BharatNotes 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
Shadow formation: Requires:
- A source of light
- An opaque object
- 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 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 44.61 GW of solar in FY2025-26 (Apr 2025–Mar 2026) — a record single-year addition
- 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
Reflection of Light
Laws of Reflection:
When light bounces off a surface, two laws always hold:
- The angle of incidence = the angle of reflection (both measured from the normal — an imaginary line perpendicular to the surface at the point where light hits)
- The incident ray, the normal, and the reflected ray all lie in the same plane
Key terms:
- Incident ray: The light ray that hits the surface
- Normal: Perpendicular line to the surface at the point of incidence
- Reflected ray: The light ray that bounces back
- Angle of incidence (i): Angle between incident ray and normal
- Angle of reflection (r): Angle between reflected ray and normal
- Always: i = r
Types of Reflection:
| Type | Surface | Result | Example |
|---|---|---|---|
| Regular (Specular) reflection | Smooth, polished | Parallel rays reflect parallel → clear image | Mirror, calm water, polished metal |
| Diffuse reflection | Rough, irregular | Parallel rays scatter in all directions → no clear image | Paper, walls, road, skin |
Why can we see objects that don't produce light? They reflect diffuse light from other sources (sun, lamps) toward our eyes. Every object we see (except light sources) is visible through diffuse reflection.
Image in a Plane Mirror
Properties of image formed by a plane (flat) mirror:
| Property | Description |
|---|---|
| Virtual | Image appears to be behind the mirror; cannot be projected on a screen |
| Erect | Image is upright (not inverted top-to-bottom) |
| Laterally inverted | Left and right are swapped — your right hand appears as the left hand in the mirror |
| Same size | Image height = object height (magnification = 1) |
| Same distance | Image appears as far behind the mirror as the object is in front |
Lateral inversion explains:
- Why ambulance has "AMBULANCE" written reversed (reads correctly in a car's rear-view mirror)
- Why text in a mirror appears reversed
- Why your face in a mirror differs from photos (camera captures left-right correctly)
Applications of mirrors:
- Periscope: Uses two plane mirrors at 45° angles — allows viewing over obstacles (used in submarines, tanks, crowd surveillance). Light enters top mirror → reflects 90° → travels down → reflects again 90° → reaches the eye
- Kaleidoscope: Multiple mirrors at angles create beautiful symmetrical patterns (used in toys, art, textile design)
- Rear-view mirrors in vehicles: Flat mirror (for accurate size judgment); some vehicles use convex mirrors for wider field of view
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
Making a pinhole camera: Take two boxes, one slightly larger than the other. Make a pinhole in the smaller box. Cover the open end of the larger box with translucent paper (screen). Slide the smaller box inside. Point the pinhole at a bright object — inverted image appears on the screen.
[Additional] 11a. Refraction and Total Internal Reflection
The chapter mentions refraction in the property table but never explains it or its applications. Refraction is the bending of light when it changes speed as it passes from one medium to another — and it underlies eyeglasses, cameras, optical fibres, and mirages.
Refraction rules:
- Light bends toward the normal when entering a denser medium (air → water/glass; speed decreases)
- Light bends away from the normal when entering a less dense medium (glass → air; speed increases)
- The higher the difference in density, the more light bends
Snell's Law: n₁ sin θ₁ = n₂ sin θ₂ (where n = refractive index; θ = angle to normal)
Refractive indices:
- Air: 1.0 (reference)
- Water: ~1.33 (light slows to 75% of its vacuum speed)
- Glass: ~1.5 (light slows to ~67%)
- Diamond: ~2.42 (light slows to ~41%) — the highest of common natural materials
Everyday examples of refraction:
- Straw/pencil appears bent where it enters water
- A swimming pool appears shallower than it actually is (apparent depth < real depth)
- Convex lenses in spectacles and cameras — bend light to focus it on a point
- Rainbows: raindrops act as tiny prisms — refraction + internal reflection splits white sunlight into spectrum
Total Internal Reflection (TIR): When light travels from a denser medium to a less dense one (glass → air), and the angle of incidence exceeds the critical angle, light cannot escape — it is completely reflected back inside. No refraction occurs at or beyond the critical angle.
