Light You Cannot See
- Physics
- Ages 14–18 · Grades 9–12
- Hands-on experiment
What you’ll find out
- Visible light is a narrow band of a much wider electromagnetic spectrum.
- All electromagnetic waves travel at the same speed in a vacuum, but differ in frequency.
- Higher frequency means more energy per photon, and above the ultraviolet it can ionise atoms.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
One family of waves
Visible light is only a narrow band of a much larger family called the electromagnetic spectrum. At one end are radio waves, whose wavelengths can be metres or even kilometres long. Then come microwaves, then infrared, which we feel as warmth, and then the thin band of colours our eyes can detect. Beyond that lie ultraviolet, X-rays and gamma rays, with shorter and shorter wavelengths. They are all the same kind of wave, all travelling at about 300 million metres per second in a vacuum. What changes from one to the next is the frequency, and therefore the wavelength.
Energy, and what it does to matter
Each electromagnetic wave carries energy in tiny packets called photons, and the higher the frequency, the more energy each photon has. That decides what a wave can do to matter. Radio waves are gentle and pass through walls and people. Microwaves make water molecules rotate, which is how a microwave oven heats food. Infrared warms your skin. Ultraviolet photons carry enough energy to damage the DNA in skin cells, which is what causes sunburn and raises the risk of skin cancer. X-rays and gamma rays carry more still, and can knock electrons out of atoms, which is called ionisation. Hospitals use them carefully, in small doses.
Curriculum links: NGSS HS-PS4-4 · NGSS HS-PS4-1