Chapter 8 - Electromagnetic Waves

Humne Ampere's circuital law padha tha ∮B·dl = μ0 Ic conduction current enclosed. Par jab capacitor charge ho raha hota hai toh wires me conduction current Ic hota hai par plates ke beech gap me koi conduction nahi. Toh Ampere law fail? Kya magnetic field plates ke beech zero hona chahiye? Experiment me magnetic field milta hai gap me bhi!

Maxwell ne socha: Capacitor charging me plates ke beech electric field E change ho raha hai kyunki charge badh raha hai. Changing electric field bhi magnetic field produce karta hai jaise changing magnetic field electric field produce karta hai Faraday me. Toh ek naya current socho: Displacement current Id = ε0 dΦE/dt where ΦE = E·A electric flux through gap. Yeh conduction nahi par effect same magnetic field banata hai.

So modified Ampere-Maxwell law: ∮B·dl = μ0(Ic + Id) = μ0 Ic + μ0 ε0 dΦE/dt. Charging capacitor me wires me Ic hai plates ke beech Id hai total continuity bani rehti hai.

Maxwell ne 4 equations se pura electromagnetism samjha diya:

1. Gauss law for electricity: ∮E·dA=Q/ε0 - Electric field lines start on positive charge end on negative like roads starting from one city ending another. Total flux through closed surface ∝ enclosed charge.

2. Gauss law for magnetism: ∮B·dA=0 - No magnetic monopole exists. Magnetic field lines closed loops like circular race track no start no end. Number entering = leaving net flux zero.

3. Faraday law: ∮E·dl = -dΦB/dt - Changing magnetic field produces electric field. Move magnet near coil current induced. Negative sign Lenz law opposes change.

4. Ampere-Maxwell law: ∮B·dl = μ0 Ic + μ0 ε0 dΦE/dt - Changing electric field produces magnetic field plus conduction current produces B.

Equation 3 and 4 together show changing E creates B changing B creates E - self-sustaining loop - EM wave propagates without needing charges! Like relay race where E passes baton to B B passes to E moves forward.

EM wave transverse wave hai: Electric field E, magnetic field B, and propagation direction k (or wave vector) all mutually perpendicular. E x B direction gives propagation direction. E and B in phase - same time reach maximum zero together like two dancers dancing in sync.

Speed: In vacuum c = 1/√(μ0 ε0) = 3×10^8 m/s exactly 299,792,458 m/s. Maxwell ne calculate kiya aur dekha yeh light ki speed ke barabar - proof that light is EM wave!

In medium speed v = 1/√(μ ε) = 1/√(μ0 μr ε0 εr) = c/√(μr εr) = c/n where n = refractive index. So in glass water speed less than c.

Energy: Energy equally divided between E and B fields average energy density u = 1/2 ε0 E² + B²/2μ0 = ε0 E² = B²/μ0? Actually average half each. EM wave carries momentum p = U/c pressure on surface.

EM wave can travel vacuum no medium needed unlike sound needs air. Changing E creates B changing B creates E self-sustaining relay.

Example: Light from sun comes through vacuum space to Earth because EM wave needs no medium.

EM waves ka pura family EM spectrum kehlata hai arranged by wavelength λ or frequency ν or energy E=hν. Relation c=ν λ constant for all in vacuum.

Longest to shortest: Radio waves (λ km to 0.1m, ν 500kHz to 1000MHz) → Microwaves (0.1m to 0.1mm, 1GHz to 300GHz) → Infrared IR (1mm to 700nm) → Visible light (700nm red to 400nm violet) VIBGYOR tiny portion → Ultraviolet UV (400nm to 1nm) → X-rays (1nm to 10^-3 nm) → Gamma rays (<10^-3 nm <0.01nm) shortest highest energy.

Frequency increases left to right, wavelength decreases, energy E=hν increases. Our eyes see only visible 400-700nm tiny portion like small window in big house.

Production detection uses different for each but all are EM waves same nature speed c in vacuum.

Radio waves: Wavelength ~0.1m to few km, frequency ~500kHz to ~1000MHz. Produced by accelerated charges in LC oscillator, detection by antenna. Uses: Radio/TV broadcast, mobile phones, satellite communication, AM FM. Property reflects off ionosphere, metal, can go long distances. Analogy like throwing stone in pond ripples carry message far - radio carries news songs from tower to your home without wire.

Microwaves: Wavelength ~0.1mm to 0.1m, frequency ~1GHz to 300GHz. Produced by klystron, magnetron, Gunn diode. Detected by point contact diodes. Uses: Microwave oven heating water molecules resonate at ~2.45GHz water absorbs energy heats food quickly. Radar aircraft detection, speed gun police measures Doppler shift, satellite communication, mobile phones. Analogy: Microwave oven me paani ke molecules nachte hain jaise bheed me dhakka dene se garmi aati hai - molecules vibration se khana garam.

Both reflect off metal so used in radar.

Infrared IR: Wavelength 700nm to 1mm, produced by hot bodies and molecules vibrations. Detected by thermopiles, bolometer, IR photographic film. Uses: TV AC remote controls, thermal imaging night vision, greenhouse effect - IR trapped by CO2 warms Earth, optical fibre communication, physical therapy. Analogy IR like invisible heat hug from bonfire - you feel warmth without seeing light.

Visible light: Wavelength 400-700nm VIBGYOR, produced by atomic transitions electrons jumping orbits. Detected by human eye, photocells. Uses: vision we see world, photosynthesis plants use sunlight to make food, optical fibres light signal transmission. Analogy visible like small window we can see through whole house beyond exists but we see only window.

Ultraviolet UV: Wavelength 1nm to 400nm, produced by sun and atomic transitions. Detected by photographic film. Uses: sterilization killing bacteria in water and surgical instruments, security marks UV ink on banknotes documents glows under UV, fluorescent lamps. Hazards: skin tanning, sunburn, eye damage, skin cancer if overexposed. Ozone layer absorbs most UV protects us. Analogy UV like overexposure to sun too much leads to sunburn.

X-rays: Wavelength 10^-3 nm to 1nm (0.001 to 1nm), produced by X-ray tube fast electrons hitting metal target inner shell electrons transitions. Detected by photographic film, Geiger counter. Uses: Medical imaging bones fractures dense bones absorb X-rays flesh less so shadow of bones on film, CT scan, airport security baggage checking, material testing cracks in metal, engineering. Hazards damages living tissue cancer if overexposed need lead apron protection.

Gamma rays: Wavelength <10^-3 nm (<0.01nm) shortest highest energy, produced by nuclear reactions radioactive decay nuclei. Detected by ionization chamber Geiger counter. Uses: Cancer radiotherapy kills cancer cells high energy destroys, sterilization surgical instruments, astronomy study universe, food preservation kills bacteria. Hazards very harmful cell damage cancer genetic mutations need heavy lead shielding.

Both ionizing radiation can break molecules.