Making rainbows

Rainbows feature in a lot of books and toys for small children. They see them so many times, that when my son saw a picture of a rainbow with the colours in the wrong places the other day, he told me it was wrong. He's only ever seen the real thing twice though, and I'm not convinced he was looking the right way one of those times. Given this, and his love of colourful things, I bought a cheap prism so we could make some rainbows (well, spectra, but I'm calling them rainbows) at home and start talking a bit more about light. 

I'm not planning to try and explain why the prism works as it does, but in case you can't quite remember the physics yourself, the reason the white light splits into different colours because the light is refracted (bent) by the prism.  The white light contains the whole spectrum of colours but when seen together they look white.  Red light has a longer wavelength and is refracted less than the shorter wavelength colours (purple having the shortest wavelength of the visible light colours); because the colours are refracted by different amounts, the angle that they come out of the prism is different and they spread out.

I tried this out first on the smallest member of the family (6 months old) as he doesn't mind me faffing with things as long as he's got something to chew and I'm singing nursery rhymes. It was a dull day but I managed to get a nice spectrum (rainbow) on the wall using our LED ceiling lights. I even got applause and smiles. To be fair, it was only day 3 of him being able to clap and the bar for applause isn't very high at the moment so he might not have even noticed the rainbow...

White light from the sun split into the spectral colours on our wall

 
Our pine cone on the back doorstep was opening last night (showing the humidity is dropping) which suggested better weather today. The pine cone was right, there was sunlight streaming in through the windows this morning. I promised rainbows after breakfast and the boy was excited which was a good sign. We ventured to the living room and got some lovely spectra of rainbow colours on the living room wall. 

Catching rainbows

He enjoyed 'catching' the rainbow on his arms and a piece of paper and we did it several times. I talked about how the prism was splitting the white light from the sun into different colours (which I don't think went in) and he pointed out the red, orange, yellow, green, blue and purple.  I also tried putting a piece of red plastic between the sunlight and the prism and it cut out the colours aside from red.  He was briefly interested, but not for long - I think we could revisit this sometime in the future.

We also had a look at the shape of the prism.  It's glass, so I didn't want him running around the house with it, so I built the shape from Magformers so he could handle it more.  He wasn't keen and wanted to build a lorry instead, but when I built him a really big one and it was a 'roadblock' for his lorry he surprised me by telling me it was a triangular prism shape, so clearly he'd been listening to more than I thought!  I like our Magformers, they're really quick to put together and nice for talking about 2D and 3D shapes whilst building the sorts of things he wants to play with e.g space rockets, garages, panda houses, rabbit cars...

Triangular prisms - the real thing and two sizes made from Magformers