I used a powerful (2W) white LED in the light box. This is not a truly multicoloured white light source like the rays from the Sun for example. As a result the spectrum is not quite so perfect as you would see produced by small rain drops in the wonderful display of a rainbow.
The prism orintation makes a lot of difference to the quality of the spectrum. The angle between the light rays coming in and the spectrum coming out of the prism can be seen on the photo above and in practice adjust this for the best spectrum.
Leaving the apparatus setup as in Experiment 1 move the screen slightly further away to say 25 cm or so. Now midway between the prism and the screen insert the lens. The lens will recombine the spectrum projecting a white light band / slit onto the screen. These experiments therefore show that white light can be split into the spectrum but also that the spectrum can be reassembled into white light again !
During Newtons time there was some sceptism about what was precisely going on with the white light and the prism glass. Some thought that the spectrum was directly produced by the glass rather than a seperation of colours brought about by the different degrees of angle deflection (refraction). So Newton devised this simple experiment (called 'Newton's Experimentum Crucis') to prove the case.
Set up as in Experiment 2 and introduce a second slit, second prism in place of the lens. By adjusting the second slit one can chose a colour from the spectrum produced by the first prism (lets say the red light) and send this through the second prism. We dont see another spectrum being produced from this red ray, instead we just see the red ray being refracted by the second prism. We can do this for any other colour showing that the spectrum is not produced by some 'colouration' effect by the passage of the rays through the glass prism.
THE CREATIVE SCIENCE CENTRE
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