eli5 What is light made up of? Where does it come from inside a torch? Is light just un-containable luminous particles than can never be studied as a single unit inside a Microscope?

Question

eli5 What is light made up of? Where does it come from inside a torch? Is light just un-containable luminous particles than can never be studied as a single unit inside a Microscope?

In: Physics

Answer 1

Hi /u/po4165!

>What is light made up of?

The term “light” is usually used to describe the visible part of the [electromagnetic spectrum](https://en.wikipedia.org/wiki/Electromagnetic_spectrum). That is, light is a wave comprising of excitations in the electric and magnetic field, with a wavelength of roughly 400-700 nanometers. [Quantum mechanics](https://en.wikipedia.org/wiki/Quantum_mechanics) tells us, that these waves do not transmit energy in a continuous stream, but instead energy is transmitted in discrete packages. These packages are called [photons](https://en.wikipedia.org/wiki/Photon).

>Where does it come from inside a torch?

I’m not sure if you are using the word torch to describe a the candle-looking thing or a flashlight, and the answer would differ slightly depending on the meaning.

Modern flashlights use [light-emitting diodes (LEDs)](https://en.wikipedia.org/wiki/Light-emitting_diode) to generate light. LEDs typically make use a process called [electroluminiscence](https://en.wikipedia.org/wiki/Electroluminescence), where photons are emitted when electrons transition to a lower energy-level (so-called “holes”). An intuitive understanding of this process can be gained, when considering conservation of energy: when an electron is excited and at some point later drops back to a lower energy-level, the energy has to go somewhere. In the case of LEDs, the energy between the excited state and the lower energy state is emitted in the form of a photon.

>Is light just un-containable luminous particles than can never be studied as a single unit inside a Microscope?

Photons are a bit special in so far as they are mass-less particles. And as it turns out, all mass-less particles must always travel at the speed of light c (through vacuum). And crucially, it doesn’t matter how fast an observer is traveling relative to the source of the light, she will always measure the photons to travel at c.

A consequence of this is that there are no valid frames of reference, relative to which photons are at rest. While photons *do* slow down when traveling through a material, they can never be studied on their own and at rest.