What is the ‘slowest’ speed of Light?

Last time, I discussed the Speed of Gravity, and said it is equal to the Speed of Light in vacuum, but what is the Speed of Light? Easy question, 299,792,458 metres per second, or often rounded off to 300,000 km/s. This is the speed of light in vacuum, denoted by the ‘c’. This is the maximum speed at which an object can move through space.

But what is the slowest speed of light? Well you can argue that ‘It is zero, if you point a laser at a wall, light just… you know hit the wall and stop’. But in this scenario, the information is lost. You cannot start the light back from the wall, hence whatever information that light was carrying, is now lost.

Is it really possible to ‘SLOW DOWN’ light?
Yes! You see it happening all around you, but just don’t notice! Speed of light is different in different materials. At school, we learnt about Refractive Index of a materials. The property of a material that causes light to displace, appearing parallel to the original image, and causes the light to bend. Looking at the below images, in one, the pen is submerged in water, and appears to be bent at the surface. In the second image, you can clearly see that the Laser light is being bent. I am pointing the laser in one direction, but when it passes through the perspex block, the angle changes. This is refraction.


Turns out, the Refractive Index is the ratio of speed of light in vacuum to the speed of light in that medium.

For Glass, it is roughly 1.5, that means, light travels 1.5 times slower in glass, approximating to 200,000 km/s. Diamond has a refractive index of 2.4, so the speed of light through Diamond is only 125,000 km/s. Germanium slows down the light to only 75,000 km/s.

So can you slow down the speed of light even further?
Yes. Using different materials and techniques, it is possible to slow down light even further. In 1998, scientists at Harvard University and Rowland Institute for Science succeeded in slowing down light to only 17 meters per second! This is a mere 61.2 km/h! You can easily drive faster than this! This was done using a special state of matter, called as the Bose-Einstein Condensate (BEC). The same team managed to momentarily stop the light completely, and later restarted it! No information was lost!

There are some consequences  of the limited speed of light and the refractive index. The speed of light in the optic fibres is about 200,000 km/h, so information sent from India to US might get delayed by about 0.2 seconds. It takes time for communication from Earth to reach space stations and space probes. Communication is delayed from India to the Mars orbiter (Mangalyaan) by about 5-20 minutes!

If you know a bit of science and you want to win on the stock market or any live event, choose Microwaves to send information. Microwaves travel faster than light in optic fibres, so you could technically predict what’s going to happen next if you use Microwaves, compared with brokers on Optic Fibres! You can thank me later.

In the below image, you can see how due to internal reflection, light is getting bounced back inside the perspex piece, and can be transmitted through even a bent tube; this is how Optic Fibres work! Through total internal reflection. The image on the left is with lights left on to show the perspex piece and direction of laser being pointed, but you pretty much can’t see anything, and the brilliance in the right image when I turned the lights off!



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