Difficulty:   difficulty-1difficulty-1difficulty-1 difficulty-2difficulty-2

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Kit contents

2 acrylic lenses (1 convex, 1 concave)

1 black acrylic piece with slits on it

1 white LED (light emitting diode)

1 button battery

*To find out where to get these supplies, click here

Set up your optical system
  1. Peel all adhesive films from your acrylic lenses and slits.
  2. Find a dark room and a flat surface, like a table. You also need some paper, a pencil and a protractor. Blue-tack will help you keep your slits standing up perpendicular to the table.
  3. Connect your LED to the button battery (straddle the battery terminals with the ‘legs’ of the LED; tape to hold, if needed. If you don’t see light, flip the battery over and try again.)ray LED

Experiment 1:  What do the lenses do?

Put your black acrylic piece on the table so the three slits are vertical relative to the table (blue-tack helps it stand up). Shine your LED light through the slits. You should see three collimated (straight, parallel) lines of light. Play with the height of the LED and the distance from the slits, to get the lines as bright and nearly parallel as you can. Now, put the convex lens in your 3 beams. What happens? Move it closer, spin the lens around.

Switch to the concave lens. What do you see now? Move it closer, spin the lens ray croppedaround. What do you see? Can you re-collimate the light using both lenses?

What is happening? When light enters a material, it interacts with the atoms (mostly the electrons) in that material, and that can make it change direction. The amount and direction of bending depends on the material’s refractive index. The acrylic of your lenses has a different index of refraction than air does, so your lenses bend light. In the next experiment, you are going to measure the refractive index of your acrylic lenses. This tells us the speed of light in your lens!

Keen to know more:  Try looking up refractive index and lenses. The refractive index is actually a measure of the speed of light in a material. How does that work? The speed of light is not really constant?! And do your own experiments!! Never stop asking and answering questions!!

Experiment 2:  Measure the refractive index of your lenses – and speed of light in acrylic!

This photonics experiment is done by scientists with fancy equipment. You can do a pretty good job with your acrylic slits and lenses!

Flip the black acrylic over, so that your light shines through only one slit. Put a piece of paper under it, so that you can trace the light rays with a pencil.

  1. Put your convex lens on the paper, so that your ray goes right through the middle of the flat side of the lens. Trace the outline of the lens on the paper, and the rays going in and out (hint: just use dots and connect them later with a ruler).
  2. Rotate your lens a bit, and do it again (the light into our out of the lens should still go through the middle of the flat side)! Use coloured pencils and labels so you don’t mix them up. Do this several times. Repeat your measurements, and take care to be accurate.

Ok, now you should have a whole lot of labelled drawings that look something like this.

       ray drawing          ray definitions

 It’s time to do your measurements and analysis. Use your protractor to measure the incidence angle and refraction angle for each pair of lines you’ve drawn. These are defined in the drawing above. Pay attention to the normal line. Enter your data in the first two columns below:

Incidence Angle (degrees) Refraction Angle (degrees) sin(incidence angle) sin(refraction angle) Refractive Index of Acrylic


Now, calculate the sine of the incidence and refraction angles and put them in columns 3 and 4. Finally, divide the sin(incidence angle) / sin(refraction angle) and put that in the last column. Snell’s Law tells us that this gives the refraction angle, if we are doing our measurement in air.

How did you do? If the measurements were perfect, your refractive index values would be identical. Are they? Why not? Google the refractive index of acrylic online – did you get this value?

The index of refraction tells us that light travels more slowly in a material (like acrylic) than it does in a vacuum. It is a ratio of these speeds. What is the speed of light in acrylic, then?

You can make a decent lens out of jelly and a circular cookie cutter. You can also make a lens out of a round water glass. Can you measure water’s refractive index? jelly? sugar water? salt water? What hypotheses can you come up with for the differences in refractive index of these materials? Why would light travel at different speeds?

Keen to know more:  Try looking up Try looking up refraction and Snell’s Law. There are links to some good sources below.  And do your own experiments!! Never stop asking and answering questions!!

Some recommended resources

HyperPhysics has good information lots of stuff.  You can find out about refraction, the index of refraction and Snell’s law here.

There is some very interesting stuff on Synopsis’ website about lenses here.

A good, though somewhat higher level (more maths) site is Newport’s here.

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