Fish Tank Optics:
Learning How Light Travels

This lesson developed by Reach Out!

Recommended Ages: Early Elementary, Later Elementary, Middle School

Guiding Question

How does light travel? What happens when light moves through or hits different materials?



Light moves through or bounces off different materials in different ways





  1. Different sizes of flashlights
  2. Fish tank filled halfway with water
  3. White and dark paper
  4. Large can or other nonfloating object

Room Preparation

Put fish tank on a table so students can stand around it, see, and do experiments

Safety Precautions

No safety precautions necessary

Procedures and Activity


How does light travel? What happens when light hits or moves through different objects? Today, we will learn about light waves and see how they travel. Light moves in waves, which can bounce off of or go through materials.


  1. Have students shine flashlight beam through their hands. We can see that flesh and bone won't allow light to pass through. Hand turns pink—evidence that light is bouncing off. This bouncing of light off a surface is called reflection.

  2. Shine flashlight beam through fish tank of water. Hold dark paper at outside end of tank to see evidence that light is coming through the tank. Look down into the water and see reflection in it. Experiment with different sizes of beams and flashlights and document what you see.

  3. Put can or other object in the middle of the tank. Shine light through tank and observe what happens when beam passes through water and hits object. Do light waves pass through the object or bounce (reflect) off of it?

  4. Next, place dark sheets of paper along the sides and end of the tank. Focus a beam on the far end of the tank and observe how light shining in at one end hits mostly, but not entirely, on the other end. Refraction causes some light waves to bend and pass through the side walls.

  5. Shine light through air in tank. Observe that light has no reflection or refraction because the medium is just “air,” so there is no material to reflect or refract the beams.

Closing - Original Question

Ask again:

What happens when light moves through or hits different materials?

Share what we have learned and observed.


Have students demonstrate, draw, or tell how light waves travel through air, water, paper, glass, hands, etc. Listen for evidence that they understand reflection and refraction.

Extension Ideas

  1. Have students try shining flashlight beams through other media—glass, plastics, wood, cardboard, cloth—of different colors and thicknesses. Have them record information about their observations. Do they see light absorbtion or scattering?

  2. Shining light through a short length of fiber optic cable is great fun! Give students a piece of fiber optic cable and a flashlight and let them see how light can travel and be bent in any direction. Share how this is useful in fiber optic applications such as communication (long-distance signals that do not degrade) and medicine (minimally invasive exploration of our internal organs).

  3. Try filling the fish tank (or a smaller glass container) with different materials, such as vegetable oil, jello, milk, sand. Experiment and record results.

  4. Try coloring the water in the fish tank with food coloring. Does light travel differently through different colors of water? What if you use different colors of light, too?

Careers Related to Lesson Topic

Prerequisite Vocabulary

Colored surfaces absorb some of the light that hits, scattering other parts of the spectrum

Behaves like waves with direction, amplitude, reflection, color, and interference

image of light reflecting
A material or substance; in this experiment, see which media light passes through and which it bounces off of

The bouncing of light waves off of a surface

image of light refracting

The bending of light when it passes through a substance; its deflection from a straight path

Rough surfaces have a complex surface that reflects light in all directions

Light travels in waves, much as the water in the ocean moves in waves

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