Filters

Materials:
Lights to Fill the Planetarium with Color.
The simplest way to do this, if you have fulldome video, is to use the red, green, and blue in the video projectors. This can be easily programmed in many systems, or create a graphic consisting of a pure color to project through the system. Three-color cove lighting systems can work also.
For conventional planetariums, make a light system of three colored flood lights (red, green and blue). The system can be as simple as clamp-on light fixtures with metal reflectors. To get as pure color light as possible, secure color filter gels over the reflectors with tape. If the filters are good enough, ordinary white flood bulbs may be used in the fixtures. Dichroic bulbs may also be used. We recommend filters gels designed to work with high-powered theater spotlights, as they are fire resistant. These are available from theater supply stores.
    Medium Red (RoscoLux #27)   Kelly Green (#94)   Medium Blue (#88) 

2. See Color Analyzers in Diffraction Gratings.


Presenter's Script:

We will now look at how light goes through certain kinds of windows. I am handing out some devices that we call “color analyzers.” When you get yours look for four windows, each of them labeled with the letters A, B, C, and D.
Hand out the color analyzers, keeping one for yourself. Make sure only the red light is on.
I’d like you to hold your color analyzer up and look at the ceiling through the windows.
Demonstrate with yours.

Which window looks brightest?
[D.] Which is second brightest? [A.]
Notice windows B and C look black or very dark. Each window has a different color of plastic in it.
Can you tell what color each window is?
What color is window A? . . . B? . . . C?

Let’s go to a different star, say a green star.
Switch off red, switch on green. If using the movie, keep a still image of the color analyzer in red light on
Now, which window looks brightest? [D.] And the second brightest? [Window B, but window C is also bright.]
Also, notice that window A looks dark.
Do you want to revise your guess as to what colors the windows are?
Let’s go to a blue star.
Switch off green, turn blue on. If using movie, keep still image of color analyzer in green in separate place on dome.
Again, which window looks brightest? [D.] And the second brightest? [Window C, and now window B is bright but dimmer.]
Notice again that window A looks dark.
Now, do you think you can guess what colors windows A, B, C and D are?
Turn off blue; turn on red. If using movie, refer to red and green still images on the dome. You can use the blue still image, but the movie goes directly to the view with white light after blue, so make sure to pause the movie for discussion about what the filter colors are.
What color do you think window A is? [Red.] And window B? [Green.]
Turn off red; turn on green.
And window C? [Blue.]
Turn off green; turn on blue.
And window D? [Clear, transparent.]
Turn on red, green and blue.
The plastic in the windows we have been looking through are called filters. Filters can also be made of other materials such as glass.
A red filter lets red light through and absorbs light of other colors. A blue filter lets blue light through and absorbs red and all other colors. A green filter lets through only green light, etc. In summary, a filter lets through light of one color (its own) but absorbs the rest.
The rules for filters are very similar to the rules for the reflected light from other objects; filters let light of one color pass through to our eyes while colored objects reflect light of their color to our eyes.


Optional — A deeper analysis of filters for older groups
Rarely do we ever see absolutely pure colors of light. There is no such thing as a “perfect” filter; a perfect filter lets through only one color of light. Our green filter lets through a little bit of blue light, in addition to the green light. Let’s see which of the filters in our paddles is nearest to “perfect.”
Switch on blue light only.
Do the red or green filters let through any of the blue light? [The green filter does. The red does not.]
Turn off blue; turn on green.
Do the red or the blue filters let through any of the green light? [The blue filter does. The red is still black.]
Turn off green; turn on red.
Do the green or the blue filters let through any of the red light? [Not much.]
Which is the best filter?
[Red—lets through only red light.]
Which filter is worst?
[Blue—lets through a lot of green.]

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