Why this chapter matters for UPSC: Basic optics — properties of light, reflection, solar/lunar eclipses — are tested in Prelims science questions. Understanding light is also foundational for solar energy (GS3) and astronomical phenomena.

PART 1 — Quick Reference Tables

Materials and Light

Type Definition Examples
Transparent Allows light to pass through completely; objects clearly visible Glass, clean water, air
Translucent Allows some light through; objects not clearly visible Frosted glass, oiled paper, thin cloth, fog
Opaque Does not allow light to pass through Wood, metal, stone, cardboard

Properties of Light

Property Description Example
Rectilinear propagation Light travels in straight lines Shadows, beams of sunlight, laser pointer
Reflection Light bounces off a surface Mirror, shiny metals, moon's reflected sunlight
Refraction Light bends when passing from one medium to another Straw appears bent in water; lenses in spectacles
Absorption Light absorbed by a surface (converted to heat) Black surfaces absorb more; white reflects more

PART 2 — Notes

Shadow Formation and Eclipses

Explainer

Shadow formation: Requires:

  1. A source of light
  2. An opaque object
  3. A screen

The shadow forms on the side of the object away from the light source. Shadow size depends on:

  • Distance of object from light source (closer = larger shadow)
  • Size of object

Umbra and Penumbra:

  • Umbra: Region of complete shadow (total darkness); receives no direct light
  • Penumbra: Region of partial shadow; receives some direct light

Solar Eclipse:

  • Moon comes between Earth and Sun
  • People in the Moon's umbra see a total solar eclipse (Sun completely blocked)
  • People in penumbra see a partial solar eclipse
  • Rare annular eclipse: Moon is at far end of elliptical orbit → appears smaller than Sun → a ring (annulus) of sunlight visible around Moon

Lunar Eclipse:

  • Earth comes between Sun and Moon
  • Moon enters Earth's shadow
  • Total lunar eclipse: Moon turns red-orange ("Blood Moon") — red wavelengths of sunlight refracted through Earth's atmosphere illuminate the Moon

Annular solar eclipse of 2031: Will be visible from India (South India path) — UPSC sometimes asks about upcoming astronomical events.

Solar Energy — Light Absorption

UPSC Connect

UPSC GS3 — Solar energy:

The principle of light absorption underlies solar energy technology:

  • Photovoltaic (PV) cells: Light photons absorbed by silicon semiconductors → release electrons → electric current (photoelectric effect)
  • Solar thermal: Dark surfaces absorb light → heat; used in solar water heaters, concentrated solar power (CSP) plants

India's solar targets:

  • National Solar Mission (PM's Council on Climate Change): Target 500 GW renewable energy capacity by 2030 (including 280 GW from solar)
  • PM-KUSUM (Pradhan Mantri Kisan Urja Suraksha evam Utthaan Mahabhiyan): Solar pumps for agriculture; solar panels on barren lands; solar power plants
  • ISTS waiver: Inter-State Transmission System charges waived for solar/wind projects (reduces cost)
  • India's solar installed capacity: 143.60 GW (MNRE, February 2026); India deployed 37.9 GW of solar in 2025 alone
  • Total renewable energy capacity: ~263 GW (January 2026); India crossed 50% non-fossil installed capacity milestone in June 2025 — five years ahead of NDC target

Pinhole Camera

A pinhole camera demonstrates rectilinear propagation of light:

  • Light from top of object passes through pinhole → hits screen at bottom
  • Light from bottom of object → hits screen at top
  • Result: Inverted image on the screen
  • This is exactly how the human eye (and modern cameras) work — the lens focuses light onto the retina; the image is inverted and the brain corrects it

Exam Strategy

Prelims traps:

  • Moon does NOT produce its own light — it reflects sunlight
  • Solar eclipse: Moon is between Earth and Sun (new moon day); happens rarely because Moon's orbit is inclined ~5° to Earth's orbital plane
  • Lunar eclipse: Earth is between Sun and Moon (full moon day)
  • Blood Moon (red colour during lunar eclipse) = Earth's atmosphere refracts red light onto Moon
  • Translucent ≠ Transparent — translucent allows diffuse light but not clear image

Previous Year Questions

Prelims:

  1. During a total lunar eclipse, the Moon appears reddish because:
    (a) The Moon absorbs blue light
    (b) Dust on the Moon reflects red light
    (c) Earth's atmosphere refracts red wavelengths of sunlight onto the Moon
    (d) The Moon enters the Sun's shadow

  2. A solar eclipse occurs when:
    (a) The Moon comes between the Earth and the Sun
    (b) The Earth comes between the Sun and the Moon
    (c) The Sun comes between the Earth and the Moon
    (d) The Moon's orbit intersects Earth's orbit

  3. The image formed in a pinhole camera is:
    (a) Upright and virtual
    (b) Inverted and real
    (c) Upright and real
    (d) Inverted and virtual