How Does the Average Speed of Light in Glass Compare With Its Speed in a Vacuum?

The speed of light in glass is slightly lower than the speed of light in vacuum. Galileo was the first person who took an attempt to measure the speed of light. Speed of light is the speed at which the waves propagate through different mediums. The speed of light is considered as the fundamental constant of nature because it’s importance is far broader than our imaginations because it serves as the single limiting speed or velocity in the universe.

Speed of Light in Glass Compare With Its Speed in a Vacuum

Measurement of light has been a challenge for the scientists but now it’s been discovered and proved that nothing can move faster than the speed of light.

Average Speed of Light in Glass Compare With Its Speed in Vacuum

In the medium, light does not travel at speed equal to its speed in vacuum which is equal to c. The measurement of light’s speed in different mediums with respect to its speed in vacuum is often represented in the terms of refractive index.

1. Refractive Index

It is defined as the ratio of speed of light in vacuum to it’s speed of light in specific medium. Refractive index is represented by symbol n.

Refractive Index Formula

The formula of refractive index describes the comparison of the speed of light with different medium either glass, water or any other medium.


n = refractive index

c = speed of light

v = speed of light in specific medium (phase velocity of light)

Refractive index helps us to measure the exten?

t of bending or refraction of light ray when it enters into some specific medium.

2. Dependence of Refractive Index

Wavelength is the factor on which refractive index depends and hence describes the angle of refraction and angle of incidence which causes the changes in the speed of light in different mediums with respect to speed of light in vacuum. The wavelength of light in the specific medium is given by

Dependence of Refractive Index

There are no such particles found in mediums so light passes out there without any scattering or bending but in the case of a specific medium as here we are discussing glass particles cause the slight bending or refraction of the light which decreases its wavelength and speed of light in glass reduces.

3. Calculation of Refractive Index

Refractive index can be calculated by using snell’s law which describes as  n1 sinθ1 = n2 sinθ2


n1 and n2  = refractive indices of the medium

Θ1 = angle of incidence when light enters medium

Θ2= angle of refraction when light enters the medium

This law explains the behavior of light ray crossing through two mediums, vacuum or any specific medium.  The refractive indices in the snell’s law describes the angle of incidence when a light ray reached between interface of medium and the angle of reflection that is the amount of light reflected back due to the particles in the medium.

Refractive index of every different nature of the medium is different but it may stay constant for a specific medium under the same conditions of temperature and pressure. Usually the refractive index of the transparent materials for example glass occurs greater than 1.

Speed of Light in Glass and Vacuum: W.R.T. Refractive Index

We can find the accurate refractive index of glass by using the minimum deviation method in which we place an isosceles triangular prism made of hgh optical glass in the air. The refractive index of glass is 1.517 and vacuum is 1.

By depending upon these refractive indices, light approximately travels at the speed of 300,000 kilometers per second in the vacuum with refractive index of 1. And this speed of light slows down in the glass and travels at a speed of 200,000 kilometers per second with a refractive index of 1.5 for the visible light.

Hence it is proved that the average speed of light in glass is slightly lower in comparison to the speed of light in vacuum due to the difference of refractive index in vacuum or in a specific medium glass.

Is the Speed of Light Really Constant?

The speed of light depends on the nature of the medium through which it is moving. When light passes through a vacuum its speed is always constant because in vacuum there are no particles.

But in comparison to some other medium like water, glass, diamond, oil and any other thing the speed of light varies in different mediums because particles cause the scattering of light in different directions which slows down the speed of light.

For instance: Speed of light in the atmosphere is as fast as it is moving in vacuum but in different mediums like diamond the speed becomes half of that.

Final Thoughts

So here I gave you information about the speed of light which depends upon the refractive index directly the nature of medium. When light moves in vacuum its speed remains constant either actually calculated in vacuum or in a box placed in a rocket moving in the vacuum. But in case of some other specific mediums light reflects back because of particles moving in that medium which reduces the speed of light. It is just all because of the differences of the refractive indices. And it’s really very interesting to know facts about the speed of light you can go through!!

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