Light is everywhere around us. From daylight out in the Sun to the light coming out of our mobile screens, light helps us see and observe the world around us. However, it’s not enough to simply name this incredible phenomenon as “light” and leave it at that. In Physics, we love to pay attention to detail and understand things on the minutest scale possible. Pick a big thing from your daily life, anything, and you’ll find that it’s actually just a collection of a bunch of small things called atoms. What’s the difference between graphite and diamond? They are both made of the same stuff – atoms of carbon -, yet one is much rarer than the other. The only fundamental difference between them is that one has a different arrangement of these tiny atoms than the other.
Similarly, when it comes to light, it is natural to assume that it must also be made of up some small stuff like every other thing in the universe. And since we have already established that everything is made up of atoms, is light too made of atoms? As it so happens, light is an exception to the rule. Instead of tiny particles called atoms, light is made up of even smaller particles called photons of light.
A photon is a packet of energy that is very small in size. For perspective, let us take a standard light bulb (100-W). Take a good look at the amount of light coming out of the bulb in a single second. By using some simple math, we know that the light bulb emits about 3 x 1020 photons per second. That’s about three hundred billion, billion particles of light being produced by the bulb in just one moment!
Let us look at some basic facts about photons:
- They are the fastest moving things in the universe. They have a constant speed of 3×108 m/s, which is also called the speed of light and is unsurpassable by anything else in the universe.
- Distances in space are measured with reference to how long it will take a photon to move through a distance. These distances are called light years – where one light-year is the distance light can travel in one year through space.
- Finally, photons are one of the only two known things in the universe with no mass (the other being the “gluon”).
At first, it seems counterintuitive that anything in the universe can be without mass. After all, mass is what gives things weight, gives them structure, and makes them visible. However, as we are going to find out, this does not necessarily have to be the case.
If we want to answer the question, “How can photons exist if they have no mass?” we need to acknowledge the fact that, generally, in order for something to exist, it must have some energy. Any particle without energy cannot exist – it would be equivalent to being a “nothing.” So, the question we actually need to ask ourselves is: “Do photons have energy?”
In order to understand this concept, we are going to make use of three terms associated with light; momentum (quantity of moving mass), speed, and mass. Remember, even though a photon has no mass, it has a definite speed and momentum.
To discuss the nature of light, let us consider the 20th-century physicist Albert Einstein’s equation on light which is given as:
Energy = m2c4 + pc2
Where ‘p’ is the momentum, ‘m’ is the mass, and ‘c’ is the speed of a photon.
This basically means that the energy of a photon depends upon its mass, momentum, and speed. Now, as we know, a photon has zero mass (m = 0). So, our equation becomes,
Energy = pc2
This means that the energy of a photon only depends upon its momentum and its speed. Now, we know that light always has a constant speed. And we also know that light has a definite momentum. So, this means that it must also have some energy. To conclude, anything in the universe that has energy must be something. As light also has energy, therefore, it must be something real and existing.