The Science Fair

Today, Left Brain gets to introduce Kid #3's Science Fair project:  Rainbows and the Yellow Filter.


Left Brain:  Kid #3 (daughter) is in Kindergarten, and she loves rainbows.  She learned a song about them that says all the different colors of the spectrum.  For the Science Fair this year, we decided to explore rainbows a bit using a prism.  What resulted was something we both found to be a fun Science Fair project, as well as something interesting for photography.

Left Brain:  First, the setup.  We sat next to a large west-facing window one afternoon, and let the sunlight shine through the prism.  For those who haven't tried fiddling around with a prism, it is highly recommended, and it isn't as easy as it looks.  You have to hold it at just the right angle.  However, when you do hold it properly, you see a rainbow on the wall.  The photo below is a picture we took of one of the rainbows that we saw.

Spectrum produced from afternoon sunlight

 Left Brain:  You can see a wide variety of colors in the rainbow produced by the prism.  For those who aren't aware, sunlight contains even amounts of all the colors mixed together.  It is referred to as "white" light, because it mixes all the colors of the visible light spectrum together.  A prism bends the light coming into it, having the effect of spreading the incoming light out into its constituent colors.


Left Brain:  Next, I presented to Kid #3 my yellow filter.  A yellow filter is used in Black & White photography to make the sky not appear blank.  It livens up B&W photos, and makes them more interesting.  I suggested to Kid #3 that we see what happens when we have the sunlight go through the yellow filter, and then through the prism.  The photo below shows the setup (Kid #3 holding the yellow filter and prism).

Prism and Yellow Filter

Left Brain:  Because this was a Science Fair project, I asked Kid #3 what she thought would happen when we looked at the resulting rainbow (before we actually saw any rainbows).  She said that she thought the rainbow would be all yellow.  The photo below is what we actually saw.

Spectrum produced by sunlight filtered through yellow filter

 Left Brain:  The spectrum produced by the prism from the filtered light is different from the unfiltered.  First, we notice that it is definitely not all yellow as Kid #3 had supposed.  Second, we notice that there are no blue or purple colors.  I asked Kid #3 what happened to the blue and purple, and she thought it was very weird how they had disappeared.  We repeated the experiment by moving the yellow filter and replacing it, and every time the yellow filter was used, there was no blue or purple.


Left Brain:  Kid #3 came to the conclusion that the yellow filter doesn't turn the light all yellow, but actually removes blues and purples.  It turns out that she is correct.  That's exactly what a yellow filter is supposed to do.  It removes these high frequency colors from the spectrum.  

Left Brain:  So why does the yellow filter do what it says it is supposed to do, darkening skies in a black and white photo?  B&W photos are interesting entities.  In regions of the scene where the photo was taken where there is no light present, the photo looks dark (we're talking a print, not a negative).  In regions where there is a ton of light, we see areas in the photo that look almost white.  By removing blue light from the spectrum, we actually remove the light that composes the sky, since the sky is blue.  That makes that region of the photo appear dark (or at least darker than it otherwise would).


Left Brain:  For those of you who were curious how a yellow filter works, I hope that helps.  For those of you interested in a simple science fair project for your kindergartener, this worked very well.