Science (086) — AI-generated practice question

Stars twinkle because they are extremely far away and act as point-sized sources of light. As starlight travels through the earth's atmosphere, it undergoes continuous refraction through layers of varying refractive index. Since atmospheric conditions keep changing, the amount of starlight reaching our eye fluctuates — the star appears alternately brighter and fainter, causing the twinkling effect.

Planets do not twinkle because they are much closer to the earth and appear as extended sources (a collection of many point-sized sources). The variations in light from all these individual points average out to zero, nullifying the twinkling effect.

Source: Chapter 10, Section 10.5 — Atmospheric Refraction

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• The key contrast examiners look for: point source (star) vs extended source (planet).
• Mention atmospheric refraction and changing refractive index for stars — these are the cause.
• For planets, the phrase "average out to zero" (or equivalent) is important to include.
• Do not confuse twinkling with scattering — it is a refraction phenomenon.
• 3 marks are typically split: 1.5 for stars + 1.5 for planets, so give roughly equal weight to both.