They emit light of
their own. The light from stars comes to the Earth after traveling millions of miles and passing
through the Earth’s atmosphere. Hot and cold air pockets blow continuously in the atmosphere,
and these pockets of air cause the refraction of light rays at different angles, due to which
stars look brighter and fainter, i.e. twinkling of stars.
- The star appears above than its actual position. The apparent position of star changes continuously with the changing physical conditions of the Earth’s atmosphere due to same reason given above.
- Stars closer to the horizon appear to twinkle more than stars that are overhead. This is because the light from stars near the horizon travels through more air than the light of the stars overhead. Thus, there is more refraction- more twinkling.
- Moonlight or light from planets doesn’t twinkle or planets generally don’t appear to twinkle. Light from planets as well as the moonlight refracts as it passes through the atmosphere, but since the moon and the planets appear so large and twinkling is so slight that it is not noticeable.
- Stars do not appear to twinkle when viewed from the moon. This is because the moon doesn’t have an atmosphere surrounding it.
(b) Advanced Sunrise and delayed Sunset
The Sun is seen a few minutes (about 2 minutes) before it rises above the horizon and after it goes below the horizon.
Due to refraction, the Sun’s rays are bent by the Earth’s atmosphere before it rises above the horizon. When it is just below the horizon, the light rays of the Sun get refracted in the Earth’s atmosphere, and they reach the areas where the Sun has not yet risen. Thus, an observer in such areas appears to see the Sun before it rises above the horizon.
The same phenomenon is observed during evening at the time of sunset. The Sun is visible for some time even after it has gone below the horizon, due to refraction of light.
Thus, advanced sunrise and delayed sunset increases duration of day by about 4 minutes.
(c) At dusk or dawn the Sun appears to be larger than at noon.
This is because when the Sun is near the horizon, the rays of light coming from the Sun have to pass through layers of air of increasing density. Due to continuous bending of light the Sun appears to be larger. At noon, the Sun appears to be smaller. This is because the rays of light that fall normally on the surface of the Earth do not get refracted.
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