Waves, Light and the Electromagnetic Spectrum

Waves carry energy

A wave transfers energy from one place to another without moving matter along with it. In a transverse wave the vibrations are at right angles to the direction the wave travels, like the ripples spreading across water or any light wave. In a longitudinal wave the vibrations are along the direction of travel, like a sound wave squashing and stretching the air as it passes. Every wave has an amplitude, a wavelength and a frequency, and these describe how big it is and how often it repeats. This module covers those wave properties, how to find a wave speed, how light reflects and refracts, and the whole electromagnetic spectrum with its uses and its dangers.

Wave words

Learn these five wave properties before you measure and calculate with them.

Match each wave to its everyday use

Transverse against longitudinal

Waves come in two kinds, told apart by the direction the vibrations point.

Reading a wave diagram

On a wave diagram you measure from the middle rest line straight up to the very top of a crest. Which property have you just measured?

• The amplitude
• The wavelength
• The frequency
• The period

The order of the spectrum

The electromagnetic spectrum is a family of transverse waves that all travel at the same speed through empty space. In order from the longest wavelength and lowest frequency to the shortest wavelength and highest frequency they run: radio waves, then microwaves, then infrared, then visible light, then ultraviolet, then X-rays, then gamma rays. As you move along that order the frequency rises and the energy each wave carries rises with it, which is why the last three can damage living cells. A good way to keep the order in your head is to make up a sentence of your own whose words start with those same letters.

Pick the true EM facts

Select every statement about the electromagnetic spectrum that is true.

• Every electromagnetic wave is transverse
• Electromagnetic waves all travel at the same speed through empty space
• Gamma rays carry more energy than radio waves
• Sound is part of the electromagnetic spectrum
• Radio waves have a higher frequency than X-rays

Order the spectrum by frequency

Put these electromagnetic waves in order from the lowest frequency to the highest frequency.

• radio waves
• microwaves
• infrared
• ultraviolet
• gamma rays

Complete the wave facts

A wave carries _____ from place to place without moving matter along. The number of complete waves passing each second is the _____, measured in hertz. The greatest distance a point moves from its rest position is the _____. Sound travels as a _____ wave.

A stone dropped in a pond

Picture dropping a small stone into a still pond. Rings spread outward across the surface, moving further and further from where the stone landed, yet a leaf floating nearby only bobs up and down on the spot. It does not travel outward with the rings. That is the whole idea of a wave: something moves outward across the pond, but the water itself, and the leaf resting on it, stay where they are. What travels is the disturbance, passed on from one bit of water to the next, and never the water itself. Every wave you meet, from a sound in the air to light from a distant star, works in this same way.

Tap the transverse waves

Tap every example that is a transverse wave.

• light from a lamp
• ripples on water
• sound in the air
• an X-ray

Work out the wave speed

A water wave has a frequency of 5 hertz and a wavelength of 3 metres. Wave speed is frequency times wavelength. What is its wave speed?

Apply your wave knowledge

Read each situation and choose the best answer.

• A doctor needs an image of a bone that may be broken. Which wave is used?
• You want to remember the order of the spectrum in the exam. What is the smart way?
• Why can ultraviolet, X-rays and gamma rays harm living cells?

Build a wave point

Choose the word for each gap to complete one accurate point about waves.

Explain waves and the spectrum

A friend asks you what a wave really is and why some waves in the electromagnetic spectrum are more dangerous than others. Explain it clearly using what you have learned.

• Explain what a wave does (it transfers energy without moving matter)
• Explain the difference between a transverse and a longitudinal wave, with an example of each
• Name the electromagnetic spectrum in order
• Explain how to work out a wave speed
• Explain why higher-frequency waves such as ultraviolet, X-rays and gamma rays can be dangerous