# Light contain of what substance?

1. Sep 5, 2015

### jianshing

Light have particle? If have particle do the light produced pressure when sunlight shine on the planets? if do not contain particle what make the light has colour? As we know most of the energy do not have colour.

2. Sep 5, 2015

### Staff: Mentor

Light can be described with the concept of particles. It does not have to.
Light shining on a surface exerts a pressure, yes.
Light of different wavelength reacts differently with the light-sensitive parts of our eyes. Well call this difference "color".
That statement does not make sense.

3. Sep 5, 2015

### vanhees71

Light can NOT be described by particles in the usual sense. What we call light is an electromagnetic wave in a certain range of frequencies (or equivalently wave lengths between about 400-800 nm). What we perceive as color is not a physical quality of electromagnetic waves but a physiological of our eyes and is due to the different excitation of the cells in the retina by electromagnetic waves of different frequencies.

With this classical picture of light as classical electromagnetic waves you come very far; most of the usual optical phenomena in everyday life can be described in this way. However, it's not the full story. E.g., when physicists in the late 19th century tried to figure out, which spectral decomposition light emitted from a hot metal might have, they had big trouble using the known laws of electromagnetism (Maxwell's equations) and thermodynamics/statistical physics. Finally, Planck found out in 1900, that the classical theory cannot account for the observed black-body spectrum, and he had to introduce the assumption that electromagnetic wave of frequency $\omega$ can only be absorbed and emitted by matter in portions of energy of the size $\hbar \omega$, where $\hbar$ was a new natural constant.

This lead to the development of quantum theory. In its relativistic form, it is a socalled quantum field theory. The electromagnetic field and the charged particles interacting with the electromagnetic field and among themselves by the electromagnetic interaction mediated by this field has to be described in terms of this quantized version of the classical Maxwell equations. This theory is called quantum electrodynamics, and within this very abstract and mathematical theory you have some configurations of the electromagnetic field that can be interpreted as "light quanta", also called "photons", which have some particle-like properties, but particularly photons which have vanishing invariant mass are on the other hand very far from what we associate with massive classical particles. It's not even possible to define the position of a photon in the usual way. You can localize them only by measuring them by letting them interact with something sensitive to it, e.g., a photo plate or our eyes. Then you can say, there was an interaction of a photon in a certain region of space, but you can never precisely know, where a photon will appear, even if you know its state completely, but only probabilities, where they might be detected.

4. Sep 5, 2015

### Staff: Mentor

At the level that you're asking, we don't need to worry about quantum electrodynamics and other esoteric subjects: Light is electromagnetic waves, and color comes from the ways that our eyes and brain respond to different frequencies of those waves. Try googling for "What is light" and "What is color" and you'll find a number of good explanations.