- Critical angle for glass–air: ~42°
- Critical angle for diamond–air: ~24° (diamond's very high refractive index means light is trapped more easily — creating the sparkle and fire unique to diamonds)
Applications of TIR:
- Optical fibres (light guided through glass core by TIR)
- Endoscopes (flexible TIR tubes for medical imaging)
- Prism binoculars (total internal reflection prisms replace mirrors; more compact than lens systems)
[Additional] Mirages — Refraction in Nature:
A mirage is a naturally occurring optical phenomenon caused by the refraction of light through layers of air at different temperatures (and therefore different densities).
Inferior mirage (hot road/desert):
- Air near a hot road or desert surface is much less dense than air above
- Light from the sky curves downward as it enters the hotter, less dense air near the surface
- Observer sees what appears to be water or a wet road — it is actually a reflected image of the sky
- Common in Rajasthan's Thar Desert and on hot summer roads
Superior mirage (cold surfaces):
- Over cold water (Arctic Ocean, cold lakes), cold dense air near the surface bends light upward
- Distant objects appear to float above their actual position — ships seem to "float" above the horizon
[Additional] 11b. Optical Fibres and BharatNet — TIR in Digital India
Optical fibres use total internal reflection to transmit data as pulses of light over thousands of kilometres with minimal signal loss.
How they work: A thin glass (silica) core is surrounded by a cladding with a lower refractive index. Light entering the core at a shallow angle hits the core-cladding boundary at angles greater than the critical angle → TIR → light bounces down the fibre indefinitely. Data is encoded as patterns of light pulses.
Advantages over copper wire:
- Carries far more data simultaneously (higher bandwidth)
- Light not affected by electromagnetic interference
- Signal travels at ~2/3 the speed of light (~200,000 km/s)
- Much lower signal loss per km
[Additional] BharatNet — India's Optical Fibre Connectivity Mission (GS3 — Digital Infrastructure):
BharatNet (formerly National Optical Fibre Network) aims to connect all gram panchayats with high-speed broadband using optical fibre cable (OFC), enabling e-governance, telemedicine, e-education, and digital payments in rural India.
Progress (as of 2025):
- ~6.93 lakh km (6,93,303 km) of OFC laid (PIB, May 2025)
- ~2.14–2.18 lakh gram panchayats made service-ready (PIB, March 2025)
- BharatNet Phase 3: Cabinet approved total project cost of ₹1.39 lakh crore (August 2023) — to connect government secondary schools, PHCs, and remaining rural areas
- Union Budget 2025-26: Annual allocation of ₹22,000 crore for BharatNet — a 238% jump over the previous year's revised estimate
- Implemented by Bharat Broadband Network Limited (BBNL), now merged with BSNL (July 2023); USOF (Universal Service Obligation Fund) provides funding
Physics of BharatNet: Each OFC cable carries data using TIR — meaning India's digital infrastructure directly depends on the same physics principle (total internal reflection) as a medical endoscope or a diamond's sparkle.
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
- Laws of reflection: angle of incidence = angle of reflection — both measured from the normal, NOT from the surface
- Regular reflection = smooth surface → clear image; Diffuse reflection = rough surface → scattered light — walls and paper show diffuse reflection; both obey the laws of reflection at each microscopic point
- Plane mirror image is virtual and laterally inverted — it CANNOT be formed on a screen; left/right swap but not top/bottom
- Periscope works on double reflection — two plane mirrors at 45° each; used in submarines
Practice Questions
Prelims:
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 shadowA 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 orbitThe image formed in a pinhole camera is:
(a) Upright and virtual
(b) Inverted and real
(c) Upright and real
(d) Inverted and virtualWhen light falls on a rough wall, it undergoes diffuse reflection. This means:
(a) The law of reflection is violated at each point
(b) The angle of incidence exceeds the angle of reflection
(c) The law of reflection holds at each point but microscopic surface irregularities cause rays to scatter in all directions
(d) Light is absorbed and re-emitted in random directionsThe image of an object in a plane mirror is:
(a) Real, inverted, and same size as the object
(b) Virtual, inverted, and same size as the object
(c) Real, erect, and same size as the object
(d) Virtual, erect, laterally inverted, and same size, at the same distance behind the mirror
