EM waves: transverse waves of oscillating electric and magnetic fields; need no medium; produced by accelerating charged particles; speed c = 3 × 10⁸ m/s in vacuum.
EM spectrum in order of increasing wavelength (decreasing frequency and energy): gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves. (Reverse for increasing frequency.)
Photon: a massless packet (particle) of light energy. Light shows both wave and particle nature. Radiation pressure: pressure light exerts because photons carry momentum. Solar sail: spacecraft pushed by radiation pressure (no fuel).
Scattering of light: light waves sent in different directions by particles of the medium.
Formulas
c = fλ. Photon energy E = hf. Photon momentum p = h/λ = E/c. h = 6.63 × 10⁻³⁴ J s. Energy depends on frequency.
Uses and hazards
Radio waves: radio and TV broadcast, astronomy (longest wavelength). Microwaves: radar, Wi-Fi, Bluetooth, satellite TV, ovens. Infrared: remote controls, heaters, thermal imaging, intruder alarms, optical fibres.
Visible light: vision (about 400 nm violet to 750 nm red). Ultraviolet: sterilising water, detecting fake notes, security marks; overexposure causes skin cancer. X-rays: medical imaging, security scanners, cracks in metal. Gamma rays: cancer treatment, sterilising food and equipment, most penetrating.
X-rays and gamma rays are ionising and are dangerous to living tissue; radio waves and microwaves are non-ionising.
Must-know points
Blue sky: short wavelengths (blue) are scattered most by air molecules, so the scattered blue light reaches the eye from all directions.
Sunset is red or orange: sunlight travels a longer path through the atmosphere, blue is scattered away and mostly red remains. Clouds look white because large droplets scatter all colours about equally.
Particle nature of light: light is made of photons with energy hf and momentum; evidence is radiation pressure; application: solar sail (or solar cells).
Past-paper questions with model answers
Name any six regions of the EM spectrum in increasing order of wavelength (every set): gamma rays, X-rays, ultraviolet, visible, infrared, microwaves (and radio waves).
What is the colour of the sky on a clear day? How does scattering determine it? Blue, as above (1+2).
Why does the colour of the sky change at sunset? Name the phenomenon: scattering (explain longer path).
Particle nature of light with one application (2+1).