The portion of wavelength $4000$ to $8000$ (angrstom) in electromagnetic spectrum is visible light. The light itself is invisible and with it things can be seen.

Views on the light are as follows :

$(1)$ Corpuscular model of light (Newton's particle theory of light) :

– The corpuscular theory of light set by Descartes in $1637$ and derived Snell's law and explained the laws of reflection and refraction of light at surface that separates two mediums.

The corpuscular model predicted that if the ray of light (on refraction) bends towards the normal then the speed of light would be greater in the second medium. According to this theory, the speed of light is lower in rarer medium and higher in dense medium.

– The corpuscular theory of light was considered Newton's corpuscular theory.

In this theory light is considered to be composed of corpuscular.

$(2)$ Huygen's wave theory : In $1678$, Christian Huygen's put the wave theory of light.

This theory explain the phenomena of reflection and refraction and it predicted that on refraction if the wave bends towards the normal then the speed of light would be less in the second medium. This is in contradiction to the prediction made by using the corpuscular model of light.

In $1850$, Foucault carried out the experiment and predicated that the speed of light in water is less than the speed in air.