Why do stars twinkle?
The twinkling of stars is due to atmospheric refraction of starlight. When starlight enters the Earth's atmosphere, it undergoes continuous refraction before reaching the observer, as the atmosphere has a gradually changing refractive index.
Since the physical conditions of Earth's atmosphere are not stationary, the apparent position of the star keeps fluctuating. The path of rays of light coming from the star varies slightly, causing the amount of starlight entering the eye to flicker.
As a result, the star sometimes appears brighter and at other times fainter. This is the twinkling effect. Since stars are very distant, they approximate point-sized sources of light, making this effect clearly visible.
Explanation
This answer is based on NCERT Class 10 Science textbook content on atmospheric refraction. The key points covered are: (1) atmospheric refraction causes twinkling, (2) continuously changing refractive index of atmosphere, (3) fluctuating apparent position due to non-stationary physical conditions, and (4) stars as point-sized sources making the twinkling noticeable. The explanation connects all these concepts logically for a 2-mark answer.