Question

A few phenomena of light are given below :
(I) Advance sunrise and delayed sunset
(II) Twinkling of stars
(III) Wavering of objects seen through a turbulent stream of hot air
(IV) Blue colour of sky
Which of the above phenomena are due to atmospheric refraction?

• A. (I), (II) and (IV)
• B. (I), (II) and (III)
• C. (I), (III) and (IV)
• D. (I), (II), (III) and (IV)

Hint:

Any optical phenomenon involving "twinkling," "bending," "shifting of position," or "apparent wavering" of light source positions in air is caused by refraction.
Phenomena involving "color" (like blue sky or red sunset) are usually caused by scattering.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question list four optical phenomena and asks us to identify which of them are caused by the refraction of light in the Earth's atmosphere.

Step 2: Key Formula or Approach:
We analyze the physical mechanism behind each listed phenomenon.

Step 3: Detailed Explanation:

• (I) Advance sunrise and delayed sunset: Caused by the bending of solar light rays as they enter and pass through the increasingly dense layers of the Earth's atmosphere. This is due to atmospheric refraction.

• (II) Twinkling of stars: Starlight passes through turbulent atmospheric layers with varying temperatures and densities (hence fluctuating refractive indices). This causes continuous changes in the apparent position and intensity of the starlight, leading to twinkling. This is due to atmospheric refraction.

• (III) Wavering of objects through hot air: Hot air above a fire or stream is less dense and has a lower refractive index than the cooler air above it. The physical movement of this hot air causes the refractive index to fluctuate rapidly. Light passing through this turbulent air refracts randomly, causing objects behind it to appear to waver. This is a local refraction effect.

• (IV) Blue colour of sky: This is caused by the scattering of sunlight by fine air molecules in the atmosphere (Rayleigh scattering), where shorter blue wavelengths are scattered much more than longer red wavelengths. It is not caused by refraction.

• Therefore, phenomena (I), (II), and (III) are due to refraction.


Step 4: Final Answer:
The phenomena due to atmospheric refraction are (I), (II) and (III).