Question: How can you use light beams to physically pick up objects?

Susan Skelton answered on 19 Nov 2013:

Really great question, delaram and noorpreet!

It seems really counterintuitive, but light can move objects because it exerts a force on them. We can imagine a light beam as being made up of a stream of little particles, called ‘photons’. When these photons hit an object, they push against it.

Think about what happens when the white ball knocks against a coloured ball in a game of snooker. The white ball bounces off the coloured ball and some of the white ball’s momentum passes to the coloured ball. The coloured ball then starts moving and hopefully goes in the pocket!

We can do exactly the same thing with light. When a photon bounces off an object, it passes some momentum to the object. Now photons are really small so hardly have any momentum at all. But if we have loads and loads of photons (which we achieve by using a really strong light beam like a laser), then the light has enough momentum to push a small object along.

The good thing about light is that we can concentrate it down to a really small spot. When we do this, the light is not parallel anymore; it is focuses at a very large angle. This means that the light hits our object from all directions which holds it in place. We can move the object around by moving the position of our light beam!

This technique is called “optical tweezers” because it allows us to pick up small things, like tweezers do! It is very useful for biology and medicine research because we can pick up things like cells and viruses without touching them, so there is no risk of contamination.

I am looking at developing this technique to hold and study smaller objects like single molecules, so that we can learn more about how to design new technology on the nanoscale.