Atmospheric Refraction - PRACTICE SET
Chapter: Human Eye and Colourful World | Topic: Atmospheric Refraction
ATMOSPHERIC REFRACTION - PRACTICE SET
Topic: Atmospheric Refraction
Multiple Choice Questions
Q1. Stars appear to twinkle because:
- Their light intensity actually fluctuates
- Atmospheric refraction causes the apparent position of the star to change continuously
- Stars move very fast
- Our eyes cannot process steady light
Q2. Planets do NOT twinkle like stars because:
- Planets produce their own light
- Planets are much larger and act as extended sources — slight variations in one part are averaged out
- Planets are closer to Earth
- Planets reflect only a single colour
Q3. The atmosphere acts as an optically denser medium near the Earth's surface. As a result, light from a star bends:
- Away from the normal (away from Earth's surface)
- Toward the normal (toward Earth's surface)
- Horizontally across the sky
- Randomly in all directionsOption D
Q4. We can see the Sun for about 2 minutes AFTER it has geometrically set below the horizon because:
- The Sun actually rises and sets twice
- Atmospheric refraction makes the Sun appear slightly above the horizon even when it is below
- Light from the Sun travels around the curvature of the Earth
- Reflection from clouds extends visibility
Q5. The apparent position of a star is:
- Its true position in space
- Always shifted toward the zenith (overhead) due to atmospheric refraction
- Shifted away from the zenith
- The same as its true position
Q6. Which phenomenon is responsible for the advanced sunrise and delayed sunset?
- Scattering of light
- Dispersion of light
- Atmospheric refraction
- Total internal reflection
Short Answer Questions
Q7. What is atmospheric refraction? Why does it occur — describe the optical property of the atmosphere that causes it.
Q8. Why do stars twinkle but planets do not? Explain using the concept of point sources vs extended sources.
Q9. Explain how atmospheric refraction causes an 'advance sunrise'. By how many minutes does the Sun appear to rise before it actually crosses the horizon?
Q10. The apparent position of stars as seen from Earth differs from their true (actual) position. Explain why, and state in which direction the apparent position is shifted.
Q11. Why does the Sun appear oval (elliptical) just at sunrise or sunset rather than perfectly circular?
Q12. Describe one phenomenon in the Indian night sky or monsoon season that can be explained by atmospheric refraction. Explain it using the physics of refraction.
Long Answer Questions
Q13. Explain atmospheric refraction and its effects in detail. Your answer must cover:
(a) why the atmosphere acts as a non-uniform refracting medium (decreasing density with altitude),
(b) how light from a distant star or the Sun bends as it passes through layers of increasing density,
(c) why this causes the apparent position of a celestial body to be different from its actual position,
(d) the advance sunrise and delayed sunset phenomenon (about 2 minutes each), and
(e) why this effect is more pronounced for objects near the horizon than for objects overhead.
Q14. Explain why stars twinkle and planets do not. Your answer must include:
(a) the mechanism of twinkling — why the apparent position and brightness of a star changes rapidly,
(b) how the Earth's turbulent atmosphere causes these rapid fluctuations,
(c) why a star is a point source while a planet is an extended source as seen from Earth,
(d) why the fluctuations from an extended source average out and appear steady, and
(e) why stars nearer the horizon twinkle more than stars overhead.
Q15. Using the concept of atmospheric refraction, explain the following observations that an astronomy enthusiast in Ladakh might make on a clear night:
(a) Stars near the horizon appear to shimmer and shift, while stars overhead appear steady.
(b) The crescent Moon seen near the horizon appears slightly distorted in shape.
(c) During a sunset, the setting Sun looks flattened (oval).
(d) A star that appears to be just above the horizon has actually already set below it.
(e) During the golden hour in Indian summers, the sky near the horizon is bright and warm-coloured — connect this briefly to atmospheric refraction and scattering.
Numerical / Application-Based Problems
Q16. The Sun has an angular diameter of about 0.5° as seen from Earth. Atmospheric refraction near the horizon is approximately 0.5°.
(a) By how much does atmospheric refraction shift the apparent position of the Sun near the horizon?
(b) If the Sun geometrically sets at 6:00 PM and atmospheric refraction gives an extra 0.5° elevation, estimate the extra time the Sun appears visible. (Hint: the Sun moves 0.5° across the sky in about 2 minutes.)
(c) How much total extra daylight do we get from atmospheric refraction at sunrise and sunset combined?
Q17. A stargazer in Chennai notes that a star appears to be at an elevation angle of 10° above the horizon. Due to atmospheric refraction, the true elevation is 9.5°.
(a) By how much is the star's apparent position shifted?
(b) If refraction shifts a star's apparent position by 0.5° per degree of elevation below 15°, at what true elevation would a star appear to be at 5° above the horizon?
(c) Why does the atmosphere not shift the apparent position of stars near the zenith (overhead)?
Q18. The Moon's diameter subtends an angle of about 0.5° at the Earth. A student observes the Moon rise.
(a) Does the Moon twinkle like stars? Explain using angular size.
(b) Atmospheric refraction near the horizon compresses the vertical angular diameter of the Moon by about 0.1°. What percentage of its vertical diameter is affected?
(c) This gives the Moon a flattened appearance near the horizon. Without refraction, the Moon would appear circular. Describe how this distortion looks to the observer.