# What kind of particle is light

## Wave model of light

Isaac Newton had a great influence on the scientific world of his time through the development of his axioms on mechanics and the formulation of the law of gravitation. This is why his concept of particles of light prevailed over the wave concept of light (which goes back to Huygens to a large extent). Some fundamental phenomena of optics can be interpreted with the particle concept as follows:

#### Straight light propagation

Newton's corpuscular theory says that light consists of tiny particles (corpuscles) that are ejected by the luminous bodies at great speed in all directions and move in a straight line in empty space.

#### reflection

When reflecting on a mirror, the velocity component \ ({c_ \ parallel} \) parallel to the mirror is retained, the velocity component \ ({c_ \ bot} \) perpendicular to the mirror changes its sign. In this way we get \ (\ alpha = \ alpha '\).

#### refraction

The refraction of light in the particle model was imagined as follows: In the transition from the optically thinner (white) to the optically denser medium (gray), the horizontal component of the particle velocity is retained. However, the component perpendicular to the interface is increased. In this way, the refracted light is refracted towards the normal of incidence. However, this would have to increase the speed of light in the optically denser medium. Some time after Newton, around 1820, Fresnel found that the speed of light in the optically denser medium, however, is lower.

#### Problems of the model

The realization that the speed of light in the optically denser medium is smaller and not greater, as well as difficulties in explaining diffraction, led to the fact that the wave conception prevailed over the corpuscle theory up to approx. 1905.

Hints:
In some passages of Newton's work, the corpuscles were also assigned wave properties in order to arrive at a better explanation of the phenomena.
In Newton's work, the different colors of light were explained by light particles of different sizes.