 You may remember from junior science the anatomy of a wave. The distance a wave wiggles from its centre point is called its amplitude. All of these waves here have roughly the same amplitude, but if I were to draw one with a much bigger amplitude it would look like this. The length from crest to crest of a wave, or from trough to trough, is called its wavelength. The number of waves that pass by a fixed point in any given time is called its frequency. I should say passed by a fixed point in one second, in fact. If this is zero seconds and this is one second later, then the orange wave has a frequency of 4.5 waves per second. We call this 4.5 hertz. The unit of waves per second has its own special name known as the hertz. The blue wave, if you count them up, has a frequency of about 8.5 hertz. Looking carefully you may be able to see that wavelength and frequency are inversely related. That's the same as saying that a wave with a long wavelength has a low frequency, while a wave with a short wavelength has a high frequency. Wavelength, frequency and speed are related by this equation. C equals F lambda, or F times lambda. C is the speed of light. In all forms of light everything on the electromagnetic spectrum has the same speed, as you measure it in a vacuum. It's 3 times 10 to the 8 meters per second. If C is a constant, you can see from that equation that if the wavelength goes down, then the frequency must go up, so that their product remains constant. Now in a sound wave, the wavelength determines pitch. Sound waves with different wavelengths will have different pitches. Short wavelengths give you a high-pitched sound. Long wavelengths give you a low-pitched sound. In a light wave, the wavelength instead determines color, or more generally the kind of wave. So from this diagram you can see that radio waves have very long wavelengths, 100 meters or longer, and hence quite low frequencies. X-rays on the other hand have very short wavelengths, around 1 nanometer or the size of an atom, and hence they have very high frequencies. The light that's visible to us, the light that our eyes have the right sensors for, has a wavelength of between about 400 and 700 nanometers. That's about the size of a small bacterium, or a large virus, or about 250 times thinner than the average human hair.