Atmospheric refraction: twinkling stars, early sunrise and delayed sunset
Atmospheric refraction is the bending of light by air layers. It makes stars twinkle and lets us see the Sun about 2 minutes before actual sunrise and after actual sunset.
What atmospheric refraction is
Air is not the same everywhere. Hot air is lighter (less dense) and has a slightly lower refractive index than the cooler air around it. Light passing through layers of air with a changing refractive index bends a little at each layer. This is atmospheric refraction. Because the air keeps moving, the bending keeps changing. Look at things through the hot air above a fire, a tandoor or a hot road: they seem to waver or flicker. This is atmospheric refraction on a small scale.
Effects of atmospheric refraction
| Effect | What you see | Why it happens |
|---|---|---|
| Twinkling of stars | A star seems to flicker, brighter one moment and fainter the next | Stars are so far away that they act as point sources of light. Moving air keeps changing the path of their light, so the amount reaching the eye keeps changing |
| Planets do not twinkle | Planets shine steadily | Planets are much closer, so they look like extended sources (many points together). The changes from all the points average out to zero |
| Stars appear higher | A star near the horizon looks a little higher than it really is | The atmosphere bends starlight towards the normal as it comes down into denser air |
| Early sunrise and delayed sunset | We see the Sun about 2 minutes before it actually crosses the horizon, and about 2 minutes after it actually sets | The atmosphere bends sunlight from the Sun below the horizon towards us, so the Sun appears above the horizon |
How to draw: early sunrise
- 1Draw a large arc for the surface of the Earth. Mark the observer O on it.
- 2Draw a straight line touching the Earth at O. Label it horizon.
- 3Draw a wider arc above the Earth for the edge of the atmosphere.
- 4Draw the actual Sun just below the horizon line.
- 5Draw a ray from the Sun that enters the atmosphere and curves gently down to O.
- 6Extend the last part of this ray backwards as a dotted line, up above the horizon. Draw the apparent Sun there and label it.
How to draw: the apparent position of a star
- 1Draw a line or a gentle arc for the Earth's surface, with an observer on it.
- 2Draw a few curved layers above it for the atmosphere. Write refractive index increases with an arrow pointing down towards the ground.
- 3Draw the real star high up on one side. Draw its ray bending slightly at each layer as it comes down to the observer.
- 4Extend the last part of the ray backwards as a dotted line. Mark the apparent star on it, a little higher than the real star.
On a clear night away from city lights, the stars twinkle, but a bright planet such as Venus or Jupiter shines steadily. The few extra minutes of daylight at dawn and dusk come from the same bending of sunlight by air.
1 marks: “By about how many minutes does atmospheric refraction lengthen the day?” About 4 minutes: about 2 minutes at sunrise and 2 at sunset.
2 marks: “Why do stars twinkle but planets do not?” Stars are point sources, so changing atmospheric refraction makes their light flicker. Planets are extended sources, so the changes from their many points average out.
3 marks: “Explain, with a diagram, why the Sun is visible about 2 minutes before actual sunrise.” Atmospheric refraction bends light from the Sun below the horizon towards the observer, so the Sun appears above the horizon. Diagram: Earth, horizon, atmosphere, curved ray, actual Sun below and apparent Sun above the horizon.
Stars are small points, so they shiver (twinkle). Planets are patches of light, so they stay put.