Rainbows keep falling on my head

Yesterday Vera guided the 4-6 graders in exploring how different
colors of light mix.  As you may recall, in her previous outing she
showed how light from a white bulb can be split into various colors
like a rainbow.  This shows that we perceive as white is actually a
mixture of all colors, so a natural question to explore next is: What
happens if we mix different combinations of colors, like red+green,
green+blue, etc?  This is a lot like mixing colors of paint, but not
exactly because many colors of paint mixed together approaches the
appearance of black, but many colors of light mixed together
approaches the appearance of white. 

She directed the students to play with this computer simulation which you are free to do at home with your child.  After about 5 minutes of just playing
with it, the students were guided through a set of thought-provoking
questions which made them go back and test the hypotheses they had
formed regarding light.

The concepts are similar to many I wrote about in my last visit to Primaria, so I won't belabor them here.  I will remark instead on the
importance of testing students' mental models.  It's one thing to say
that "white light contains all colors" but it's quite another to probe
what students really think that means in practice.  If you put white
light into red glass, did the glass turn all the other colors into
red, or did it just block all the non-red colors from passing through?
The answer is the latter, but the most interesting thing is the
thought process which leads to that.  How would we devise an
experiment to distinguish between these two hypotheses?  By passing
the resulting red light through blue glass: if the conversion
hypothesis is true, a lot of blue light will come out, but if the
blocking hypothesis is true, basically no light will come out (because
the blue glass will block the red light).  It's not enough just to
tell kids that "white light contains all colors."  If you tell that to
7 kids, they will have 7 slightly different ideas about what that means
in practice. So a teacher has to find ways for students to test and
modify their mental models.  Not only does this lead to a better
mental model, but it make students practice that most important skill:
using evidence to improve their model of how the world works.