The last three topics showed that light interferes, diffracts and polarizes, so it is a transverse wave. That leaves an obvious question: a wave of what? Water waves are moving water and sound is moving air, but light crosses the vacuum of space from the Sun with nothing to move. The answer is that light is a travelling pattern of electric and magnetic fields, and visible light is only a thin slice of a much wider family of waves that includes the signals carrying your phone calls and the X-rays in a dentist's office.
Where electromagnetic waves come from
In the 1860s James Clerk Maxwell combined the laws of electricity and magnetism and found they predicted waves of electric and magnetic field travelling at a speed that matched the measured speed of light. In the late 1880s Heinrich Hertz generated and detected such waves in his laboratory using electric sparks, confirming the prediction. The source of every electromagnetic wave is an accelerating charge. A radio transmitter is the clearest case: an alternating current drives electrons up and down an antenna, making an oscillating electric dipole — the top of the antenna is positive while the bottom is negative, then the reverse, many times a second. The changing electric field around it creates a changing magnetic field, which creates a changing electric field, and the disturbance moves outward at , oscillating at the same frequency as the charges.
| Band | Approximate wavelength | Typical source | Common use |
|---|---|---|---|
| Radio | longer than about | Electrons oscillating in an antenna | AM and FM broadcasting, communication |
| Microwave | about to | Electrons oscillating in a magnetron or a small antenna | Microwave ovens, Wi-Fi, radar, cell networks |
| Infrared | about to | Vibrating charges in any warm object | Remote controls, thermal imaging, night vision |
| Visible | about (violet) to (red) | Electron energy transitions in atoms; very hot objects | Vision, lasers, fibre optics |
| Ultraviolet | about to | Electron transitions; the Sun | Sterilization, fluorescence |
| X-ray | about to | Fast electrons suddenly stopped in a metal target | Medical and dental imaging, security scanners |
| Gamma ray | shorter than about | Changes inside atomic nuclei | Cancer treatment, sterilizing medical equipment |
Refraction and dispersion
In a material, light slows to , where is the refractive index, and it bends at a boundary according to . The key detail is that is slightly different for each wavelength. In glass and water, violet light travels a little slower than red, so it has a slightly higher and bends more. This spreading of white light into its colours is dispersion: it is how a prism makes a spectrum, and it is half of how a rainbow forms (the other half is reflection inside each raindrop). Refraction also explains a mirage. Air just above a hot road is less dense and has a slightly lower index than the cooler air above it, so light from the sky curves gradually upward toward your eye, and you see an image of the sky on the road that looks like water.
Diffraction limits resolution
Every lens, telescope, camera and pupil is an opening, and light diffracts through openings. A distant point of light therefore never focuses to a perfect point, but to a tiny blurred disc. When two points are so close together that their discs overlap too much, they look like one. The smallest angle between two points that can just be told apart is approximately
Technologies that use the wave nature of light
| Technology | Wave property | How it works |
|---|---|---|
| Spectroscope | Diffraction grating or prism dispersion | Spreads light into a spectrum. Each element emits its own set of wavelengths, so the lines identify what a lamp, flame or star is made of. |
| CDs, DVDs and Blu-ray discs | Diffraction and interference | Data is stored in a spiral track of tiny pits. The closely spaced tracks act as a reflection grating, which is why a disc shows rainbow colours. |
| Holograms | Interference | Film records the interference pattern between a reference beam and light scattered from an object. Lighting the pattern rebuilds the original wavefronts, so the image has depth. They are hard to copy, so they are used as security features on cards and bank notes. |
| Polarized lenses | Polarization by reflection and filters | Vertical transmission axes block horizontally polarized glare from water, snow and roads. |
| Anti-reflection coatings | Thin-film interference | A quarter-wavelength layer makes the two reflections cancel, so more light is transmitted. |
| Night vision | The infrared part of the spectrum | Thermal cameras detect the infrared emitted by warm bodies; image intensifiers amplify the faint light that is present. |
| Wireless networks | Microwave electromagnetic waves | Wi-Fi uses bands near and ; the longer wavelength of the 2.4 GHz band diffracts around obstacles and passes walls more easily. |
These technologies have real costs and benefits worth weighing. Fibre-optic cables and wireless networks carry enormous amounts of information and let publications and services move online, which can reduce paper and physical shipping — but data centres and networks consume electricity, and short-lived devices add to electronic waste. Discs are giving way to streaming, which changes where the energy is used rather than removing it. Polarized lenses and anti-reflection coatings improve safety and efficiency for drivers and solar panels. When you assess a technology, name the physics it relies on, then trace its effects on people and on the environment in both directions.