The twinkling of a star is due to the atmospheric

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diffraction of starlight

reflection of starlight

refraction of starlight

dispersion of starlight

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UPSC NDA-1 – 2022

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The twinkling of stars is caused by the atmospheric refraction of starlight. As starlight passes through the Earth’s atmosphere, which has layers of varying density and temperature, it undergoes continuous refraction. The turbulent movement of air causes fluctuations in the refractive index of the atmosphere along the path of light. This leads to variations in the apparent position and intensity (brightness) of the star as perceived by an observer, resulting in the twinkling effect.

Atmospheric refraction causes starlight to bend as it passes through layers of air with different optical densities, leading to fluctuations in apparent brightness and position.

Planets are closer to Earth than stars and appear as extended sources rather than point sources. The light from planets undergoes similar refraction but the effect is averaged out over their larger apparent disk, which is why planets typically do not twinkle.

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