Light is transported by particles, called photons. They have no mass, but carry sizeable amount of energy. When hitting a photomultiplier, a highly sensitive light detector, this energy can set an electron free, which – after multiplication – can be measured as a current impulse. If this current impulse is sent to a loudspeaker, it makes a click. If the light beam hitting the detector is gradually dimmed, the clicks still have the same loudness but their frequency gets smaller, see Fig. 1. This is one of the simplest manifestations of the particle nature of light. In vacuum, photons fly with the highest speed possible in nature, with the speed of light, see box.

Photons also show wave-like properties, as it becomes apparent when they can reach a detector via different paths, see Fig. 2. If many photons are emitted, which “wave” in unison, as it happens in a laser, a wave consisting of an oscillating electric and magnetic field emerges, see Fig. 3, which exerts measurable forces on charged particles. In visible light, these fields oscillate several hundred trillion times in a second, with the oscillation frequency determining the colour of the light wave. In infrared and ultraviolet light, the oscillation is slower and faster, respectively.

The energy carried by a photon is proportional to the number of field oscillations per second, called frequency. The energy of infrared photons, which can build up a wave oscillating at a low frequency relative to visible light is therefore much smaller than the energy of ultraviolet or x-ray photons, which can result in waves oscillating much faster than visible light. The more photons cross a unit area per second the larger the amplitude of oscillations of the electric and magnetic fields and the higher the intensity of the light wave.