Light is a particular type of electromagnetic radiation or EMR. Electromagnetic radiation is composed of an electric field and a magnetic field. These fields are perpendicular to each other and the direction of propagation for light – so as light travels, it moves through time as well as space! In this way, light has a “warp factor” – tachyons are tardy particles that don’t experience this temporal component of light.

The Speed of Light: 186,000 miles per second (299,792.458 km/sec) – in vacuum (empty space). It’s an amazing property that light has. When traveling through material substances, its speed is reduced the denser the material.

For example, in water, light travels at about 1/3 of its vacuum value. So if you swim towards a flashlight holding it above your head in an average depth pool, you can have quite a time seeing it before it hits your eyes!

In diamond, however, which has one of the highest refractive indices, the speed of light is reduced to about 1/7th its vacuum value. It’s possible to see diamonds ring in the sun since the almost 180 degree angle between sunlight and our line of sight means less than half of all photons are absorbed!

The light was first measured using a water-filled telescope by Ole Rømer in 1676. The first to measure it without using any material medium was Hippolyte Fizeau in 1849. He used a rotating cogwheel with 720 teeth per rotation. He took 5 minutes to see the light flash from one tooth to an adjacent one!

In a vacuum, the speed of light is always constant. But in materials, the speed decreases with increasing refractive index, which also varies slightly with frequency. However, since this variation is only 5 parts out of 108, one can treat ‘c’ as constant even inside material substances for all practical purposes!

Bonus Trivia: The mass-energy equivalence equation, first postulated by Albert Einstein, is: E=mc^2

Where ‘m’ stands for mass and ‘c’ stands for the speed of light in a vacuum. The equation says that energy (E) and mass (m) are different forms of the same thing. As an example, whenever particles such as electrons or protons move with a high velocity, they “appear” to have a mass greater than their rest masses.

In fact, according to the Special Theory of Relativity, even photons have momentum and inertial mass! Another interesting consequence of this is that an object’s energy content can be increased by speeding it up. As Einstein himself said: “Put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute. THAT’S relativity!”