STEM Challenge: Light Reflecting Rainbows

Thursday 2nd August 2018    

Today we used some recycled CDs taken from the pedagogical exchange in Richmond. We thought of concluding our learning about the weather with a lesson on rainbows. This STEM (Science, Technology, Engineering and Maths) activity using a blank CD and paper is an interesting way to create rainbow reflections with CDs and using paper to alter the patterns of the light.

How it works is that, like water drops in falling rain, the CD separates white light into all the colours that make it up. The colours you see reflecting from a CD are interference colours, like the shifting colours you see on a soap bubble or an oil slick. You can think of light as as being made up of waves-like the waves in the ocean. When light waves reflect off the ridges on your CD, they overlap and interfere with each other. Sometimes the waves add together, making certain colours brighter, and sometimes they cancel each other, taking certain colours away. A CD is a mirrored surface with spiral tracks or pits. These tracks are evenly spaced and diffract the sunlight (separating the colours). Because the CD’s surface is mirrored, the light is reflected to your eye.

We found an area outdoors to do the activity. We wanted to access sunlight with our CDs and have an area in the shade to reflect the rainbows. (If you don’t have a blank wall, you can use a piece of white poster board.) We took the CD out of its case and looked at the blank side (the side that doesn't have any printing on it). We saw the bands of shimmering colour. As we tilted the CD back and forth we saw the colours shift and change. Our children helped us with this. We then held the CD in the sunshine. When it became cloudy, we turned out the lights and shone our flashlight at the CD. We could see the reflections of the CD by also holding a piece of white paper so that the light reflecting off the CD shines onto the paper.

“It's a rainbow” Said Ava. “I’m following it.” Said Sophie, as she attempted to trace the colours from the reflections. Next, we tipped the CD to see how that changes the reflections. We also changed the distance from the CD to the paper. We asked, “What happens to the colours? “It is long. Bigger rainbow.” Said Mickey. “Long rainbow” Said Finnley.

To extend on this experience, we may buy a set of "rainbow glasses" at a science shop. Through these glasses, all lights look like rainbows. The glasses are made with diffraction gratings, clear plastic that is etched with many lines. This would be another way to show how light reflections work.