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?

The phenomena of the size of the sun at dusk and the twinkle of stars are classic optical illusions. The apparent enlargement of the sun or moon near the horizon (the 'Moon illusion') is a psychological perception where the brain misinterprets size due to the presence of terrestrial reference points or the Ponzo illusion, as physical measurements show no change in angular diameter [6]. The twinkling of stars is an optical effect caused by atmospheric scintillation, where light is refracted by turbulent air layers. The reddish colour of the sun at dawn is a physical phenomenon caused by Rayleigh scattering, where shorter wavelengths are scattered away, leaving longer red wavelengths [1]. The moon being visible at dawn is a geometric reality of its orbit and position relative to the sun [4]. The Polestar's visibility is a physical fact of its position above the Earth's axis [5]. Thus, only 1, 2, and 4 are optical illusions.

Sources

1. [6] https://en.wikipedia.org/wiki/Moon_illusion
2. [1] Science , class X (NCERT 2025 ed.) > Chapter 10: The Human Eye and the Colourful World > Advance sunrise and delayed sunset > p. 168
3. [4] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 11: Keeping Time with the Skies > Activity 11.1: Let us explore* > p. 172
4. [5] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 19: The Motions of The Earth and Their Effects > Midnight Sun > p. 253