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. The question asks to identify which of the listed phenomena are optical illusions or effects. 2. Size of the sun at dusk (1): The sun (and moon) appearing larger near the horizon than when high in the sky is a well-known optical illusion, often called the Moon illusion. It's primarily related to perception and context rather than a physical magnification. 3. Colour of the sun at dawn (2): The reddish/orange colour is due to Rayleigh scattering of sunlight by the Earth's atmosphere, which scatters shorter wavelengths (blue) more effectively, allowing longer wavelengths (red/orange) to dominate when light travels through more atmosphere at sunrise/sunset. While a real physical effect, it alters the perceived colour compared to when the sun is overhead and is often discussed under atmospheric optics. 4. Moon being visible at dawn (3): The visibility of the moon at certain times depends on its orbital position and phase. This is a normal astronomical occurrence, not an optical illusion. 5. Twinkle of stars in the sky (4): Stars appear to twinkle due to the atmospheric refraction of starlight as it passes through turbulent layers of air with varying density. This causes rapid fluctuations in the light's path and intensity reaching the eye. This atmospheric optical effect leads to the perception of twinkling. 6. Polestar being visible in the sky (5): The Polestar's visibility is due to its position near the celestial pole and its brightness. It's a standard astronomical observation, not an illusion. 7. Considering optical illusions broadly to include perceptual illusions (1) and prominent atmospheric optical effects altering perception (2 and 4), statements 1, 2, and 4 describe such phenomena.