Light

UPSC GS3 — Light and optics:

Refraction: Bending of light when it passes from one medium to another (change in speed causes bending).

• Light bends toward normal when entering denser medium (air → glass/water)
• Light bends away from normal when entering less dense medium (glass → air)

Total Internal Reflection:

• When light hits the boundary from dense to less dense medium at angle > critical angle → all light reflected back
• Optical fibre: Light travels along a glass fibre by total internal reflection → basis of fibre optic communication; used in broadband internet, medical endoscopes
• BharatNet: India's optical fibre network to connect all panchayats

Dispersion (Spectrum): White light = mixture of all colours When white light passes through a prism:

• Different colours refract by different amounts (violet most, red least)
• Produces VIBGYOR spectrum: Violet, Indigo, Blue, Green, Yellow, Orange, Red
• Rainbow: Natural dispersion; water droplets in air act as tiny prisms; sunlight dispersed into spectrum

Scattering of light:

• Why sky is blue: Shorter wavelengths (blue, violet) scattered more by air molecules; we see scattered blue light → sky appears blue
• Why sunrise/sunset is red/orange: When sun is at horizon, light travels through more atmosphere; blue light scattered away; longer wavelengths (red, orange) remain → red/orange sky
• Why clouds are white: Clouds have large water droplets that scatter all wavelengths equally → appears white

Applications:

• Solar concentrators: Concave mirrors focus sunlight → high temperature → cooking, steam, electricity
• Telescopes: Hubble Space Telescope, James Webb Space Telescope use mirrors to collect light from far objects
• ISRO's GSLV/PSLV: Carry Earth observation satellites with optical sensors

[Additional] LiDAR — Laser Eyes for Autonomous Systems and Remote Sensing — GS3 (S&T / Geospatial):

LiDAR = Light Detection And Ranging

Principle:

• A LiDAR system emits rapid pulses of laser light (usually near-infrared) toward a surface
• The laser pulses reflect back; the time-of-flight (how long light takes to return) is measured
• Since light speed is exactly known (~3×10⁸ m/s), distance = (time × speed of light) ÷ 2
• By rotating and scanning the laser in all directions, LiDAR builds a precise 3D point cloud of the surroundings — far higher resolution than microwave RADAR

Advantage over RADAR:

• RADAR uses microwave wavelengths (centimetres); LiDAR uses laser wavelengths (nanometres) → LiDAR resolution is ~1,000× finer
• LiDAR can detect individual tree branches, road edges, and facial features that RADAR cannot resolve
• Limitation: Laser light is blocked by heavy cloud cover, fog, and heavy rain (similar to visible light); RADAR works through these conditions

Key applications:

1. Autonomous vehicles (self-driving cars): LiDAR is the primary "eye" of most autonomous vehicles — builds real-time 3D map of surroundings; detects pedestrians, other vehicles, obstacles
2. Archaeology: LiDAR revealed vast hidden Maya cities under dense jungle canopy (Guatemala, 2018) — invisible to ground surveys and conventional aerial photos
3. Disaster management: Flood mapping, landslide monitoring, post-earthquake damage assessment
4. ISRO/NRSC remote sensing: Aerial LiDAR surveys for topographic mapping, urban planning, forest volume estimation; IIRS (Indian Institute of Remote Sensing) runs dedicated LiDAR training programmes (2024)
5. Defence: Rangefinders, target acquisition, terrain mapping for missile guidance

India's strategic concern — Chinese LiDAR dominance:

• China's Hesai Technology is the world's largest LiDAR manufacturer — supplies most of the global autonomous vehicle and defence LiDAR market
• In January 2024, the US Department of Defence added Hesai to its list of "Chinese military companies," raising national security concerns: autonomous vehicles and defence systems in multiple countries (including India) rely on Chinese-made LiDAR sensors
• India's dependence on imported LiDAR for defence applications is a vulnerability — indigenous LiDAR development by DRDO and Indian startups is ongoing
• This appeared in UPSC Current Affairs (2024) under GS3 Science & Technology and national security