What Color Are Your Blue Jeans?

Visit to a planet orbiting a red star

Fill the dome with red light to flood the planetarium, all other lights out.

Alternatively, if your dome has its own red-green-blue lighting system (e.g., cove lighting), you may use that instead of the red light effect.

[We are taking a liberty here: a real red or blue star would be emitting many more colors than just the red or blue, so objects on planets orbiting those stars may not look much different than they do on Earth. In these activities, we exaggerate the “purity” of light to illustrate points about the physics of light, color, reflection, and absorption.]

Look at your clothes. Notice what the colors look like now.
If you are wearing red notice if it looks bright or dark. 

Who is wearing something red? 

[Ask a volunteer wearing something red to stand so everyone can see the red clothing.]

Who is wearing something blue? 

[Ask a volunteer wearing something blue to stand so everyone can see the blue clothing.]

I’ll ask my volunteers to stand up to display their red and blue clothes so we can compare them.

Which one looks brighter?

[The red.]

What colors look bright under a red light?

[Red, white, and other light colors like yellow and pink.]

Now, let’s transport ourselves to a place near a blue star.

Are blue stars hotter or cooler than red stars?


Please fasten your seatbelt.
We will be using “warp drive with warp factor 100” leaving the red star
to go to a planet orbiting around a blue star!

Visit to a planet orbiting a blue star

Red light off; Fill entire dome with blue light.

Here we are on a planet revolving around a blue star.

What’s different now? Notice the colors around you again.

Which colors look brighter?

[Blue and white.]

I’ll ask my volunteers to stand again so we can compare their clothes under this new light.

Which one looks brighter?

[The blue.]

 It seems that blues and whites and perhaps greens look bright under a blue light; all other colors look dark. 

Let’s go back and forth between the red and the blue stars and watch what happens to our volunteers’ clothes. 

For younger groups, start out with the phrase “bounces off” for “reflect” and “is trapped” or “soaks in” for “absorbed.”
DIGITAL EFFECT: Alternate Red/Blue Light
Alternate red and blue lights a few times and finish, leaving red lights on.

Ask volunteers to be seated.

To understand the effects we just saw, we need to trace the path of light.
All the light in this room is coming from this red light in the center of the planetarium
[or lights at the edges of the dome, depending on your system].

Where does the light go next?

[Everywhere; upwards to the top of the dome.]

When the light hits the top of the dome it bounces off in all directions. We call this bouncing-off reflecting. So, the light from the red bulb bounces off the dome and reflects all over, hitting other things and reflecting off of them. But when light hits an object, it doesn’t always bounce off or reflect—sometimes it is absorbed. Just like a sponge can absorb water, things can absorb light. When an object reflects light, some of that reflected light comes into our eye and we see the object bright. When an object absorbs light, the light doesn’t reach our eye and it looks dark. Look at the red clothing in this light.

Does red light reflect from it into our eyes, or does the red clothing absorb the red light?


Look at blue clothing.

Does the red light reflect from it into our eye or does the blue clothing absorb the red light?


Now, let’s do the same experiment in blue light.

Turn off red; turn on blue.

Does the blue light reflect off of red objects or is it absorbed?


And does the blue light reflect off of blue objects or is it absorbed?


This is why things appear to be colored; blue objects reflect mostly blue light and absorb most other colors, while red objects reflect mostly red light absorbing most other colors. Objects with colors like white and yellow reflect most colors of light.

Optional: Switch to green light and note how red objects look dark because they absorb green light. Blue objects will not look as dark because the green light has some blue light in it too.
Turn on red light. Any other colors off.