The electromagnetic spectrumIB MYP Sciences: Revision notes

The electromagnetic (EM) spectrum is a family of waves. In order of increasing frequency (and decreasing wavelength):

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

Visible light is only a small part of the spectrum. Waves at the gamma end have very short wavelengths and carry the most energy.

All EM waves:

• are transverse waves
• travel at the same speed in a vacuum, 3 × 10⁸ m/s
• transfer energy and can travel through empty space (no medium needed)
• obey v = f × λ

Worked example: a microwave has a frequency of 3 × 10⁹ Hz. λ = v ÷ f = 3 × 10⁸ ÷ 3 × 10⁹ = 0.1 m.

• Radio waves: broadcasting TV and radio, communications
• Microwaves: cooking food, mobile phones, satellite communications. Hazard: they heat body tissue, so ovens are shielded.
• Infrared: remote controls, heaters, thermal cameras, grills. Hazard: too much causes burns.

• Visible light: seen by the eye; used for photography and in optical fibres.
• Ultraviolet (UV): used in sun tanning, security marking and sterilising water. Hazard: UV damages skin cells, causes sunburn and skin cancer, and can damage the eyes. Sun cream and sunglasses reduce the risk.

• X-rays: used to see bones and for airport security. Bones absorb more X-rays than soft tissue.
• Gamma rays: used to sterilise instruments and food, and to treat cancer.

X-rays and gamma rays are ionising: they knock electrons out of atoms, which can damage or mutate DNA and cause cancer. Doses are kept as small as possible, and workers wear lead aprons or stand behind lead screens.

When judging uses of radiation, weigh the benefits (diagnosis, treatment) against the risks (cancer) and the costs.