Waves & Optics: EM Spectrum, Fiber Optics, Polarisation, Diffraction

Waves and advanced optics questions appear in Kerala PSC Science sections. This note covers wave fundamentals, the full EM spectrum, optical phenomena, and fiber optics — topics beyond the basic light/sound coverage.

1. Wave Basics

Property	Detail
Wave	A disturbance that transfers energy without transferring matter
Wavelength	Distance between two consecutive crests (or troughs)
Frequency	Number of complete oscillations per second (unit: Hertz, Hz)
Amplitude	Maximum displacement from equilibrium position
Velocity	v = frequency x wavelength
Time period	Time for one complete oscillation; T = 1/frequency

Types of Waves

Type	Particle Motion	Example
Transverse	Perpendicular to direction of wave propagation	Light waves, water surface waves, seismic S-waves
Longitudinal	Parallel to direction of wave propagation	Sound waves, seismic P-waves, compression in spring

Type	Medium Required?	Example
Mechanical waves	Yes	Sound, water waves, seismic waves
Electromagnetic waves	No (can travel through vacuum)	Light, radio, X-rays, microwaves
Matter waves	Associated with particles	De Broglie waves (electrons)

2. Electromagnetic (EM) Spectrum — Complete Table

Listed in order of increasing frequency (decreasing wavelength):

EM Wave	Wavelength Range	Frequency Range	Discovered By	Key Applications
Radio waves	More than 1 m	Less than 300 MHz	Heinrich Hertz	Radio, TV broadcasting, communication
Microwaves	1 mm to 1 m	300 MHz to 300 GHz	—	Microwave ovens, radar, satellite communication
Infrared (IR)	700 nm to 1 mm	300 GHz to 430 THz	William Herschel	Night vision, remote controls, thermal imaging
Visible light	380 nm to 700 nm	430 THz to 790 THz	—	Human vision; VIBGYOR (Violet to Red)
Ultraviolet (UV)	10 nm to 380 nm	790 THz to 30 PHz	Johann Ritter	Sterilisation, vitamin D synthesis, fluorescence
X-rays	0.01 nm to 10 nm	30 PHz to 30 EHz	Wilhelm Rontgen	Medical imaging, airport security
Gamma rays	Less than 0.01 nm	More than 30 EHz	Paul Villard	Cancer treatment, nuclear reactions, sterilisation

Key Facts about EM Spectrum

• All EM waves travel at the speed of light (3 x 10^8 m/s) in vacuum
• VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red
• Violet has the shortest wavelength and highest frequency among visible colours
• Red has the longest wavelength and lowest frequency among visible colours
• Ozone layer absorbs most UV radiation from the Sun

3. Optical Phenomena

Interference

Fact	Detail
Definition	Superposition of two or more waves resulting in a new wave pattern
Constructive	Waves in phase; amplitudes add up; bright fringes
Destructive	Waves out of phase; amplitudes cancel; dark fringes
Young’s Double Slit Experiment	First demonstrated interference of light (1801); proved wave nature of light
Thin film colours	Oil on water shows colours due to interference

Diffraction

Fact	Detail
Definition	Bending of waves around obstacles or through narrow openings
Condition	Most noticeable when opening/obstacle size is comparable to wavelength
Single slit diffraction	Produces central bright band with fainter side bands
Diffraction grating	Device with many parallel slits; used to separate wavelengths; produces spectra
Application	CD/DVD surfaces show rainbow colours due to diffraction

Polarisation

Fact	Detail
Definition	Restriction of wave vibrations to a single plane
Applies to	Transverse waves only (not longitudinal — sound cannot be polarised)
Unpolarised light	Vibrates in all planes perpendicular to direction of travel
Polarised light	Vibrates in only one plane
Polaroid	Material that polarises light; invented by Edwin Land
Applications	Sunglasses (reduce glare), LCD screens, 3D movie glasses, photography filters
Brewster’s angle	Angle of incidence at which reflected light is completely polarised
Malus’s Law	Intensity of polarised light through analyser: I = I0 x cos^2(theta)

4. Total Internal Reflection and Fiber Optics

Concept	Detail
Total internal reflection (TIR)	Light travelling from denser to rarer medium reflects completely when angle of incidence exceeds critical angle
Critical angle	Angle of incidence at which refracted ray travels along the boundary
Conditions for TIR	(1) Light must travel from denser to rarer medium; (2) Angle of incidence must exceed critical angle
Mirage	Caused by TIR in layers of hot air near ground surface
Diamond sparkle	Due to high refractive index (2.42) and low critical angle (24.4 degrees); multiple TIR inside

Optical Fibers

Feature	Detail
Principle	Total internal reflection
Structure	Core (high refractive index glass) + Cladding (lower refractive index glass) + Protective jacket
Types	Single-mode (long distance, thin core) and Multi-mode (short distance, thick core)
Applications	Internet/telecom cables, endoscopy (medical), decorative lighting
Advantages	High bandwidth, low signal loss, immune to electromagnetic interference, lightweight
Inventor	Narinder Singh Kapany (Indian-American physicist, “Father of Fiber Optics”)

5. Scattering of Light

Phenomenon	Explanation
Rayleigh Scattering	Scattering by particles much smaller than wavelength; shorter wavelengths scatter more
Blue sky	Blue light scattered more than red by atmospheric molecules (Rayleigh scattering)
Red sunset/sunrise	Light travels longer path through atmosphere; blue scattered away, red reaches observer
Tyndall Effect	Scattering of light by colloidal particles; beam visible in dusty room
Raman Scattering	Inelastic scattering; discovered by C.V. Raman (1928); won Nobel Prize 1930

6. PSC Quick Revision — One-Liners

• Sound is a longitudinal wave; light is a transverse wave
• EM waves do not need a medium; they travel at 3 x 10^8 m/s in vacuum
• Infrared was discovered by William Herschel; UV by Johann Ritter
• X-rays discovered by Rontgen (1895); used for medical imaging
• Fiber optics work on total internal reflection
• Narinder Singh Kapany — “Father of Fiber Optics”
• Polarisation proves light is a transverse wave
• Young’s Double Slit Experiment (1801) proved wave nature of light
• Raman Effect discovered by C.V. Raman in 1928 (Nobel Prize 1930)
• National Science Day (28 February) celebrates discovery of Raman Effect
• VIBGYOR: Violet has shortest wavelength, Red has longest