Is light matter or not, why?

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Can light depend on gravity?

Light is affected by gravity, but only in a very specific way. Light always travels through open spaces at the same speed, and that doesn't change with gravity. However, gravity can change the direction of a beam of light by slowly bending it, or rather the space through which the light is traveling. A simple analogy would be a train traveling on some tracks - the train always maintains its speed, but if something (gravity for example) bends the tracks left or right, the train will have to go along with it. This light-bending effect is very useful to astronomers looking in the sky to see where there might be something very massive that produces a strong gravitational force - the gravity is directly visible because the light from the stars in the sky looks like it bends, as if you were looking at the sky through a magnifying glass.

Gravity can also change the color of light - the closer a beam of light is to a heavy object, like a planet or a star, the more blue it will be. The farther away it is, the more red it will be.

We know these effects from a physics theory called General Relativity.

Is electron matter or not?

We call something matter if it occupies some volume and weight. An electron is a very small part of an atom, but it certainly takes up space - it is very small, but it has a definite volume. It also has mass, just a very small one. For these reasons, an electron is matter.

Can we see an electron?

An electron is too small to be seen directly. What you can definitely see though is how an electron affects things around it, or when something happens to the electron to cause it to emit light - literally "light up". For example, when electrons move around, they feel a type of friction. This friction heats up the space around the electrons, and it can even become glowing hot. This is what happens with lightning: electrons moving from the clouds to the ground heat up the air and create the flash of light that you see. Old light bulbs also worked this way - the electrons moving through the wire in the light bulb caused the wire to heat up and glow.

Another way you might see an electron is when it collides with special chemicals called phosphors that are designed to glow when hit with an electron. Old TVs and computers (the kind that were very thick and bulky) worked using this effect - electrons would hit phosphor chemicals on the screen, and they would emit light up so you could see where the electron is.

A wave is matter or not?

The requirement for something to be matter is that it should have some volume and some weight: it is an object.

In contrast, a wave (for example, a wave in water, or on a rope) is made up of objects that move around, and maybe bump into something else and cause that something else to start moving: a wave is a type of energy. For this reason, we would say a wave is energy and not matter.

This definition works well for large objects and waves that you can see around you every day. Once we start looking at very small objects: atoms, electrons, and even smaller particles, we find out that the distinction between matter and energy disappears, and these small particles start behaving as if they were waves. At such a small scale, it would be correct to start saying that a wave is both matter and energy at the same time!

Why can't we see behind a wall if the wall is made of atoms?

It is actually possible to see behind a very thin wall. While atoms take up some space, there is quite a bit of empty space between them. What is important is how well atoms absorb light that is nearby, which depends on the type of atom and how it is connected chemically to atoms around it. In some cases it is hard for light to get through even a thin layer of atoms, because certain types of atoms are very good at absorbing light that is nearby. Still, if you had a thin wall that was made of a single layer of atoms, you would almost certainly be able to see through it. However, most walls are made of an enormous number of atomic layers, so that even if the light gets through the first few layers, it eventually runs into enough atoms and is reflected back or absorbed. Because light cannot get through this thick wall, you cannot see through it. The exception is transparent materials - for example glass. These materials are made of atoms that do not absorb or reflect light, so it doesn't matter if the light eventually does run into an atom or how thick the wall is - the light can still get through the glass and you can see through.