newton's experiments on light

8/13/2019 Newton's Experiments on Light

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Newton's Prism Experiments

In 1665, Isaac Newton was a young scientist studying at Cambridge University in England. Hewas very interested in learning all about light and colors. ne bright sunny day, Newton dar!enedhis room and made a hole in his window shutter, allowing "ust one beam o# sunlight to enter theroom. He then too! a glass $rism and $laced it in the sunbeam. %he result was a s$ectacular

multicolored band o# light "ust li!e a rainbow. %he multicolored band o# light is called a colors$ectrum.

Newton believed that all the colors he saw were in the sunlight shining into his room. He thoughthe then should be able to combine the colors o# the s$ectrum and ma!e the light white again. %otest this, he $laced another $rism u$side&down in #ront o# the #irst $rism. He was right. %he bando# colors combined again into white sunlight. Newton was the #irst to $rove that white light ismade u$ o# all the colors that we can see.

Newtons Experiments - light and prismsThis is Newtons famous experiment where a triangular prism is used to 'split' white light

into the colours of the spectrum. In this apparatus we have a mains powered light box tocreate parallel white light beams, two prisms, various slits to help collimate the beams as

well as small white projection screens. A solid wooden base is provided so that

everything can be easily aligned correctly.

Experiment 1e set up the light box !left hand side of above photo" to produce a slit of white light that

will fall on the prism and about #$ cm away. In between the two set up a slit to ma%e sure

that the light is as collimated as possible. #$ cm or so away from the prism on the otherside of the table set up a screen. Adjust !rotate" the prism until a spectrum of colours can

be seen on the screen. &ou may need to dim the room lighting so that it is clearly visable.



&ou can move the slit closer to the prism to improve the colour seperation. The light

from the light box should be composed of white rays all in parallel, but in practice there

will be some spread of angles coming from the box. If these unparallel rays go into the prism they will each create a spectrum of their own and cause a jumbling of colours and

so spoil the show. ith the slit moved near to the prism it reduces the spread of rays so

they are all nearly parallel it ma%es a lot of difference to the spectrum projected on thescreen but we do lose a little light in the process and so the spectrum is not (uite so

bright.



The white light composed of many colours is going into the prism. )very colour will be

bent !refracted" slightly differently by the materail properties of the glass. *ed light is

bent least while the blue colours are bent greatest. As a result white light going in to the prism on one side emerges as a refracted spectrum of colours.

I used a powerful !+" white )- in the light box. This is not a truly multicolouredwhite light source li%e the rays from the un for example. As a result the spectrum is not

(uite so perfect as you would see produced by small rain drops in the wonderful displayof a rainbow.

The prism orientation ma%es a lot of difference to the (uality 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.

Experiment 2

eaving the apparatus setup as in )xperiment # move the screen slightly further away tosay +$ 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 thatthe spectrum can be reassembled into white light again 0



Experiment 3

-uring Newtons time there was some sceptism about what was precisely going on with

the white light and the prism glass. ome 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". o Newton devised this simple experiment!called 'Newton's )xperimentum 1rucis'" to prove the case.

et up as in )xperiment + and introduce a second slit, second prism in place of the lens.

2y adjusting the second slit one can chose a colour from the spectrum produced by thefirst prism !lets say the red light" and send this through the second prism. e dont see

another spectrum being produced from this red ray, instead we just see the red ray being

refracted by the second prism. e can do this for any other colour showing that thespectrum is not produced by some 'colouration' effect by the passage of the rays through

the glass prism.

newton's experiments on light

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