Physics Glitch! Why Does Light Appear to Slow Down When it is Not in a Vacuum?

Light is the fastest thing in the Universe and has the same speed regardless of the reference frame. So why do we say it slows down when it's not in a vacuum?

Why does light seem to slow down from half to half?
Why does light seem to slow down depending on the environment it is in?

Our way of observing the Universe is through light using its entire spectrum, ranging from gamma rays to radio. Furthermore, light is also the form of communication between antennas and even between satellites. One of humanity's greatest advances was understanding Electromagnetism and better understanding light.

One of the discoveries of Electromagnetism was that light has a constant speed in a vacuum. Light has the maximum speed limit in the Universe with a speed of c = 299972 km/s. This discovery was the basis for Albert Einstein to introduce relativity and conclude that the speed of light is the same for every reference frame.

However, we learn from an early age that light slows down when it passes from one medium to another. For example, the speed of light in air is greater than the speed of light in water. The value known as c being possible only in a vacuum. But why does the light seem to slow down halfway through? And how does Physics explain this phenomenon?

Wave-particle duality

One of the most interesting phenomena in Physics is wave-particle duality. This phenomenon describes which properties of Quantum Mechanics can behave as a particle or as a wave. Electrons, protons, photons are examples of components that behave like a wave or particle.

The most famous experiment is the photoelectric effect that won Albert Einstein a Nobel Prize in Physics.

The best-known component with this property is the photon, which is the particle associated with light. We also know that light has wave properties and we have even divided the electromagnetic spectrum into wavelengths and frequencies. Depending on the type of observation, light can be treated as either one or the other and the results will converge.

Snell's Law

In Optical Physics, one of the main laws is the famous Snell's Law. This law describes how the behaviour of light changes from one medium to another. The formula is quite simple: we know the angles, wavelengths or indices of refraction of the light before it enters the medium and then it is possible to find changes in the speed of light.

Example of Snell's law when a ray of light leaves one medium for another. Credit: BYJUS
Example of Snell's law when a ray of light leaves one medium for another. Credit: BYJUS

Snell's law is important in several areas, such as the construction of glass that makes up car windows, for example. In our daily lives, we observe Snell's law when we are in the pool and the position of something in the water appears different depending on where it is observed. But what draws the most attention is the difference in speeds.

Using the law, it is possible to see that the speed of light in air is different from the speed of light in water, considering the example of a swimming pool. This is true for different mediums as well. We know that the maximum speed of light happens when it is in a vacuum and that is why it is common to hear that light is only the fastest thing in the Universe in a vacuum.

Absorption and re-emission

When light passes through a medium composed of atoms, such as air or glass, it interacts with the atoms that make up that medium. Interactions happen through absorption and re-emission. Absorption is when photons are absorbed by atoms through passing energy to electrons that go to a higher energy level.

As everything in the Universe seeks a minimum energy level, the electron needs to lose that energy to return to its original energy level. The way the electron loses energy is by re-emitting the photon into the medium again. This process is called re-emission and it takes time from the moment it is absorbed until it is re-emitted.

Obstacle race

Because of the effect of absorption and re-emission, the photon suffers a delay in the time it would take from one point to another, making the speed appear to be slower. It's like an obstacle course where the obstacles are the atoms that are present in a medium. This process of absorption and re-emission happens countless times.

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Therefore, these obstacles delay the light, making the effective speed less than the speed of light as we know it. Despite this, the speed of the photon from one atom to another remains the same and is constant regardless of the reference frame. In a vacuum, this effect does not happen because there are no obstacles to light.

Change in each medium

The effective speed that light will have in each medium varies depending on how many atoms are present in a given volume. In the air, the atoms are more spread out so there are fewer obstacles than the same volume of water. This becomes even more evident when we consider solids like glasses where the atoms are much closer together.

Therefore, the index of refraction changes from medium to medium, all of which are related to the density of each medium. Furthermore, other effects such as dispersion are also related to the process of changing the environment. These effects are studied by Optical Physics.