1. Light and Radiation
Photons (2/4)
How do we interpret the signals observed in our experiment? The pulses registered by the counter and shown on the oscilloscope leads us to assume that the detector perceives the light not as a continuous wave but rather as irregular individual occurrences, as in the case of particles!
Each pulse appearing on the screen of the oscilloscope represents an electron released by a photon on the metal surface of the detector. Such electrons are called photoelectrons. They are amplified by the detector electronics and registered as a pulse. Electrons have a negative charge and therefore appear as a negative pulse on the oscilloscope. This is the photoeffect. In 1905, Albert Einstein interpreted the photoelectric effect.
Einstein did not have an oscilloscope at his disposal. This was only invented around 50 years later! An experiment that can be carried out at school without an oscilloscope is described here.
The hypothesis that light has discrete and therefore quantised properties was first put forward by the physicist Max Planck in 1900. By quantising light, he was able to explain the brightness and shape of the spectrum of black bodies.
The photon properties of light can be seen
- in the radiation of black bodies
- in the absorption and emission of spectral lines by atoms and molecules.
The spectrum of sunlight changes as it travels through the atmosphere to the earth's surface due to absorption by components of the air. For example, UV radiation is reduced by the absorption of ozone, which would otherwise be very harmful to us. Such effects are discussed in Chapter 2.
While observing the heavens from the earth with a telescope, you may occasionally see some fast moving objects: comets! Owing to the sun’s strong radiation, comets become mechanically unstable when they come close to it and thereby lose some dust and particles. This material is then seen as bright comet’s tail.
Sometimes a double tail appears: the blue tail consists of molecules, the white tail of larger dust particles. The air molecules in our atmosphere also scatter blue, the larger water droplets in the clouds white. The comet in the picture above shows both tails very clearly. Their different orientation is often explained with the photon model: the photons of sunlight push the molecules away from the sun with their collisions, which they cannot do with the much larger and more massive dust particles! However, Maxwell had already shown in 1873 that electromagnetic waves also exert such forces.
These tails are further analysed in Supplement 1.6 and in a worksheet.
