A star appears slightly higher (above) than its actual position in the sky. Illustrate it with the help of a labelled diagram.
A star appears slightly higher (above) than its actual position in the sky. Illustrate it with the help of a labelled diagram.
The starlight, on entering the Earth's atmosphere, undergoes continuous refraction before it reaches the Earth. This refraction occurs in a medium of gradually changing refractive index.
Since the physical density of air increases as we go closer to the Earth's surface, the refractive index of the atmosphere increases progressively downwards. Consequently, the atmosphere bends starlight towards the normal. When we trace these refracted rays backward in a straight line, the star appears slightly higher (above) than its actual position, especially when viewed near the horizon.
Marking Scheme
- 1Explaining that starlight bends towards the normal due to a progressively increasing refractive index of atmospheric layers. (1 mark)
- 2Drawing a labelled diagram showing the Earth, atmospheric layers, actual star position, curved refracted ray, and the higher apparent star position. (1 mark)
Hint
The density of air increases downwards, which means light bends towards the normal. Trace the final ray entering the eye straight back to find the apparent position.
Quick Oral Answer
Starlight bends towards the normal because it enters progressively denser layers of the atmosphere with a higher refractive index. This makes the star appear slightly higher than its actual position.
Analysis & Explanation
Atmospheric refraction is the bending of light as it passes through the Earth's atmosphere. Because the temperature and density of air layers vary, the refractive index is not uniform. As starlight travels from the vacuum of space (optically rarer) into the increasingly dense layers of the atmosphere (optically denser), it continuously bends towards the normal. The human eye perceives light as traveling in a straight line along the tangent to the path of the ray entering the eye, making the star appear higher in the sky.
Common Mistakes
- 1Drawing the light ray bending away from the normal (upwards) instead of downwards towards the Earth.
- 2Placing the apparent position below the actual position.
Interesting Facts
This same atmospheric refraction is responsible for the 'advance sunrise' and 'delayed sunset', making the Sun visible to us about 2 minutes before it actually crosses the horizon and 2 minutes after it has set.
Spotted a mistake or something unclear?
Tell us — we fix reported answers fast.
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.
Which chapter does this question come from in Science?
This question is from the chapter "The Human Eye and the Colourful World" in the CBSE Class 10 Science syllabus.
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.