Flat beam

Cut through water with light!

15 minutes


  • You will use a safe Class 1 laser in the experiment. It is perfectly safe when used as outlined in the instructions.
  • Do not aim laser beam at eyes or face.
  • Never eat or drink any of the substances provided. Do not use for culinary purposes.
  • Read the "Working with Batteries" section of the safety guidelines carefully before proceeding. Always disconnect the setup after finishing the experiment.

Step-by-step instructions

Be very careful in a dark room—prepare everything you’ll need in advance; clear the path from the light switch to the table or ask someone to turn the lights off.

To observe how a light beam moves through the air, you’ll need something more sophisticated than a regular light source. A laser will provide a narrow beam of light.


Connect the batteries to power the laser.


You won't see the beam moving through the air, but you can see the light spot where the beam stops. Remember the shape the light spot makes when there is no obstacle in front of the laser.


The beam can pass not only through the air, but also through transparent objects. The light spot on the wall will show how passing through the cylinder changes the path of the beam.


Let’s use water to try to influence the path of the laser beam.


Observe how this setup changes the path of the beam and affects the light spot on the wall.


To make the path of the ray visible, you need to put many small, reflective obstacles in its path—such as a glitter suspension.


A suspension of particles will show you which path the beam is traveling. Along the thin line where the beam is shining, you can see how the particles are moving in the solution. This is called the laser sheet method.



  • Dispose of solid waste together with household garbage.
  • Pour liquids down the sink. Wash with an excess of water.
  • Dispose of used batteries in accordance with local regulations.

Scientific description

Fast fact! Here, you used a laser—a device that shines a narrow beam of light rays  in one direction.Turns out that transparent objects not only transmit light, but also change its direction. Light rays bend when they pass from one medium (air) to another (glass, water). This phenomenon is called refraction.

Due to refraction, the light spot  turns into a line when you place the transparent cylinder  in the path of the beam . Let’s talk about how this happens! Before encountering the cylinder , all the rays  are going in one direction. Then they bend twice: when they enter the cylinder, and when they exit it. In total, the rays are refracted in a way that makes them gather together behind the cylinder and then diverge to the sides , spreading like the feathers on a hand fan.

To visualize the light’s path, one can use a suspension containing tiny reflective particles . As they pass through it, the rays are reflected by the particles that they meet along the way. The reflected rays reach our eyes, which is how we see the path of the beam. From the side, it seems that the laser is cutting the glass of liquid like a knife.