The electromagnetic spectrum

Electromagnetic (EM) waves are transverse waves that transfer energy from a source to an absorber. They form a continuous spectrum, grouped by wavelength. In order of decreasing wavelength and increasing frequency:

radio waves → microwaves → infrared → visible light → ultraviolet → X-rays → gamma rays

All EM waves travel at the same speed through a vacuum (space) or air: 3 × 10⁸ m/s.

Our eyes can only detect visible light, a very small part of the spectrum. Within it, red has the longest wavelength and violet the shortest.

All EM waves have the same speed, so a higher frequency means a shorter wavelength.

Example: a radio station broadcasts at 100 MHz. Find the wavelength.

1. Convert: 100 MHz = 100 000 000 Hz = 1 × 10⁸ Hz
2. λ = v ÷ f = (3 × 10⁸) ÷ (1 × 10⁸)
3. λ = 3 m

Different materials absorb, transmit, refract or reflect EM waves in different ways, depending on the wavelength. For example, the atmosphere lets visible light through but absorbs most ultraviolet and X-rays.

Radio waves can be made by oscillations (vibrating charges) in electrical circuits. In a transmitter, an alternating current makes radio waves with the same frequency as the current.

When radio waves are absorbed by a conductor, such as an aerial, they can make an alternating current with the same frequency as the wave. This is how a radio receiver works.

Changes inside atoms and nuclei also make EM waves. When electrons move between energy levels, they give out or absorb light and other EM waves. Gamma rays come from changes in the nucleus.