Waves and the Electromagnetic Spectrum

Grade 7 · Science · Study Guide

An introduction to wave properties and the range of electromagnetic radiation from radio waves to gamma rays.

A wave is a disturbance that transfers energy from one place to another without transferring matter. Waves have several key properties. Wavelength is the distance from one crest (peak) to the next. Frequency is the number of waves that pass a point in one second, measured in hertz (Hz). Amplitude is the height of the wave and relates to its energy — bigger amplitude means more energy. Wave speed equals frequency multiplied by wavelength.

There are two main types of waves. Mechanical waves, like sound and ocean waves, need a medium (such as air, water, or solid material) to travel through. Electromagnetic waves do not need a medium and can travel through the vacuum of space. Light is an electromagnetic wave.

The electromagnetic spectrum organizes all types of electromagnetic radiation by wavelength and frequency. From longest wavelength to shortest: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. As wavelength decreases, frequency and energy increase.

Visible light is only a tiny portion of the full spectrum — the part our eyes can detect. Different colors of visible light have different wavelengths: red has the longest wavelength and lowest frequency in visible light, while violet has the shortest wavelength and highest frequency. This is why prisms and raindrops can separate white light into a rainbow.

Key terms

Wavelength: The distance between two consecutive crests or troughs of a wave, usually measured in meters or nanometers.
Frequency: The number of wave cycles that pass a given point per second, measured in hertz (Hz).
Amplitude: The maximum displacement of a wave from its rest position; related to the energy carried by the wave.
Electromagnetic spectrum: The complete range of electromagnetic radiation arranged by wavelength, from radio waves (longest) to gamma rays (shortest).
Transverse wave: A wave in which the disturbance moves perpendicular (at a right angle) to the direction the wave travels.
Electromagnetic wave: A wave that does not need a medium to travel and is made of oscillating electric and magnetic fields; travels at the speed of light.

Flashcards

Crest: The highest point of a transverse wave.
Trough: The lowest point of a transverse wave.
Hertz (Hz): The unit of frequency; one hertz equals one wave cycle per second.
Infrared radiation: Electromagnetic radiation with wavelengths longer than visible light; felt as heat.
Ultraviolet radiation: Electromagnetic radiation with wavelengths shorter than visible light; can cause sunburn and is mostly blocked by the ozone layer.
Speed of light: The speed at which all electromagnetic waves travel in a vacuum: approximately 300,000,000 meters per second (3 × 10⁸ m/s).
Mechanical wave: A wave that requires a physical medium (solid, liquid, or gas) to travel through; sound is an example.
Gamma ray: The electromagnetic radiation with the shortest wavelength and highest energy and frequency; emitted by radioactive materials.

Questions

Which type of electromagnetic radiation has the longest wavelength?
1. Gamma rays
2. X-rays
3. Radio waves
4. Ultraviolet
Show answer
Answer: Radio waves
Radio waves have the longest wavelengths in the electromagnetic spectrum, ranging from about 1 mm to more than 100 km. They carry the least energy per photon.
Sound waves can travel through the vacuum of outer space.
Show answer
Answer: False
Sound is a mechanical wave and needs a medium (like air, water, or solids) to travel. Space is a vacuum with no medium, so sound cannot travel there — hence 'in space, no one can hear you scream.'
The number of wave cycles that pass a point per second is called the wave's ______.
Show answer
Answer: frequency
Frequency measures how many complete wave cycles pass a fixed point each second. It is measured in hertz (Hz) and is inversely related to wavelength.
As the frequency of an electromagnetic wave increases, what happens to its wavelength?
1. It increases
2. It decreases
3. It stays the same
4. It doubles
Show answer
Answer: It decreases
Frequency and wavelength have an inverse relationship. Since all EM waves travel at the same speed, a higher frequency means more shorter waves pass per second — so wavelength decreases.
Visible light makes up the majority of the electromagnetic spectrum.
Show answer
Answer: False
Visible light is only a very small slice of the electromagnetic spectrum. Human eyes can only detect wavelengths roughly from 380 nm (violet) to 700 nm (red).

Get a personalized course on Waves and Electromagnetic Spectrum built around your child.