Light, in slow motion!

Photography and videography are possible because everyday objects move slowly enough and reflect enough light so it can be captured by a camera. Just enough light is let in using a mechanical or electronic shutter to capture either a snapshot (in the case of still photography) or several frames (in the case of movies). So, everyday photography is all about capturing the light reflected off a surface onto a medium.

Imagine what it is to film a ray of light itself.

A photon, the particle that constitutes light, measures about 1.6 femtometers or 1.6 x 10^-15 m across. This exceedingly small particle also travels exceedingly fast - at the rate of 3 x 10⁸ m/s. Filming or photographing the movement of a ray of light would seem to be a fool's errand - and playing it back in slow motion even more so. However, a team from MIT have used a combination of innovative time slicing techniques and a monstrously fast camera to do just that. You can now watch a ray of light in slow motion, see it produce waves, and confirm its particle nature by looking around corners and inside bodies. Femto Photography is sure to bring about a revolution in imaging, please watch:

Light: Bend it like Beckham!

Celeritas. The Latin word for speed shortened to c is used to represent the speed of light. c is a part of the most famous equation in science E = mc². The speed of light being a universal constant 3 x 10⁸ m/s is well known, and forms the crucial base on which Einstein's theory of relativity rests.

What is often left out are the words "...in space". Light has the speed of 3 x 10⁸ m/s in space, and like this post mentions, it has different speeds in different mediums. 

Eyes found in earthly creatures would not work if this were not true, and by extension, neither would human eyes; and neither would lens-based inventions such as microscopes and telescopes.

Inexplicable though it may be, light slows down when it enters a denser medium. When light travels from air into a lens, it slows down. Light then shows another interesting behavior - in what is known as Fermat's principle of least time, it alters its path to take a route that would carry it through the lens in the least possible time; this causes rays of light to bend as soon as they enter the denser medium. The rays bend towards the nearest exit to shorten their transit time, and bend again when they exit from the lens back into air.

This diagram shows a red ray of light going from a rarer medium (P) to a denser medium (Q), whereupon its velocity reduces making v₂ < v₁. n₁ and n₂ are called the refractive indices of P and Q respectively. Snell's law wraps this all up in a single line:
Sinθ₁/Sinθ₂ = v₁/v₂ = n₂/n₁



The shape of a lens is designed to use the natural bending of light to produce effects like magnification and inversion of images that form the basis for telescopes and microscopes.

Light: A riddle wrapped in a mystery, inside an enigma

The strangest thing in the universe may also be the most commonplace.

Light.

A gift from our neighborhood star, a tonic that nourishes life on Earth, and the stuff of every rainbow. It is also a thorn in the side of Science - a constant reminder of how less we know. Science hangs its failures on this honest sounding word -  "paradox". The nature of light has intrigued thinkers for millennia. Scientists who have tried to win it over have succeeded only moderately - still, the romance shows no sign of waning.

There are times when light appears to be made of particles. Newton noticed that light travelled in straight lines, as if constituted from discrete particles - he called these particles "corpuscles". The famous photoelectric effect is a natural phenomenon in which light apparently knocks electrons off their orbits to create electricity - again, an indication that it is made of particles. The photoelectric effect was explained by Einstein in 1921, years after Newton, in a Nobel prize winning effort that proposed that light travels in quanta called photons - essentially, particles. There is more support for the particle nature of light - light travels fastest in space, where there is no intervening matter. As soon as it enters the atmosphere of the Earth, it slows down a little. When it enters water it slows down even further. 

But although light slows down when it enters a medium of higher density, it does that only once, at the point of entry. It does not continue to slow down, as you would expect something made of particles that encounter more particles, to do. Paradox number 1. 

When light goes from water back to the air, or from air back into space - it instantly goes back to its original speed, then retains that speed forever until it enters a different medium. This sort of behaviour is a strike both for and against the particle theory. Paradox number 2. 

Thomas Young discovered that passing light through microsopic slits produced interference patterns, much like waves in a pond. This meant that light also behaved like a wave. The "dual nature of light" is that light is a wave and a particle - not by turns, but simultaneously! Paradox number 3. 

If that was not bad enough, it was shown that even a single particle of light passing through a double slit can "interfere with itself" and produce an interference pattern. That is, even though it is only one particle it can pretend to be a wave when confronted with a double slit, and produce interference. Paradox number 4.

There is currently a lot of fanfare about the Higgs Boson, aka the "God Particle". This discovery does not yet involve any talk about light, since photons are massless and the Higgs Boson confers mass - but I would not hold my breath.  Given the history light has had with human beings, I will just say - "watch this space", to see how that story develops. We'll keep you posted.