Q. Consider the following phenomena: - Size of the sun at dusk
- Colour of the sun at dawn
- Moon being visible at dawn
- Twinkle of stars in the sky
- Polestar being visible in the sky
Which of the above are optical illusions? (UPSC Prelims 2013)
Answer:
1, 2 and 4
Notes: The correct answer is [C] 1, 2 and 4. Optical illusions in the atmosphere occur primarily due to the refraction or scattering of light as it passes through different layers of the Earth's atmosphere.
- Size of the sun at dusk (Statement 1 is an Optical Illusion): At sunset (dusk) or sunrise, the sun appears larger and somewhat flattened. This is an optical illusion caused by atmospheric refraction. Light from the lower edge of the sun is refracted more than light from the upper edge because it passes through thicker layers of air, causing a vertical compression of the sun's image. Additionally, the "Ponzo illusion" makes objects near the horizon appear larger to the human brain compared to when they are high in the sky.
- Colour of the sun at dawn (Statement 2 is an Optical Illusion): The reddish appearance of the sun at dawn and dusk is due to Rayleigh scattering. When the sun is near the horizon, sunlight travels through a much thicker layer of the atmosphere. Shorter wavelengths (blue and violet) are scattered away by air molecules, leaving only the longer wavelengths (red and orange) to reach the observer's eye. The sun isn't actually changing color; it is an illusion of perception based on light scattering.
- Moon being visible at dawn (Statement 3 is Not an Optical Illusion): This is a physical reality based on the moon's orbit and its position relative to the Earth and Sun. Depending on the lunar phase (like the waning crescent), the moon can be physically present in the sky during the early morning hours.
- Twinkle of stars in the sky (Statement 4 is an Optical Illusion): Stars do not actually twinkle; they emit a steady light. The "twinkling" (scintillation) is an optical illusion caused by atmospheric refraction. As starlight passes through various layers of the atmosphere with different temperatures and densities, the light path is constantly bent. This causes the star's position and intensity to appear to shift rapidly to the human eye.
- Polestar being visible in the sky (Statement 5 is Not an Optical Illusion): The visibility of the Polestar (Polaris) is a physical fact. It is located nearly directly above the North Pole of the Earth's rotational axis, making it a fixed point used for navigation. Its visibility is not an illusion but a result of its astronomical position.