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?

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

Explanation

Option (c) is correct

• Size of the sun at dusk: The sun appears larger near the horizon due to atmospheric refraction, an optical illusion.
• Colour of the sun at dawn: The sun appears red or orange at dawn because of the scattering of shorter wavelengths of light (blue and violet) by the atmosphere, creating an illusion of color.
• Moon being visible at dawn: This is not an optical illusion; the moon is visible depending on its position relative to the Earth and the Sun.
• Twinkle of stars in the sky: This is an optical illusion caused by the Earth’s atmosphere, which causes the light from stars to refract and appear to twinkle.
• Polestar being visible in the sky: The polestar’s visibility is not an illusion; it remains relatively fixed in the sky.

Answer: (c) 1, 2 and 4; Difficulty Level: Easy