# If light slows down passing through water, how does it speed back up when it comes out?

As proven by scientists, light is the fastest thing on the planet Earth. If measured in miles per hour, light travels at a speed of 670, 616, 629 mph. If we ever get to travel at the speed of light, we may be able to travel around the Earth 7.5 times in just the blink of an eye. But unfortunately, nothing as fast as the travelling of light waves has yet been invented by the human race.

Light travels fastest in a vacuum. But does that mean light travels at different speeds? The answer is no. Light waves move at the same speed; however, it depends on the barriers coming in the travelling path of the light waves that determine how long it takes for them to travel through any medium. The denser the medium or the more particles a medium contains, the slower the speed of the light waves.

## Is light speed constant?

Similar to sound waves, the time it takes for light to travel also varies according to the object it is travelling through. So, let’s say you are standing behind a wall and your friend tells you something, would you be able to hear it as clear as they would have said it right in front of you, without any object in between? Of course not! Drawing a parallel to this concept, it is why light changes speed when entering or exiting a body of water. The obstructions differ between air and water which ends up in the difference of the light speed.

## Why does light slow down when travelling through water?

Imagine a big famous actor walking through a room. They travel at a constant speed that we will call A. The actor always moves at speed A, no matter what. When the room is empty, they can walk into the room and out of it, easily in a straight line.

However, if the room is full of people then the actor can’t walk in and out of the room in a straight line. They keep moving at A, but because of the people, they have to bounce around and take a much more circuitous path to get out of the room. This means that despite walking at a speed of A mph, the entire time, it took them longer to get out of the full room than the empty room.

The same is true with light. Light does not ‘slow down’ in water. The light is still moving at the same speed. Light travels in waves which means when it travels through objects, it slows down. Even the presence of dust particles in the atmosphere can slow down the speed of light.

It is the reason why light waves travel fastest through a vacuum. Following the same concept, water is much denser than air or a vacuum, so to make it through any body of water, light waves have to take a much lengthier path. Despite never changing speed, we as an outside observer perceive the light taking more time to cross through the same distance as a difference in speed, rather than what it actually is: the distance has changed.

## But then why does light travel faster when coming out of water?

Let’s continue with the same concept mentioned above. Since the speed of light stays technically the same and it is only the particles coming in its way that increase the traveling distance, the moment the light waves come out of the water they start travelling at a faster speed. Because now they are moving through a less dense medium, that is air, the light waves can manage to move at a quicker speed without any objects coming in between their path.

So, to sum up, the answer to the question we began this piece with, light travels at a constant speed. It is the medium it is travelling through which determines the length of the distance and eventually, the time it takes for the light waves to travel. This is what makes the light waves seem to travel faster or slower in comparison.