Aspirant Academy

Science and Technology

Light and Optics

Physics: Motion, Work/Power/Energy, Gravitation, Light, Heat, Electricity, Magnetism, Sound, EM Waves, Medical Diagnostics, Nuclear Fission/Fusion, Radiation Safety

Paper II · Unit 2 Section 4 of 13 0 PYQs 31 min
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Light and Optics

3.1 Properties of Light

Light is an electromagnetic wave with wavelength range 400–700 nm (visible spectrum). Speed in vacuum: c = 3 × 10⁸ m/s (exact value: 299,792,458 m/s).

Reflection: Angle of incidence (i) = angle of reflection (r). Both measured from the normal to the surface.

Mirrors:

  • Plane mirror: Virtual, erect, same-size image; image formed as far behind mirror as object is in front.
  • Concave mirror (converging): Real, inverted image for objects beyond focal length; used in telescopes, shaving mirrors (close up), car headlights.
  • Convex mirror (diverging): Virtual, erect, diminished image; always forms image between F and P; used as rear-view mirrors (wide field of view).

3.2 Refraction and Lenses

Refraction: Light bends when passing from one medium to another.

Snell's Law: n₁sinθ₁ = n₂sinθ₂.

Refractive index (n) = c/v = (speed in vacuum)/(speed in medium). For glass n ≈ 1.5; for water n ≈ 1.33.

Total Internal Reflection (TIR): When light travels from denser to rarer medium at angle greater than critical angle (θc), no refraction occurs — all light is reflected internally. Critical angle for glass ≈ 42°.

Applications of TIR:

  • Optical fibres: Light travels long distances with minimal loss — basis of broadband internet, endoscopy, telecommunications.
  • Diamond brilliance: Diamond's n = 2.42 → θc = 24.4° — most light entering undergoes multiple TIR, creating sparkle.
  • Periscopes and binoculars: Prisms use TIR instead of mirrors.

Lenses:

  • Convex (converging): Used in cameras, projectors, magnifying glasses, reading glasses (hypermetropia/long-sightedness correction).
  • Concave (diverging): Used to correct myopia (short-sightedness).
  • Power of lens (D) = 1/f (in metres): Measured in dioptres (D); positive for convex, negative for concave.

3.3 Dispersion and the Spectrum

Dispersion of white light through a prism: White light splits into VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red).

  • Violet light deviates most (highest refractive index); Red deviates least.
  • Rainbow formation: Dispersion + TIR inside water droplets.
  • Primary rainbow: 42° elevation; secondary: 51° (colours reversed).