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?

[UPSC Civil Services Exam – 2013 Prelims]

(a) 1, 2 and 3 only

(b) 3, 4 and 5 only

(c) 1, 2 and 4 only

(d) 2, 3 and 5 only

Answer: (c)        

Explanation:

• The size of the sun appears different throughout the day due to an optical illusion, with the sun appearing larger during morning time when seen with other objects, smaller during noon time due to a clear sky, and clearer during evening time.
• The color of the sun also appears to change throughout the day due to an optical illusion caused by the way light travels through the atmosphere. Red light travels farthest and is seen last, giving the sun a reddish hue during dawn and dusk, while it appears whitish or yellowish during noon time.
• The twinkling of stars in the sky is an optical illusion caused by the light of the stars passing through various layers of the atmosphere before reaching our eyes.
• The moon being visible at dawn is not an optical illusion, as it is visible during the half cycle of Full Moon to just before New Moon.
• The visibility of the Polestar in the sky is also not an optical illusion, as it is located in the North or South pole and its visibility depends on the rotation of the Earth.