What is atmospheric refraction? Explain why stars twinkle but planets do not.

1. 1 · Define in one line. Atmospheric refraction is the bending of light as it passes through layers of air whose refractive index keeps changing.
2. 2 · Explain twinkling. Starlight bends again and again in the atmosphere. Stars are very far, so they act as point sources. As the air moves, the path of their light keeps changing, so the light entering the eye flickers.
3. 3 · Draw the apparent position. [Diagram: air layers; ray from the real star bending down to the observer; dotted extension to the apparent star, slightly higher]
4. 4 · Explain planets. Planets are much nearer and look like extended sources. The flicker from their many points averages out, so they do not twinkle.
5. 5 · Daily-life link. On a clear night, a steady bright point near the Moon is often a planet such as Venus or Jupiter.
6. 6 · Close in one line. So twinkling depends on whether the source is a point or an extended one.

Scoring tip: Use the words point source and extended source: they carry the marks.