Consider the energy transmitted to the Earth from the Sun by light (or by any source of light). Light, which is also called “electro-magnetic radiation”. Why the fancy term? Because light really can be thought of as oscillating, coupled electric and magnetic fields that travel freely through space (without there having to be charged particles of some kind around).

It turns out that light may also be thought of as little packets of energy called photons (that is, as particles, instead of waves). The word “photon” derives from the word “photo”, which means “light”.  Photons are created when electrons jump to lower energy levels in atoms, and absorbed when electrons jump to higher levels. Photons are also created when a charged particle, such as an electron or proton, is accelerated, as for example happens in a radio transmitter antenna.

But because light can also be described as waves, in addition to being a packet of energy, each photon also has a specific frequency and wavelength associated with it, which depends on how much energy the photon has (because of this weird duality – waves and particles at the same time – people sometimes call particles like photons “wavicles”). The lower the energy, the longer the wavelength and lower the frequency, and vice versa. The reason that sunlight can hurt your skin or your eyes is because it contains “ultraviolet light”, which consists of high energy photons. These photons have short wavelength and high frequency, and pack enough  energy in each photon to cause physical damage to your skin if they get past the outer layer of skin or the lens in your eye. Radio waves, and the radiant heat you feel at a distance from a campfire, for example, are also forms of electro-magnetic radiation, or light, except that they consist of low energy photons (long wavelength and high frequencies – in the infrared band and lower) that your eyes can’t perceive. This was a great discovery of the nineteenth century – that radio waves, x-rays, and gamma-rays, are just forms of light, and that light is electro-magnetic waves