Lasers are brilliant, and easily one of the best things in all of science! They’re super powerful, amazingly versatile and have the ability to travel across huge distances. Not to mention they look pretty cool! It might surprise you to learn that, for all the power a laser has, it’s actually very easy to trap a laser beam in water. And what’s more, you can do this at home. Easy peasy!

Caught in a trap

To trap your laser, you will need a laser pointer and a large plastic bottle full of water. Take the bottle and poke a hole in the side, letting a stream of water arc out of the hole. Aim the laser pointer through the bottle and watch as it gets trapped within the waterfall you’ve created, arching downwards with the water’s flow!


Upon reflection

When the beam of light from the laser hits the edge of the waterspout at a certain angle, it’s forced to reflect rather than passing right through it. This reflection happens over and over again as the light repeatedly hits the edge, causing the laser to remain trapped in the waterfall. This process is called total internal reflection.


Playing the angles

In order for total internal reflection to occur, the light must be inside a denser medium as it moves towards the boundary of a less dense medium – in this case water and air. The light must also hit the boundary at an angle greater than the ‘critical angle’ – for water and air this angle is 48.6°. Once the angle increases above this the light is reflected back.


Fibre optics

The cables that give you your speedy internet also use total internal reflection. Optical-fibre cables are made up of long fibres of glass or plastic that trap the beams of light inside.

Think tank

You can also witness total internal reflection in a glass tank full of water! Send the beam at just the right angle and it will bounce off the water’s surface and back into the tank before bouncing off the bottom and out again!