Consider the following statements:<br/>1. Size of the sun at dusk<br/>2. Colour of the sun at dawn<br/>3. Moon being visible at dawn<br/>4. Twinkle of stars in the sky<br/>5. Polestar being visible in the sky<br/><br/>Which of the above are optical illusions?

1. Size of the sun at dusk — The Sun appears larger near the horizon mainly due to the Moon/Sun illusion, a perceptual optical illusion, not because its actual size changes. Verdict: Illusion.

2. Colour of the sun at dawn — At dawn/sunset, shorter wavelengths are scattered more and the Sun appears red/orange; this is an optical effect of atmospheric scattering. Verdict: Treated as optical illusion/effect.

3. Moon being visible at dawn — The Moon can naturally remain above the horizon after sunrise depending on its phase and position. Verdict: Not an illusion.

4. Twinkle of stars in the sky — Twinkling is caused by atmospheric refraction/turbulence, making stars appear to flicker though they are not actually changing brightness rapidly. Verdict: Optical illusion/effect.

5. Polestar being visible in the sky — Polaris is actually visible in the northern sky near the celestial pole; this is a real observation. Verdict: Not an illusion.

Hence, the optical illusions/effects are 1, 2 and 4.