What is the primary cause of the apparent flattening of the Sun’s disc at sunrise and sunset?

The correct Answer is Atmospheric refraction

Explanation

The phenomenon of light bending as it passes from one medium to another is known as refraction. The Earth's atmosphere, composed of layers of varying optical densities, causes light from celestial bodies to refract, leading to observable effects.

Statement-wise Analysis

The question asks for the primary cause of the apparent flattening of the Sun's disc at sunrise and sunset. Dispersion by water vapour Incorrect. Dispersion is the splitting of white light into its constituent colours. While water vapour can contribute to atmospheric effects, it is not the primary cause of the apparent flattening of the Sun's disc. Scattering by aerosols Incorrect. Scattering of light by aerosols (tiny particles suspended in the atmosphere) is responsible for phenomena like the blue colour of the sky and the reddish appearance of the Sun at sunrise and sunset. However, it does not cause the apparent flattening of the Sun's disc. Atmospheric refraction Correct. When the Sun is near the horizon (at sunrise or sunset), its light travels through a greater thickness of the Earth's atmosphere compared to when it is overhead. The atmosphere acts as a refracting medium, with denser layers closer to the Earth's surface. As light rays from the lower limb of the Sun travel through a denser medium and a longer path, they are refracted more than the light rays from the upper limb. This differential refraction causes the lower limb of the Sun to appear raised more than the upper limb, leading to an apparent flattening or oval shape of the Sun's disc. Diffraction through narrow slits Incorrect. Diffraction is the bending of light waves as they pass around obstacles or through narrow openings. This phenomenon is not responsible for the apparent flattening of the Sun's disc.

Key Takeaway

The apparent flattening of the Sun's disc at sunrise and sunset is primarily caused by the differential atmospheric refraction of light rays from its upper and lower limbs.