“The time difference between the actual sunset and the apparent sunset is about 2 minutes.” What is the reason for the same? Explain with the help of a diagram.
“The time difference between the actual sunset and the apparent sunset is about 2 minutes.” What is the reason for the same? Explain with the help of a diagram.
The Sun is visible to us about 2 minutes before the actual sunrise and about 2 minutes after the actual sunset. By actual sunrise (or sunset) we mean the instant when the Sun's upper edge actually crosses the horizon. The reason for this difference is atmospheric refraction.
The Earth's atmosphere is optically denser near the surface and rarer higher up. When the Sun is slightly below the horizon, the sunlight travelling from the rarer upper layers of the atmosphere to the denser layers near the Earth bends towards the normal continuously as it passes through layers of increasing refractive index. Because of this continuous bending, the light rays from the Sun (which are still actually below the horizon) curve into the observer's eye, making the Sun appear to be above the horizon. Hence the Sun is seen about 2 minutes earlier and about 2 minutes later than the actual time of sunrise and sunset. The apparent flattening of the Sun's disc at sunrise and sunset is also due to the same phenomenon.
Marking Scheme
- 11 mark: Correct explanation — atmospheric refraction of sunlight as it passes through layers of varying density in the atmosphere.
- 21 mark: Neat labelled diagram showing the actual position of the Sun, the apparent position, the horizon, and the bending of light rays through atmospheric layers.
Hint
Think about what happens to light as it travels from rarer to denser parts of the atmosphere — the same phenomenon that makes a coin in an empty bucket appear to rise when the bucket is filled with water.
Quick Oral Answer
Due to atmospheric refraction, sunlight coming from the Sun (which is actually below the horizon) continuously bends towards the normal as it passes from rarer to denser layers of the atmosphere, and reaches the observer's eye. This makes the Sun appear above the horizon about 2 minutes before actual sunrise and about 2 minutes after actual sunset.
Analysis & Explanation
This question tests the application of atmospheric refraction to a real-life situation. The NCERT text clearly identifies atmospheric refraction as the cause of the 2-minute time difference. The diagram must show the actual Sun below the horizon, the observer on the Earth's surface, and the apparent Sun above the horizon, with the light ray drawn bending as it traverses the atmosphere. A good diagram should label the actual position, the apparent position, the horizon, and the bending of light rays. This 2-mark question requires both the explanation (1 mark) and a labelled diagram (1 mark).
Common Mistakes
- 1Stating reflection instead of refraction as the cause.
- 2Forgetting to mark the actual and apparent positions of the Sun in the diagram.
- 3Not drawing the bending of the light ray in the atmosphere.
- 4Confusing this phenomenon with dispersion or scattering.
Interesting Facts
On a normal day the atmosphere gives us about 4 extra minutes of sunlight — 2 minutes at sunrise and 2 minutes at sunset.
The same effect is responsible for the apparent flattening of the Sun's disc (oval shape) at the time of sunrise and sunset.
The Sun is not a point source; its lower edge is refracted more than its upper edge, which is why the Sun appears flattened at the horizon.
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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.
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 "Advance sunrise and delayed sunset" 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.