Color Mixing Shadows

In our Light and Shadows class, we put up a shadow screen and use a shop light and let the kids make hand shadows, and shadow puppets, and do shadow dances. In the next class, we study Rainbows. We’ve tried color-mixing with light in two ways: putting colored plastics on an overhead projector (where you overlap a yellow and a red to make an orange shape in the projection), which works, and shining lights through colored water, which we haven’t yet gotten to work well.

This year, I wanted to try a different approach to color mixing light – I thought about using colored flashlights (or regular flashlights covered with colored cellophane.)

But then I was excited to find Exploratorium’s Science Snack called Colored Shadows. In their example, they used a power strip, three outlet to light socket adapters, and colored compact fluorescent light bulbs in blue, red, and green. A fun easy experiment, and reasonably cheap. I purchased the supplies, though I got these dimmable LED bulbs. (Please note that when you’re using paint or other substances, you mix red yellow and blue to make all the colors. With light, you use red, blue, and green. These are the primary colors of light.) Total cost: around $40.

My son and I went into a dark closet (it was a sunny summer day) to test it out. We explored colored shadows. If you hold the marker near the lights and far from the wall, you’ll see three fuzzy colored shadows.

As you move the marker away from the light and toward the wall, the shadows begin to merge, and become sharper.

When the marker is close to the wall, if you peer behind it, you’ll see where the colors have mixed and formed a rainbow like effect. The photo shows 4 colors – we were able to get more colors in our rainbow, I just didn’t capture that.

We also played with the shadows made by this rainbow fan.

And the way that colors shone on the white side of the fan.

After playing with all the colors, we tested what happened when you took away one of the colors (by unscrewing the light bulb). If we took away the green, we’d have a purplish glow from the blue and red, etc.

Then we noticed our Lego bag. We have a bright yellow plastic shopping bag from the Lego store. But when the green light is off, it doesn’t look yellow anymore! If only the blue light is on, it looks blue. If only the red light is on, it is a neutral dark color.

Then I found a bag with a Microsoft logo on it. Since it’s four colors (plus the white word) it seemed like it would be a fun thing to experiment with, and it was!

The Science Behind It:

Objects appear to be different colors based on what wavelengths of light they absorb, and what wavelengths they reflect. The colors we see are the ones that are reflected. So the top left square of the logo looks red when all three colors of light are available. It is absorbing the blues and greens and reflecting back red. As long as the red light is on, it appears to be some shade of red. But notice how when the red is turned off completely, but the blue and green are on, it looks almost black- it’s reflecting very little of the blue and green.

Here’s a video for older kids that explains the science of light: For young children, check out the “Let There Be Light” episode of Sid the Science Kid. For adults, there’s more info at Science Learning Hub and Sylvania.

Reflections on this Activity Design

So, this was easy to assemble, and reasonably affordable. My 7 year old enjoyed it, and learned from it. I showed him how to carefully unscrew a light bulb partway to turn it off, and carefully screw it back in to turn it back on. I felt like the design was safe for him to explore with minimal supervision from me. But now the next question is: do I think it’s safe for a classroom full of 3 – 7 year olds?

The good news is that these dimmable LEDs simply don’t get hot enough to burn you. After they’ve been on for an hour, I can wrap my hand tightly around one and it’s pleasantly warm. But they are glass, so there’s a risk of one getting broken by enthusiastic kids. And, there were exposed outlets on the power strip, so outlet covers on those might be a good idea.

I think this experiment will work in our class if I assign a parent volunteer to watch closely, and I may set the limit that only the grown-up can touch it, thus only a grown-up can unscrew or screw in the bulbs.

Another note is that these LEDs are REALLY bright and it is uncomfortable to look directly at them, and they leave an impression on your eyes after you look away. So, I’m considering placing the lights behind a sheer curtain, or behind a shoji screen, or perhaps creating a box like the shadow puppet theater to contain the lights.

Further Evolution of this Activity Design

I found socket adapters with On/Off switch. They’re $8.99 vs. the 3.99 for the adapters without switches. This would allow kids to turn off a light without having to unscrew a bulb, which is a plus. But I wonder if poking at a light switch would increase the chance of a broken bulb. Also, the reviews say the bendable part isn’t very rigid, and so I’m not sure what direction it would flop in.

Then I discovered there were remote control switches. Three sockets and a remote for $19.99 would allow me to flip on and off the three bulbs remotely. For my class of 3 – 7 year olds, having them push buttons instead of poking at switches near fragile lightbulbs and a power strip seemed safer. And… maybe I could find a way to set it up in my toddler class where my toddlers could push buttons, but not reach anywhere near the power strip and bulbs. (Perhaps inside of a plastic tub.)

Then, I thought… it would be even cooler if the lights were dimmable, so I could have a little blue, and a lot of red, etc. So, I purchased these wireless dimmer sockets with remote. They work great with an incandescent bulb, and with the LED light bulbs I use in my kitchen. However, they don’t work at all with the dimmable LED bulbs I have. (It says on the bulbs box that they’re not compatible with all dimmer sockets.) So, bought one of these LED Bulbs and it works great with the sockets. So, if you want the remote control dimmable option for this activity, here are the item you would need:

Total cost as of July 2018: $79

Other options for exploring color mixing light bulbs.

Note: there are, of course, color-changing LED bulbs where one light bulb can produce a broad array of colors. That would be fun for all sorts of exploration for kids. Or you could use a smartphone or other device that can project a variety of colors. But for this particular experiment, I wanted to have three separate colors of bulbs that the kids would mix. I think the physicality of the three light bulbs turning on or off really helps them to understand how each color influences the result.

For older children, search for “color mixing arduino” and you’ll see that one of the recommended early arduino programs for kids is a color mixing experiment with LED’s.

More Activity Designs

Do you like to learn not just about new activity ideas, but also about the thought process that goes into designing new activities? If so, click on the category “design process” in the right hand sidebar (or at the bottom of the page on mobile devices) to see several similar posts.


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