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PRELIMS

Light ​

Nature and Speed of Light
  • Definition: Electromagnetic wave lying between UV and IR ( to ).
  • Dual Nature:
    1. Wave Nature: Explains rectilinear propagation, reflection, refraction, interference, diffraction, and polarisation.
    2. Particle Nature (Quantum Theory): Explains Photoelectric and Compton effects. Light consists of energy packets called photons ( ).
  • Speed: Measured by Roemer (1678). Max in vacuum ( ).
    • Refractive Index ( ): (Speed in vacuum / Speed in medium).
    • Time to Earth: From Sun (8 min 19 sec); From Moon (1.28 sec).
  • Vacuum: Speed of Light (m/s): . Medium: Glass. Speed of Light (m/s):
  • Water: Speed of Light (m/s): . Medium: Terpentine oil. Speed of Light (m/s):
  • Rock salt: Speed of Light (m/s): . Medium: Nylon. Speed of Light (m/s):
Reflection and Plane Mirrors
  • Reflection: Returning of light into the same medium at an interface.
  • Plane Mirror Properties:
    1. Image is virtual, laterally inverted, and same size as object.
    2. Image distance = Object distance.
    3. If object moves at speed , image moves at relative to object.
    4. If mirror rotates by , reflected ray rotates by .
    5. Full image requires a mirror at least half the person's height.
    6. Number of Images ( ) for angle :
      • If is even: .
      • If is odd: (Symmetric) or (Asymmetric).
Refraction and Critical Angle
  • Refraction: Bending of light when entering a new medium.
    • Snell's Law: .
    • Factors: Decreases with increase in wavelength (Max for Violet, Min for Red). Decreases with increase in temperature.
    • Illustrations: Bending of dipped rod, objects appearing shallower (fish, coins), Twinkling of stars.
  • Critical Angle: Incidence angle in denser medium for which refraction angle is .
Total Internal Reflection (TIR)
  • Conditions: Light must travel from Denser to Rarer medium; Angle of incidence > Critical angle.
  • Illustrations:
    1. Sparkling of diamonds.
    2. Mirage (deserts) and Looming (cold regions).
    3. Shining of air bubbles in water.
    4. Sun visible before sunrise/after sunset.
  • Optical Fibre: Propagates light via multiple TIR with zero energy loss.
    • Usage: Optical signals, endoscopy, converting electrical signals to light for transmission.
Lenses and Power
  • Convex (Converging): Thicker at middle. Concave (Diverging): Thicker at edges.
  • Power (P): (in meters). Unit: Dioptre (D).
    • Convex lens: Positive (+) power. Concave lens: Negative (-) power.
  • Lens in Liquid:
    • If (e.g., Glass in water): increases, decreases.
    • If : is infinite, is zero (Lens disappears).
    • If : Nature reverses (Convex becomes Concave). Example: Air bubble in water behaves as concave lens.
Dispersion, Rainbow and Colours
  • Dispersion: Splitting of white light into VIBGYOR. Max deviation for Violet, Min for Red.
  • Rainbow: Formed by dispersion of sunlight by water droplets.

10. Light ​

1. Nature and Speed of Light
  • Dual Nature: Light behaves as both a wave (EM wave) and a particle (photon).
  • Characteristics:
    • It is a transverse electromagnetic wave.
    • Does not require a medium for propagation.
    • Speed in Vacuum: (Max).
    • Speed in Media: Decreases as the refractive index increases (Measured by Foucault in water).
2. Reflection & Plane Mirrors
  • Reflection: Bending of light into the same medium upon hitting a surface.
  • Plane Mirror Characteristics:
    • Image is virtual, erect, and laterally inverted.
    • Size of image = Size of object.
    • To see a full image, the mirror must be at least half the height of the person ( ).
    • If a mirror is rotated by , the reflected ray rotates by .
    • Images formed between two mirrors:
      • Parallel: Infinite.
      • At angle : (if is even).
3. Spherical Mirrors (Concave & Convex)
  • Concave: Nature: Converging. Uses / Applications: Shaving mirror, ENT doctors (head mirror), Car headlights, Solar cookers.
  • Convex: Nature: Diverging. Uses / Applications: Rear-view mirror in vehicles (provides a wide field of view), Street lights.
4. Refraction of Light
  • Definition: Bending of light when it passes from one medium to another.
  • Refractive Index (n): .
  • Common Phenomena:
    • Twinkling of stars.
    • A coin in a bowl of water appearing raised.
    • A stick appearing bent when partially immersed in water.
    • The Sun appearing before sunrise and after sunset (Time difference 2 mins).
5. Total Internal Reflection (TIR)
  • Condition: Light must travel from denser to rarer medium, and the angle of incidence must be greater than the Critical Angle.
  • Applications of TIR:
    • Sparkling of diamonds.
    • Mirage in deserts and Looming in cold regions.
    • Optical Fibers (used in telecommunications and Endoscopy).
    • Sparkling of air bubbles in water.
6. Lenses & Human Eye
  • Convex Lens: Converging; used in cameras, microscopes, and to correct Hypermetropia.
  • Concave Lens: Diverging; used to correct Myopia.
  • Defects of Vision:
    1. Myopia (Short-sightedness): Near objects are clear; corrected by Concave Lens.
    2. Hypermetropia (Long-sightedness): Distant objects are clear; corrected by Convex Lens.
    3. Astigmatism: Blurred at certain angles; corrected by Cylindrical Lens.
    4. Presbyopia: Common in old age; corrected by Bifocal Lens.
7. Dispersion & Color
  • Dispersion: Splitting of white light into 7 colors (VIBGYOR) by a prism.
    • Red: Least deviated (Maximum speed).
    • Violet: Most deviated (Minimum speed).
  • Primary Colors: Red, Blue, and Green (combine to form White).
  • Secondary Colors: Yellow, Magenta, and Cyan.
8. Wave Phenomena (Interference & Polarization)
  • Interference: Colors seen in soap bubbles and thin oil films on water.
  • Diffraction: Bending of light around sharp corners.
  • Polarization: Proves that light is a transverse wave (Sound waves cannot be polarized).
9. Mirror and Lens Formulae
  • Mirror Formula: (Sign convention: distances measured from pole; concave mirror is negative, convex mirror is positive as per Cartesian convention).
  • Lens Formula: .
  • Magnification (m): (mirrors); (lenses).
10. Electromagnetic Spectrum

Arranged by increasing frequency / decreasing wavelength:

  1. Radio waves: > 1 m (longest wavelength).
  2. Microwaves: 1 mm – 1 m (radar, satellite communication).
  3. Infrared (IR): 700 nm – 1 mm (heat radiation, remote controls, thermal imaging).
  4. Visible Light: ~400 nm (violet) to ~700 nm (red) ( to ).
  5. Ultraviolet (UV): 10 nm – 400 nm (causes skin damage/sunburn; absorbed by the ozone layer).
  6. X-rays: 0.01 nm – 10 nm (medical imaging; discovered by Roentgen).
  7. Gamma rays: < 0.01 nm (shortest wavelength, highest energy/frequency; emitted in radioactive decay).
UPSC Relevance
  • Communications: Importance of Optical Fibers (TIR) in high-speed internet (BharatNet).
  • Technological Optics: Understanding lens applications in surveillance and medical diagnosis.
  • Nature Facts: Differentiating between refraction (mirages) and dispersion (rainbows).
  • EM Spectrum: Ordering by wavelength/frequency and matching each band to its application is a recurring Prelims theme.

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