Electromagnetic WavesNEET Physics · Class 12 · NCERT Chapter 8

You can expect 1 question almost every year, occasionally 2. The chapter is short and high-yield: most questions are direct recall of EM spectrum (which radiation has the highest frequency, which is used for medical imaging, etc.), the speed-and-amplitude relation E_0 = c B_0, and properties of displacement current.

Maxwell saw that the original Ampere's law failed when applied to a charging capacitor: no real current flows between the plates, but a magnetic field is still produced. He introduced the displacement current I_D = epsilon_0 dPhi_E over dt, where Phi_E is the electric flux. With this correction, Ampere's law works in every situation, including capacitors and free space.

In an EM wave, the electric field E and magnetic field B both oscillate perpendicular to the direction of propagation, and perpendicular to each other. There is no component of E or B along the direction of travel. This is why EM waves can be polarised. Sound waves, in contrast, are longitudinal.

In vacuum c = 1 over root (epsilon_0 mu_0) ≈ 3 x 10^8 m/s. The same value for all EM waves (gamma rays, visible light, radio). In a medium v = c over n where n is the refractive index, n = root (mu_r epsilon_r). The wave slows down, frequency stays the same, wavelength shortens to lambda over n.

In free space E_0 = c B_0, where E_0 and B_0 are peak values of the electric and magnetic fields. So if E_0 = 30 V/m, then B_0 = 30/(3 x 10^8) = 10^-7 T = 100 nT. The magnetic field is much smaller in SI units, but they carry equal energy density.

The two carry equal energy on average. Energy density u_E = (1/2) epsilon_0 E^2 and u_B = B^2 over (2 mu_0). Since B = E/c, you can show u_E = u_B. Total average u = epsilon_0 E_rms^2. Intensity I = u c.

EM waves carry momentum, so they exert pressure on a surface. For a perfectly absorbing surface, P = I over c. For a perfectly reflecting surface, P = 2 I over c (factor of 2 because momentum reverses). Radiation pressure is what pushes solar sails, and is responsible for the tail of comets pointing away from the Sun.

Gamma rays (>10^19 Hz, nuclear sources, cancer treatment), X-rays (10^16 to 10^19 Hz, medical imaging), UV (10^15 to 10^16 Hz, sterilisation, vitamin D), visible light (4 x 10^14 to 7.5 x 10^14 Hz, vision), IR (10^11 to 10^14 Hz, thermal imaging, remote controls), microwaves (10^9 to 10^11 Hz, ovens, radar), radio waves (<10^9 Hz, AM/FM, TV, mobile). Wavelengths run inversely.