6A46.10 Rainbow


Dispersion of light, refraction, reflection


Light refracting and reflecting through a water-filled round-bottom flask makes a rainbow, as light through a raindrop does. A faint secondary rainbow may be visible. The flask has a reflective coating to make the primary rainbow more intense.

In this demo, you see the colour spread of a single water "droplet." In a natural rainbow, many water droplets at different heights give you different colours to build the rainbow.

In the picture above, the primary rainbow is on the right, and it should be visible in a dim classroom. The secondary rainbow on the left is significantly less intense. Note the flipped colours of the secondary rainbow.

The primary rainbow above only undergoes one internal reflection.

The secondary rainbow above undergoes two internal reflections. It is dimmer because light can also refract out of the flask with each reflection, at least with rain drops.



  • [1] Reflective round-bottom flask filled with water
  • [1] Lab stand
  • [1] 90-degree clamp
  • [1] 3-finger clamp
  • [1] Slide projector
  • [1] Slide with slit
  • [1] Wooden block
  • [1] Extension cord 
  • [1] Screen

Classroom Assembly

  1. Mount the flask on the lab stand using the clamps.
  2. Place the slide into the slide projector carousel.
  3. Plug in the projector and turn it on to fan mode.
  4. Advance the carousel until the slide is active.
  5. Turn on the projector lamp and align the projector with the flask until it makes a rainbow.
  6. Set up the screen to view either the primary rainbow or the secondary rainbow.
  7. Change the projector to fan mode.

Important Notes

  • Try not to scratch up the reflective surface on the flask.
  • When turning off the projector lamp, leave it in fan mode for a few minutes to let the lamp cool.


  1. Turn off the room lights.
  2. Turn on the projector. Point out the primary rainbow. Point out the secondary rainbow, if visible.
  3. Change the projector to fan mode.


Additional Resources



  • Don't attempt this at home!

Last revised

  • 2019


  • The projector has two brightness settings. They are only marginally different.

Related AV

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If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.