# What exactly is light? [duplicate]

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What is light in terms of physics? Whenever looking for definition on the internet getting explanation or translation of my language, a common definition/explanation for light was

Light is light. We get it from the Sun, when we flip on the switch, or turn on the flashlight. We see it! But that explanation doesn't really explain anything

This was explanation or it just was an example of light. Can anyone define what exactly light is? Is it motion? Is it energy? Or vice versa?

## marked as duplicate by RedGrittyBrick, ACuriousMind♦, Kyle Kanos, Sebastian Riese, John DuffieldOct 20 '15 at 16:23

• In physics, light is $A_\mu$. Okay, it's a bit more than that, but once you have $A_\mu$, you inevitably have light – Jim Oct 20 '15 at 13:07
Light is a phenomenon which transfers energy and momentum through many substances, including vacuum, with characteristic velocity $c$ through the vacuum and characteristic energy-to-momentum ratio $E/c.$ It differs from other similar phenomena in many other characteristic ways: it can bend through glass, it has a 2D polarization transverse to its propagation (so there exist polaroid filters which work on it), it can be emitted and absorbed by antennas, it is absorbed (at least at visible wavelengths) by our eyes, and it tends to cause electrons and protons to vibrate, but not isolated neutrons: its effect is proportional to a strongly-related physical phenomenon called "electric charge." We like to think of it as a wave in the "electromagnetic field" but ultimately it comes in lumps of energy where the energy per lump depends on the frequency via Planck's constant, $E = h~f,$ and these diffract due to the rules of quantum mechanics.