UPSC CSE Prelims
Light and Optics Previous Year Questions (PYQs)

Detailed Explanation:

The correct answer is 1, 2 and 3 (Option 3). Einstein's General Theory of Relativity states that gravity bends light, matter curves space-time, and its equations also allow an expanding universe, which was later confirmed by observations.

General Theory of Relativity (1915) explains gravity as the curvature of space-time caused by mass and energy. It predicts phenomena like gravitational lensing, black holes, gravitational waves, and the expanding universe.

Detailed Explanation:

Answer: Option 3 — 1, 2 and 3

✅ Statement 1 – Correct: OLEDs use thin films of organic compounds that can be deposited on flexible plastic or metal foil substrates, unlike LCDs which require rigid glass panels. This enables bendable and foldable displays.

✅ Statement 2 – Correct: The inherent flexibility of OLED technology allows fabrication of roll-up displays that can be embedded in clothing, wearables, and other flexible surfaces—applications impossible with rigid LCD technology.

✅ Statement 3 – Correct: OLED pixels emit light independently without requiring a backlight, and the organic layers can be made semi-transparent. This enables creation of transparent displays where the screen is see-through when not displaying content, unlike LCDs which are always opaque due to their backlight and liquid crystal structure.

All three statements correctly identify key advantages of OLED over LCD technology.

Detailed Explanation:

1. Size of the sun at dusk – Optical illusion: The sun appears larger near the horizon due to the moon illusion, a psychological effect where our brain misinterprets distance cues, though its actual angular size remains constant.

2. Colour of the sun at dawn – Optical illusion: The sun appears red/orange because its light travels through more atmosphere, causing Rayleigh scattering that removes blue light, creating a perceived color change.

3. Moon being visible at dawn – NOT an illusion: This is a real astronomical event where the moon's orbital position allows it to be visible during daylight hours.

4. Twinkle of stars – Optical illusion: Stars don't actually twinkle; their light is refracted by turbulent atmospheric layers, causing rapid brightness fluctuations.

5. Polestar being visible – NOT an illusion: The Pole Star (Polaris) is genuinely visible due to its alignment with Earth's rotational axis, making it appear stationary.

Answer: Option 3 (1, 2 and 4) are optical illusions.

Detailed Explanation:

Refraction occurs when sunlight enters and exits the raindrop, bending light at air-water boundaries.

Dispersion separates white light into constituent colours as different wavelengths refract at different angles inside the droplet.

Internal reflection bounces the dispersed light off the rear inner surface of the raindrop back toward the observer, creating the visible rainbow arc.

All three phenomena—Dispersion, Refraction, and Internal Reflection—work together to form a rainbow.