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Atmospheric Refraction and Scattering of Light

Air is denser near the ground, so light from space bends gradually as it comes down (atmospheric refraction). This makes stars twinkle and look higher, and lets us see the Sun about 2 minutes before it rises and after it sets. Tiny particles in air throw light sideways (scattering). Very small air molecules scatter blue far more than red, so the sky is blue; at sunrise and sunset sunlight crosses so much air that the blue is scattered away and the Sun looks red.

šŸŽ¬ Step-by-step story

  1. The Earth wears a blanket of air that gets thinner as you go up. Starlight bends little by little on its way down, so your eye sees the star a bit higher than it really is.
  2. The air above us keeps moving and changing, so the bending keeps changing. The star's position and brightness flicker: it twinkles. A planet is a small disc, not a point, so its flickers average out and it stays steady.
  3. At dawn the Sun is still just below the horizon, but its light bends over the curve of the air to reach you. You see the Sun about 2 minutes early, and it sets about 2 minutes late.
  4. A torch beam in clear water is hard to see, but in milky water its path lights up, because tiny particles throw light sideways. This is the Tyndall effect.
  5. Air molecules are much smaller than the wavelength of light. They scatter blue light far more than red, and that scattered blue reaches us from every direction: the sky is blue.
  6. Near sunrise and sunset the light crosses a much longer path of air. Most blue is scattered out on the way, so the Sun looks orange-red. Drag the Sun height slider and watch the sky and Sun change.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

šŸ¤” Common doubts, cleared

Is the star really where I see it?

Not quite. Starlight bends as it enters denser air, and your eye traces the last bit of the ray straight back, so the star looks higher.

Why does a planet not twinkle if its light also passes through the same air?

A planet is a small disc, not a point. Flickers from different parts of the disc cancel out, so its total light stays steady.

How can I see the Sun when it is still below the horizon?

Its light bends along the curved air layers and reaches your eye; you see it along the last part of that bent path, above the horizon.

Why can I see a torch beam in fog but not in clean air?

Fog droplets are big enough to scatter a lot of light sideways into your eyes. Clean air scatters very little, so the beam's path stays invisible.

If violet scatters even more than blue, why isn't the sky violet?

Sunlight has less violet than blue, some violet is absorbed high up, and our eyes are more sensitive to blue. So we see the sky as blue.

Why is the Sun white at noon but red in the evening?

At noon light crosses a short path and keeps most of its blue. In the evening the path is many times longer, so blue is scattered away and red is left. Slide the Sun down to see it.

Atmospheric refraction

The air around the Earth is not the same everywhere: it is densest near the ground and thinner higher up. Denser air bends light more (higher refractive index). So light coming from space enters layers of ever-denser air and bends a little at each one. This gradual bending by the atmosphere is atmospheric refraction.

Why stars twinkle but planets do not

Stars are so far away that they act as point sources of light. The air along the path keeps moving and its density keeps changing, so the amount of bending keeps changing. The apparent position of the star wobbles and the amount of light entering our eye changes: the star twinkles.

Planets are much nearer and look like tiny discs. A disc is a collection of many point sources; their flickers go up and down at different moments and cancel out. So planets shine steadily.

Advance sunrise and delayed sunset

Because of atmospheric refraction, we see the Sun about 2 minutes before it actually crosses the horizon in the morning and about 2 minutes after it has actually set. The day is about 4 minutes longer than it would be without air. The Sun also looks slightly flattened near the horizon, because its lower edge is lifted more than its upper edge.

Scattering of light and the Tyndall effect

When light hits very small particles it is sent off in many directions. This is scattering. The colour that gets scattered depends on particle size:

Tyndall effect

The path of a beam becomes visible when it passes through a colloid such as smoke, fog, mist or milky water, because the particles scatter light towards our eyes. Examples: sunbeams in a forest through the canopy, a projector beam in a dusty hall.

Why the sky is blue

Sunlight contains all colours. As it passes through air, the tiny Nā‚‚ and Oā‚‚ molecules scatter blue (short wavelength) much more than red (roughly 5ā€“6 times more). This scattered blue light reaches our eyes from every part of the sky, so the sky looks blue.

Above the atmosphere there is nothing to scatter light, so astronauts and people in very high-flying aircraft see a dark sky even in daytime.

Colour of the Sun at sunrise and sunset

At noon sunlight crosses a short path of air, so only a little blue is lost and the Sun looks white-yellow. Near the horizon the light travels through a path many times longer. By the time it reaches us most of the blue and green has been scattered out of the beam, and mainly orange-red is left. So the Sun and nearby clouds look reddish at sunrise and sunset.

Exam tips: questions often ask why stars twinkle, why planets don't, why the sky is blue, why danger signals are red, why the sky is dark in space and what the Tyndall effect is (2ā€“3 marks each).

Key formulas and definitions

Worked examples

1. Why do stars twinkle?

Starlight passes through air layers whose density keeps changing. The bending of light keeps changing, so the star's apparent position and brightness fluctuate. Because a star is a point source, these changes are noticed as twinkling.

2. Why are danger signal lights red?

Red has the longest wavelength of visible light, so it is scattered least by fog, smoke and dust. It travels farthest without fading and can be seen from a long distance.

3. Why do clouds look white while the sky looks blue?

Cloud droplets are much larger than the wavelength of light, so they scatter all colours about equally and the mix looks white. Air molecules are tiny and scatter mainly blue.

4. What would the sky look like if the Earth had no atmosphere?

There would be no scattering, so the sky would look black even in daytime, with the Sun as a bright disc. Sunrise and sunset would also not be advanced or delayed.

Common mistakes

Practice quiz

1. Stars twinkle because of:
2. Because of atmospheric refraction the Sun is seen about ___ before actual sunrise.
3. The sky appears blue because blue light is:
4. The Tyndall effect can be seen in:
5. To an astronaut in space the sky appears:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is atmospheric refraction?

It is the bending of light as it passes through layers of the Earth's air that have different densities. It causes twinkling of stars and early sunrise and late sunset.

What is the Tyndall effect?

It is the scattering of light by particles of a colloid (like smoke, fog or milky water), which makes the path of a light beam visible.

Why is the sky blue?

Air molecules are much smaller than the wavelength of light and scatter blue light much more than red. The scattered blue reaches our eyes from all over the sky.

Where this is taught

CBSE (India)Class 10Natural Phenomena

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