Consider the following phenomena 1. Size of the sun at dusk 2. Colure 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 correct answer is C, that is 1, 2 and 4. The sun's apparent size at dusk is larger than its size overhead, a classic case where the brain judges size against the horizon and the eye is fooled. The sun's reddish colour at dawn is the result of atmospheric scattering, which removes the shorter blue wavelengths along the long path through the air, and the perceived colour is treated as an optical effect here. The twinkling of stars comes from atmospheric refraction and turbulence that shift the light rapidly, so the star looks like it is flickering.

The other two are real. The moon is physically visible at dawn when it is still above the horizon in the morning sky, and the Pole Star lies very close to the celestial north pole, so it is always visible in the northern sky from the northern hemisphere.

• Core concept: the difference between a true physical phenomenon and a perceptual or atmospheric effect that misleads the observer.
• Mains link: this sits under the basics of optics in Physical Geography and under atmospheric phenomena in Environment.

Trap: assuming that anything that looks unreal is an illusion. The moon and the Pole Star are visible in the sky as expected, so they are not illusions.