The Human Eye and the Colourful World · Lesson 6 of 8
Atmospheric Refraction
“The moving atmosphere quietly shifts stars and borrows a few minutes of sunlight.”
• Explain atmospheric refraction through air layers of changing refractive index. • Relate the wavering of objects above hot surfaces to changing air conditions. • Explain the apparent position and twinkling of stars. • Explain why planets generally do not twinkle noticeably. • Describe advance sunrise, delayed sunset and apparent flattening of the Sun.
Air often seems optically empty, but it is not always uniform. Watch an object across the hot air rising from a road, fire or radiator and its outline may waver. The object is steady; the air between it and the observer is changing. Light continually refracts through layers whose density and refractive index vary, so the apparent direction of the object fluctuates.
The refraction of light as it passes through the Earth’s atmosphere, whose refractive index changes from place to place and with height.
Air just above a hot surface becomes warmer, expands and is less dense than cooler air above it. Its refractive index is therefore slightly different. Because rising air is turbulent, the boundaries and refractive conditions do not remain stationary. Successive rays from the same object follow slightly different paths, making the object appear to shift or flicker. This local effect provides a small-scale model for optical effects across the full atmosphere.
Problem
Why does an object viewed above a fire appear to waver even when neither object nor observer moves?
- 1.Hot and cooler air form irregular regions with slightly different refractive indices.
- 2.The regions move and mix, so the light path changes from moment to moment.
- 3.The eye traces each arriving ray backward and assigns a slightly changing apparent position to the object.
Twinkling Of Stars
Starlight travels through an enormous thickness of atmosphere before reaching the eye. As it descends, it encounters air of gradually changing refractive index and undergoes continuous refraction. The atmosphere generally bends the light towards the normal as it enters denser lower layers. The final ray reaches the observer from a direction that makes the star appear slightly higher than its actual position, especially near the horizon.
The atmosphere is never perfectly steady. Temperature, density and air movement change continuously, so the refracted path and the apparent position of a star also change slightly. Because a distant star is effectively a point-sized source, small changes in its ray path alter the amount of starlight entering the eye. The star appears brighter at one moment and fainter at another. This rapid fluctuation is the twinkling effect.
The apparent fluctuation in the position and brightness of stars caused by continually changing atmospheric refraction of light from point-sized stellar sources.
The star does not jump through space every time it twinkles. The changing atmosphere alters the ray direction reaching the observer, and the eye assigns that ray a changing apparent origin.
Why Don’t The Planets Twinkle?
Planets are much closer to Earth than stars and appear as extended sources rather than single point-sized sources. Even if the planet looks small to the unaided eye, its visible disc can be treated as a collection of many point sources. Atmospheric changes may brighten one part while dimming another at the same time.
When the contributions from all parts of the disc are added, the variations largely average out. The total amount of light reaching the eye stays much steadier, so noticeable twinkling is usually absent. The difference is therefore not that planetary light avoids refraction; it is that fluctuations from an extended source cancel one another.
| Feature | Star | Planet |
|---|---|---|
| Apparent source | Point-sized | Extended disc |
| Atmospheric path changes | Affect the unresolved source strongly | Affect many source points differently |
| Combined brightness | Fluctuates noticeably | Variations largely average out |
| Usual appearance | Twinkles | Steady |
Problem
Both starlight and planet light cross the atmosphere. Why is ‘planets do not refract’ an incorrect explanation for their steady appearance?
- 1.Atmospheric refraction affects light from both stars and planets.
- 2.A star behaves as one unresolved point source, so its intensity variation remains visible.
- 3.A planet supplies light from many points across an extended disc.
- 4.The separate variations average out, producing a nearly steady total brightness.
Advance Sunrise And Delayed Sunset
Actual sunrise is the moment when the Sun physically crosses the horizon. Yet the Sun becomes visible about two minutes before that event. When the Sun is still slightly below the horizon, its light passes obliquely through atmospheric layers and bends towards the denser lower atmosphere. The observer receives the bent ray and traces it backward in a straight line, locating an apparent Sun above the horizon.
The same process allows the Sun to remain visible for about two minutes after actual sunset. Atmospheric refraction therefore advances the apparent sunrise and delays the apparent sunset. It also produces the apparent flattening of the Sun’s disc near the horizon because light from its lower and upper edges is refracted by slightly different amounts.
Problem
Actual sunrise at a location is 6:10 a.m. Using the approximate atmospheric effect described here, when may the upper edge of the Sun first appear? What assumption is being made?
- 1.The Sun is visible about two minutes before actual sunrise.
- 2.Subtract two minutes: 6:10 a.m. − 2 minutes = approximately 6:08 a.m.
- 3.The estimate assumes ordinary atmospheric conditions and uses the stated approximate time difference; actual observations can vary slightly.
Wavering above hot air, the raised apparent position of a star, twinkling, advance sunrise, delayed sunset and apparent solar flattening all arise because light travels through air whose refractive conditions vary along the path.
Quiz
What causes the wavering of an object seen through rising hot air?
Why does a star appear slightly higher than its actual position near the horizon?
Which property makes stellar twinkling noticeable?
Why do planets usually appear steady?
How long is the Sun approximately visible before actual sunrise because of atmospheric refraction?
Practice Problems
- Explain why hot air above a road can make a distant object appear unsteady. Answer: Temperature differences create moving air regions with changing refractive index. The light path varies, so the object’s apparent position fluctuates.
- Why is the apparent position of a star near the horizon higher than its actual position? Answer: Atmospheric layers continuously bend starlight. The eye extends the final arriving ray backward in a straight line and locates the star above its true position.
- Compare the source-size explanation for stars and planets. Answer: A star acts as one point-sized source, so atmospheric intensity changes remain visible. A planet acts as many point sources across a disc, and their variations largely cancel.
- If actual sunset is at 6:42 p.m., estimate the apparent sunset under the stated approximation. Solution: Atmospheric refraction delays sunset by about two minutes, so the Sun may remain visible until approximately 6:44 p.m.
- Connect apparent solar flattening with atmospheric refraction. Answer: Light from different parts of the Sun’s disc passes through slightly different atmospheric paths and is refracted by different amounts, so the vertical appearance is distorted near the horizon.
Key Takeaways
• Atmospheric refraction occurs because the atmosphere has a changing refractive index. • Turbulent hot air changes the light path and makes an object’s apparent position waver. • Atmospheric refraction makes a star near the horizon appear slightly higher than its actual position. • Changing ray paths and brightness from a point-sized star produce twinkling. • Planetary light comes from an extended disc, so individual fluctuations largely average out. • The Sun is visible about two minutes before actual sunrise and after actual sunset. • Different refraction across the solar disc causes apparent flattening near the horizon.