Beginner Optics Project Rainbow

Pint Glass Beam Bending

Have you ever wondered how the light from a Laser pointer gets from your finger to a dot on the wall?
The light is invisible right? Just because you can’t see it, doesn’t mean it isn’t there…
This activity will give you the opportunity to make ‘invisible’ laser light ‘visible’, and also show you how to bend the Laser beam using only simple household items; a curved pint glass (or any other curved glass), and some washing up liquid!

Check out the video below :

The Activity

What you will need:

  • A Laser Pointer (a cat Laser toy, for example);
  • A Pint Glass (ideally curved – this is so that you can bend the beam on the curve of the glass);
  • Washing up liquid;
  • A Stirrer (a spoon or a straw).

What you need to do:

  1. Fill your pint glass to almost the top with water;
  2. Add a teaspoon full of washing up liquid to the water, and give it a stir – this is to make the water ‘cloudy’, but take care, you don’t want any bubbles!
  3. So that you can see the beam when you shine it into the water, you’re going to need a really dark room, so turn off the lights, and close your blinds;
  4. Shine the Laser beam straight down into the water from the top of the glass. If you can’t see the beam, and a little bit more washing up liquid;
  5. Now, shine your laser towards the curve of the glass (similar to the figure) – can you see the Laser beam bend?
  6. (Optional) – move the Laser pointer around to see if you can change the angle of the bend.

Fun Fact…

Did you know that LASER is an acronym for:


Amplification by


Emission of


Share your attempts with me directly, or via the School’s pages:

By Daniel Roberts

I am a Coleg Cymraeg Cenedlaethol Lecturer in Electronic Engineering at the School. I completed my PhD in Laser Physics at the School in 2016 researching Semiconductor Devices for Generating Terahertz (THz) Radiation.

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