# How does the Average Speed of Light in Glass Compare with its Speed in a Vacuum?

Do you know about the average speed of light compared to different mediums? Do you know in which medium-light moves faster or slower? Well, I will explain that all, but before let’s have a brief look at the early history of the speed light.

## Early History of the Speed of Light

Here is a brief look at the history of the research of speed of light.

• Empedocles of Acragas, around 450 BC, was one of the early philosophers who speculated that light traveled with a finite velocity.
• Around 525 AD, Anicius Boethius documented the speed of light. But, unfortunately, he was decapitated after being accused of treason and sorcery.
• During the middle of the Thirteenth Century, the Chinese secrets behind explosives made their way to the West.
• The Arabic scholar Alhazen, around 1000 AD, was the first optical severe scientist who suggested that light had a finite speed.
• By 1250 AD, a pioneer in British optics Roger Bacon recorded that the speed of light was finite but rapid.
• In 1572, a Danish astronomer Tycho Brahe was the first to describe a supernova in the constellation Cassiopeia.
• In late 1604, the German physicist Johannes Kepler speculated that the speed of light was instantaneous.

## Speed of Light in Air and Water

When light enters a different medium while traveling through the air, the light’s speed and wavelength are reduced,d, but the frequency remains unchanged. Check out the speed of light and reflective indexes in different mediums listed below:

• Light travels at approximately 300,000 km/sec in a vacuum, with a refractive index of 1.0
• 225,000 km/sec in water
• 200,000 km/sec in glass with a refractive index of 1.5
• In diamond, the speed of light is reduced to a relative crawl of 125,000 km/sec with a relatively high refractive index of 2.4

A light-year equals 9.5 trillion kilometers or about 5.9 trillion miles. Thus, the distance from Earth to the next nearest star beyond our sun, Proxima Centauri, is approximately 4.24 light-years. Comparing the speed of light in Milky Way galaxy to Andromeda, the former is estimated to be about 150,000 light-years in diameter, and the latter is approximately 2.21 million light-years; this implies that light left the Andromeda galaxy 2.21 million years ago and is just arriving at earth until and unless it meets a reflecting celestial body or refracting debris that causes it to delay.

## The Speed of Light

Have you heard these statements before? For example,  “Nothing can travel faster than the speed of light.” OR “Light always travels at the same speed.”

Unfortunately, these statements are somewhat misleading as they are often quoted as results of Einstein’s theory of relativity. Let’s clarify these statements first:

1. Earth can travel at a faster speed as compared to light in a vacuum
2. Speed of light in a vacuum always remain the same

By definition, a vacuum is a region with zero percentage of matter. So a vacuum would not contain any dust particles Light traveling through anything other than a perfect vacuum will scatter whatever particles exist below. In vacuum the speed of light is c = 2.99792458 x 108m/s.

Where c is the speed of light in a vacuum, as discussed in the theory of relativity, the speed inside a vacuum remains the same no matter who and from where it is measured. For example, suppose a vacuum is inside a box, and the box is in a rocket traveling away from the earth. Now, an astronaut and a hypothetical observer standing somewhere on earth will measure the speed of light. The light moving through that box will be exactly c. None of them will measure a faster speed. Therefore, we can say that c is the ultimate speed limit of the universe. So it’s better not to say that nothing ever travels faster than light.

On the contrary, as light travels through different materials, it scatters the molecules in the material and slows down. In some materials, the light will slow down more than electrons will, water, for instance. That means an electron in water can travel faster than light in water. But nothing ever travels faster than c. The index of refraction describes the amount by which light slows in a given material. The speed of light defines the index of refraction of materiality in vacuum c divided by the speed of light through the material v:

n = c/v

The values of n depend on wavelength to some extent. Therefore, unless you are told otherwise, assume the index of refraction given to you is appropriate for the wavelength of light you are considering.

Even though light slows down in matter, it still travels at a fantastic speed, even though a dense material such as lead still travels at a fantastic speed.  (Although light does not travel far through lead before being absorbed, high-energy gamma rays can travel a centimeter through lead at the speed calculated here.  Thus, using the definition of n, we can find the speed of light through lead:

vlead = c/nlead= (2.99792458 x 108  m/s) /(2.6) = 1.2 x 108 m/s = 2.6 x 108 miles/hr

Light still travels at the speed of 260 million miles per hour despite being slowed by lead. That’s 10,000 times greater than the speed of a space shuttle orbiting the earth.

## Bottom Line

The above discussion concluded that light could travel faster in a vacuum because it contains no dust particles. We also learned about the ultimate speed possible in the universe.  I hope this article has fully explained the topic in detail. It will also solve some of the misconceptions related to light and its speed. As time passed, many concepts changed and lost their originality and authenticity. Therefore, it is necessary, specifically in scientific topics, that you should not believe in words of mouth. Instead, do thorough research and try to extract information from practicals and experiments. For research purposes, do not compromise on the authenticity of the source of information, as it can ruin the hard work and energy you put into the project.

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