A star sometimes appears brighter and some other times fainter. What is this effect called ? State the reason for this effect.
A star sometimes appears brighter and some other times fainter. What is this effect called ? State the reason for this effect.
- This effect is called the twinkling of stars (or twinkling effect).
- Reason for this effect:
- The twinkling of a star is due to the atmospheric refraction of starlight.
- Starlight undergoes continuous refraction as it passes through the Earth's atmosphere, which has a gradually changing refractive index.
- Since the physical conditions (such as temperature and density) of the Earth's atmosphere are not stationary but keep changing continuously, the path of light rays coming from the distant point-sized star varies slightly over time.
- Consequently, the apparent position of the star fluctuates, and the amount of starlight entering our eyes flickers—making the star appear brighter at some moments and fainter at others.
Marking Scheme
- 1Identify the effect as the twinkling of stars: [0.5 marks]
- 2State that it is caused by atmospheric refraction of starlight: [0.5 marks]
- 3Explain that the physical conditions of the atmosphere are not stationary, causing the refractive index to change: [0.5 marks]
- 4Explain how the path of light fluctuates, causing the flickering in brightness: [0.5 marks]
Hint
Think about how the Earth's atmosphere is not stationary and how it bends light coming from very distant point sources.
Quick Oral Answer
The twinkling of stars is caused by atmospheric refraction. As starlight passes through the turbulent layers of Earth's atmosphere, its path is constantly bent, causing the amount of light reaching our eyes to fluctuate rapidly.
Analysis & Explanation
Stars are extremely far away from the Earth, so they approximate point-sized sources of light. As their light travels through the turbulent layers of the atmosphere, it continuously bends towards the normal. Because the atmosphere is dynamic, the refractive index of the air layers fluctuates. This causes the light rays to shift slightly, changing the intensity of light entering our eyes and producing the flickering (twinkling) effect.
Common Mistakes
- 1Attributing twinkling to the star's own changing brightness.
- 2Confusing atmospheric refraction with scattering of light.
Interesting Facts
Planets do not twinkle because they are much closer to Earth and act as extended sources of light, meaning the variations from different points average out to zero.
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Frequently Asked Questions
How many marks does this question carry in CBSE Class 10 Science 2012?
This question carries 2 marks in the CBSE Class 10 Science 2012 examination.
What topic does this question cover in Science?
This question covers the topic "Atmospheric Refraction" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2012 paper?
This is a Short Answer question from Section A in the CBSE Class 10 Science 2012 paper.