Newton’s Prism Experiments

Purpose:
To recreate several of Newton’s experiments on light using prisms to determine how they showed that white light was a composite of different colors.

Background:

Excerpt from “BBC History: Sir Isaac Newton”:

"People were using prisms to experiment with colour, and thought that somehow the prism coloured the light. Newton obtained a prism, and set up his so that a spot of sunlight fell onto it. In their experiments, Descartes, Robert Hooke and Edward Boyle had put a screen close to the other side of the prism and seen that the spot of light came out as a mixture of colour. Newton realized that to get a proper spectrum you needed to move the screen a lot further away. In the study upstairs at Woolsthorpe, he used the 22 feet from the window to the far wall to project a beautiful spectrum."

A. Forming a Spectrum with a Prism

Materials:
source of a thin slit of white light
prism
convex lens and clay to make a base to hold it
block to elevate prisms if necessary
white paper to make a screen and to put under the prism on the lab table

1. Use the materials above to produce a sharp spectrum. With white paper under your light source and prism you should be able to see the path of the light beam. What did you have to do to produce a sharp spectrum?

2. Draw a sketch of your set-up, showing the path of the light beam. Use colored pencils to draw the spectrum.

B. Newton’s Experimentum Crucis

Newton performed a “crucial experiment” to prove that the different colors were actually components of white light itself and that the light was not somehow colored by the prism.

Materials:
source of a thin slit of white light
2 prisms
convex lens and clay to make a base to hold it
blocks to elevate prisms if necessary and to use as barriers
white paper to make a screen and to put under the prism on the lab table

3. Open the animation at “Newton’s Prism Experiments”. What happens when you put the second prism in the spectrum produced by the first?

4. Using the materials in the list above, try to recreate this experiment. You may need to pass the spectrum through a lens to focus it before passing it through the second prism. Sketch your set-up.

5. Newton then isolated one color of the spectrum before passing it through the second spectrum. Following the diagram below, use barriers to allow only one color of light to pass through. Place a second prism in that beam of monochromatic light. Record what you see on the paper target.

7. Open the website, Experiment 33: Prism:. Move the yellow hand on the tuneable laser so that red light is going through the prism. Click "Tools" and use them to measure the angle of the red light as it leaves the prism as shown in the diagram below. Repeat for green and blue light. Notice that the beam enters the prism at the same angle for all colors.

color of light           angle at which it leaves the prism

red

green

blue

8. Are all colors bent (refracted) the same amount when passing through a prism? If not, which is bent the greatest amount?

9. Click the “hold down for white light” button. From the results of this lab, explain how a prism produces a spectrum from white light.


D. Let’s Play!

10. View the following four black and white arrangements through a prism. Using colored pencils, sketch what you see (you can do this right on the diagrams).

11. What do you think causes the orders of colors you observed in #9?