UPSC Prelims 2013 Q89: Science & Technology – Physics
Consider the following phenomena:
1. Size of the sun at dusk
2. Colour of the sun at dawn
3. Moon being visible at dawn
4. Twinkle of stars in the sky
5. Polestar being visible in the sky
Which of the above are optical illusions?
- (a) 1, 2 and 3
- (b) 3, 4 and 5
- (c) 1, 2 and 4
- (d) 2, 3 and 5
Answer: (c) 1, 2 and 4
Explanation
Which of these are optical illusions?
An optical illusion makes us see something different from what is really there.
| Phenomenon | Optical illusion? | Reason |
|---|---|---|
| 1. Size of the sun at dusk | Yes | The sun only looks bigger near the horizon |
| 2. Colour of the sun at dawn | Yes | Scattering removes blue light, so the sun looks red |
| 3. Moon being visible at dawn | No | The moon is really there; nothing is distorted |
| 4. Twinkle of stars in the sky | Yes | Atmospheric refraction makes steady light seem to flicker |
| 5. Polestar being visible in the sky | No | A real star seen in its real place |
Effects of the atmosphere on light:
| Effect | Cause |
|---|---|
| Red sun at sunrise and sunset | Scattering (Rayleigh) |
| Blue sky | Scattering of blue light |
| Twinkling of stars | Refraction through moving air layers |
| Early sunrise, late sunset | Refraction, by about two minutes each |
Answer: (c) 1, 2 and 4.
Why each option is right or wrong
- (a) Incorrect. The moon at dawn is real, not an illusion; twinkling is left out.
- (b) Incorrect. The moon at dawn and the Polestar are real sights.
- (c) CORRECT. The sun's size at dusk, its colour at dawn and the twinkle of stars are illusions.
- (d) Incorrect. The moon and the Polestar being visible are not illusions.
Key point
Optical illusions from the atmosphere: large sun at the horizon, red sun at dawn (scattering), twinkling stars (refraction). Seeing the moon or the Polestar is real.
Remember
Illusions: big sun, red sun, twinkling stars.
Common mistakes
- Treating any unusual sight as an illusion.
- Forgetting that the red sun is due to scattering, not a change in the sun.
- Thinking planets twinkle like stars; they do not.
Exam tip
Illusion = appearance differs from reality. Merely seeing a real object is not one.
Sources
- NCERT Class X — Science, Ch 10 (The Human Eye and the Colourful World)
- NCERT Class XII — Physics Part II (Ray Optics and Optical Instruments)