\nTwinkling of stars (scintillation) is caused by atmospheric refraction.
\n– Light from distant stars travels through the Earth’s atmosphere before reaching our eyes.
\n– The atmosphere is not uniform; it consists of layers with varying temperatures and densities.
\n– Variations in temperature and density cause variations in the refractive index of the air.
\n– As light from a star passes through these turbulent layers with changing refractive index, it undergoes continuous refraction in random directions.
\n– This causes fluctuations in the apparent position and brightness of the star as seen from Earth. These rapid fluctuations are perceived as twinkling.
\n– Planets, being much closer, appear as extended sources of light rather than point sources. The light from different parts of a planet’s disc undergoes similar but independent variations, which average out, so planets do not twinkle noticeably.<\/p>\nOption A correctly identifies the cause: changes in atmospheric refractive index due to temperature variations (and hence density variations) lead to varying refraction of starlight.
\nOption B is incorrect; stars emit light continuously.
\nOption C is incorrect; twinkling is an effect on light from a single star due to atmospheric effects, not interference from different stars.
\nOption D is incorrect; while diffraction occurs, twinkling is primarily an effect of refraction due to atmospheric turbulence.
\n<\/section>\n