**Aspects of the Wave model**

**wavelength**, the distance between two consecutive crests or troughs of the wave. Hence wavelength is a length and is expressed in metres. We can also count how many complete waves pass at a particular point each second and we call that **frequency**, expressed in cycles per second or Hertz (Hz). If we multiply the two, then we calculate what length of waves passes in one second, thus we obtain the **speed** of the wave in metres per second.

Light or EM-radiation travels very fast and at a constant speed (in vacuum) of 299,792,458 metres per second. This number is exact, because the definition of the **metre** as a unit of length has been adapted to this value for the speed of light. To have an idea how fast this is: light travels about 7.5 times around the world at the equator in one second.

With a constant speed of the travelling wave, frequency and wavelength are inversely proportional. This means that when the frequency e.g. is doubled, then the wavelength will be halved and the other way around.

**Aspects of the Quantum model**

If we think about the quantum model for light we can talk about the (photon) **energy** of light that is proportional to the frequency. In other words if the frequency is doubled then the energy is doubled as well, when the frequency is halved then the energy is halved, etc.

In summary, when the wavelength is long, the frequency and energy are small, when the wavelength is short, the frequency and energy are large. |