Filtered Rainbows: a Simple Experiment on Light & Colors

The following instructable describes a very minimal setting to play with and understand the colors of the rainbow.

It might be usable for educational purposes or just for pure fun.

A bit of theory: sunlight in the visible spectrum consists of a mixture of light of all colors, from violett to dark red. Water drops can work like a set of prisms and split the white light into its components, which then results in a rainbow. For more information on prisms and rainbows I would suggest to have a look at the corresponding Wikipedia sites.

Color filters work by eliminating parts of the spectrum by a process called absorption. So what you see as yellow may just be white minus blue and violett, red being white minus green and so on. If the remaining spectrum is limited to a narrow range around a certain wavelength, the colors are very clean, as primary colors.

So I used a prism to split sunlight into an indoor "rainbow", or technical terms a spectrum. Then I placed a number of color filters in front of the prism, in a way that only one half has been covered. This allows to have a direct comparison between the color intensities of the spectra with and without filters.

I used a set of Rosco color filters that come with an absorption spectrum graph for each filter, allowing an easy comparison of expected and detected spectra.

I then took photographs with my mobile phone of the settings and resulting spectra. The quality of the images is not as good as I hoped for, but I hope they are good enough to give you a first impression.

Using this simple combination of a prism and color filters, the theoretical stuff becomes, well, obvious to the naked eye. You may find some ideas on how to actually measure colors and spectra within expensive devices in some of my previous instructables. Have a look here or here.

A cheap 30 x 30 x 100 mm glass prism (Amazon.de, about 11 Euro).

A ROSCO filter sample block, I had bought some years ago at Modulor, Berlin.
It is similar to this one.

A part of the styrofoam piece from the package of the prism, as prism holder.

A room with a west, south or east looking window and a white wall on the opposite side.

Sunlight, coming from an angle of about 45° (e.g. at about 9:00 am or 15:00).

Smartphone with camera.

I had placed the prism on the windowsill of a west-looking window at about 9:00, using pieces of the styrofoam piece that came with the prism as a holder. The prism was placed on one of its edges, so the rainbow was projected to a door about 4 meters away. Then I placed the filters before one side of the prism and took photos of the filter setting and the resulting spectra.

For the some examples, please have a look on the attached photos. It turned out that the camera of the phone is not a good tool to document the spectra, especially if they are relatively bright. The best results were seen when I used dark color filters.

As expected the red filter blocked the green and parts of the blue light, a light green filter blocked violet, blue and red, while a raspberry filter (-> "Vanity Fair") blocked everything between blue and yellow, leaving red and violet. Etc, etc, …

While this is not really rocket science, this simple setting allows to show children how color, especially color subtraction, works.