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At some stage, we all might have wondered why do stars twinkle? But in reality, stars don’t twinkle. They just appear to twinkle when viewed from the ground. The twinkling of stars is the result of the earth’s atmosphere. When the starlight enters the earth’s atmosphere, the winds and areas with different temperatures and densities affect it. As a result, the starlight appears to twinkle when seen from the ground.
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Keyterms: Air, Atmosphere, Earth, Stars, Light, Winds, Twinkle, Starlight, Atmospheric refraction, Temperature, Water
Phenomenon behind twinkling of the stars
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Stars twinkle due to atmospheric refraction. The air above us is not very dense. It has a small bending effect on the light when the light passes through it. The atmosphere is always moving, and winds at every level will cause slight differences. This will eventually result in the bending of the light.
Also, different densities of winds bend light at different amounts. As a result, the starlight will appear to move in a zigzag way and give us an illusion of twinkling.
During clear nights, there will be a lot of air movements in the atmosphere, and the stars will appear to twinkle more. On other nights, when the upper air is calm, not much twinkling will be noticed.
Phenomenon behind twinkling of the stars
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Phenomenon similar to the twinkling of the stars
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This similar effect can be seen when we are at the bottom of a swimming pool during bright sunlight. We find a lot of moving light and dark patterns on the pool floor as the water surface is wavy due to the airy atmosphere. If there is no disturbance and the surface of the water is stable, and there is no wind, then the moving pattern will disappear. Similarly, the stars appear to twinkle in the airy nights.
Why don’t planets twinkle?
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Planets do not twinkle because of the following reasons-
- Planets reflect the low-intensity light reaching them; they do not have the light of their own.
- The shift in atmospheric refraction is less as planets are much closer to earth compared to stars.
- Since the planets are closer, they appear larger than stars, so the shift is never enough for planets to appear to twinkle.
If we see stars and planets from outer space, both will shine steadily since there will be no atmosphere to disturb the steady streaming of their light. The observers who have experience can observe the difference between the stars and planets. But to recognize a planet for the first time, one should notice the steadiness of the light of a planet by contrasting it to a nearby star.
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Things to Remember
- Stars twinkle due to atmospheric refraction.
- The atmosphere of the earth is affected by wind, temperature and different densities.
- When the light enters the earth’s atmosphere, it undergoes refraction many times.
- Some of the light rays coming from the star reach us directly while some bend away or towards us. This happens so fast that the stars give a twinkling effect.
- This phenomenon can also be observed when we are at the bottom of the swimming pool during a bright sunny day.
- Planets do not twinkle because they reflect the low-intensity light reaching them.
- If we see stars and planets from outer space, both will shine steadily since there will be no atmosphere to disturb the steady streaming of their light.
Sample Questions
Ques: Why do different components of white light deviate through different angles when passing through a triangular glass prism? (All India 2010, 1 Mark)
Ans: The different components of white light deviate through different angles when they are passed through a glass prism because of the change in refractive index of the prism and the atmosphere.
Ques: What will be the colour of the sky, when it is observed from a place in the absence of any atmosphere? Why? (All India 2012, 1 Mark)
Ans: The sky appears dark when it is observed from a place in the absence of the atmosphere.
The reason behind this phenomenon is that in the absence of the atmosphere, the sunlight would not scatter.
Ques: The sky appears dark instead of blue to an astronaut. State its reason. (Delhi 2012, 1 Mark)
Ans: The sky appears dark instead of blue to the astronaut because the scattering of light does not take place at high altitudes due to the absence of atmosphere.
Ques: Why does the sun appear reddish at sunrise? (All India 2011, 2 Marks)
Ans: At sunrise, the sun appears reddish because only the red colour ( λb < λr ) has the least scattering and due to this it is received by our eye before any other colour. Hence, the appearance of the sun near the horizon may look almost reddish at sunrise.
Ques: What is the colour of the clear sky during day time? Give a reason for it. (All India 2012, 2 Marks)
Ans: Clear sky appears blue.
Reason: The atmosphere contains molecules of air and other fine particles whose size is smaller than the wavelength of visible light. When sunlight passes through the atmosphere, the molecules and particles present in the atmosphere scatter the blue colour more strongly than the other colours of the spectrum because the wavelength of blue colour is more than other colours. This scattered blue light enters our eye due to which the colour of sky appears blue to us during daytime.
Ques: What is meant by the spectrum of white light? How can we recombine the components of white light after a prism has separated them? Draw a diagram to illustrate it. (Delhi 2011, 2 Marks)
Ans: Spectrum of white light is the coloured pattern VIBGYOR formed by a prism by splitting the incident white light. If we place two prisms inverted to each other, then the scattered light will be recombined into the white light again.
Components of white light after a prism
Ques: Explain why the planets do not twinkle but the stars twinkle. (Delhi 2011, 3 Marks)
Ans: Planets are larger in size and can collect a great amount of light which nullifies the twinkling effect.
The starlight undergoes multiple refractions due to different conditions of the atmosphere. As a result, the path of the star varies slightly while passing through the atmosphere. Therefore, the apparent position of the star fluctuates and the amount of light that is coming to us changes continuously. The star sometimes appears brighter and some other times, it appears fainter. This causes the twinkling of stars.
Ques: A star appears slightly higher (above) than its actual position in the sky. Illustrate it with the help of a labelled diagram. ( All India 2012, 3 Marks)
Ans: The changes in the refractive index of the atmospheric layers due to the various conditions causes atmospheric refraction. When the light coming from the stars enters the atmosphere, it gets refracted continuously. The air that is at higher level acts as a rarer medium while the air near the surface of the earth acts as a denser medium.
So, the atmosphere bends the starlight towards the normal due to this, the apparent position of the star is slightly different from its actual position. Thus, the star appears slightly higher than its actual position in the sky.
A star appears slightly higher (above) than its actual position in the sky
Ques: A star sometimes appears brighter and some other times fainter. What is this effect called? State the reason for this effect. (Delhi 2012, 3 Marks)
Ans: A star sometimes appears brighter and some other times fainter. This effect is called the Twinkling effect.
Reason: It is due to the atmospheric refraction of starlight. The atmosphere has different layers. The change in the refractive index of different layers of the atmosphere causes atmospheric refraction and the light of the stars undergoes multiple refractions. So, the fluctuation in the position of the star occurs continuously due to the changing amount of light coming to us. The star sometimes appears brighter and at some other times, it appears fainter. This causes the twinkling of stars.
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