Frequency and wavelength in waves
On the previous page you saw that a wave is energy that travels. In a mechanical wave the energy needs a medium to travel through, like water, air or solid materials. Electromagnetic waves are different in this respect. They can travel through a medium, but they can also travel through empty space, which is nice, because it allows for wireless communication via satellites in space.
Frequency and wavelength
Mechanical waves and electromagnetic waves are different, but they have some important similarities too. They both have a frequency and wavelength. When we talked about the basics of radio you saw that the frequency and wavelength of a wave determine the characteristics of the wave.
In electromagnetism waves of different frequencies have entirely different characteristics and uses. Waves with a low frequency and long wavelength (the size of cars or buildings) are radio waves, while waves with a high frequency and a very short wavelength (the size of atoms) are known as gamma radiation.
In sound waves, waves with a high frequency sound like high pitched tones to your ears, while waves with a low frequency sound like low pitched sounds.
In electromagnetic waves withinin the specturm of visible light, waves with a long wavelength are red light, while waves with a short wavelength are colored blue.
If your eyes perceive color differences this means that they are receiving electromagnetic radiation in the visible light spectrum of different frequencies.
How waves are represented
Waves are usually represented by a wavy line, resembling the mechanical waves you see in water. But don't be fooled. The wavy line representation of waves is just a model. Electromagnetic waves for example don't look like wavy lines, neither do sound waves. In physics scientists use models to illustrate the characteristics of natural phenomena.
In the picture below you see three waves of different wavelengths and frequencies.
The top of a wave is called a crest, while the bottom is called a trough. The distance between two crests (or troughs) is the wavelength of the wave.
If you imagine the wave travelling you can see how the uppermost wave "changes" once every second, while the lowermost wave "changes" three times in a second. We say that the uppermost wave has a long wavelength and low frequency (1 Hz, more on the Hz notation on the next page) and the lowermost wave has a short wavelength and a high frequency (3 Hz).
In the picture, you can also see that wavelength and frequency are related. Waves with a long wavelength have a low frequency and waves with a short wavelength have a high frequency.