UPSC Prelims 2013 · Question 39 of 99

UPSC Prelims 2013 question on Optical Illusions Sun Moon

Consider the following statements:

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?

  1. 1, 2 and 3
  2. 3, 4 and 5
  3. 1, 2 and 4
  4. 2, 3 and 5
Show answer

Answer: C. 1, 2 and 4

Analysis

Statement 1 is an optical illusion because the Sun appears larger near the horizon due to the brain comparing it with surrounding terrestrial objects. In reality, its size remains nearly constant.

Statement 2 is also related to atmospheric optical effects. The reddish appearance of the Sun at dawn and dusk occurs because shorter wavelengths like blue light are scattered more, while red light reaches the observer.

Statement 4 is an optical effect caused by atmospheric refraction. Starlight continuously bends while passing through Earth’s turbulent atmosphere, making stars appear to twinkle.

Statement by statement

Statement 3 is not an optical illusion because the Moon can naturally remain visible during dawn depending on its phase and position.

Statement 5 is not an optical illusion because the Pole Star appears nearly fixed due to its alignment close to Earth’s rotational axis. Therefore, statements 1, 2, and 4 are optical illusions or optical atmospheric effects.

Extra UPSC info

Planets generally do not twinkle as much as stars because they appear as larger discs rather than point sources of light.

More questions on Physics

All 17 questions on Physics →

← Full 2013 question paper · All Science and Technology questions