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Q & A: Newton's rings

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Most recent answer: 10/22/2007
What are newtons rings?
- Anonymous
Newton's rings are a fun demonstration of the wave nature of light, and also of the fact that white light is made up of many components of different wavelengths.

Newton observed these rings when placing a spherically shaped piece of glass (a lens) touching a flat piece of glass. The spherically shaped piece of glass, for this purpose, should not be very curved -- it should have a large radius of curvature so that the surface is almost, but not quite, flat. The pieces of glass will touch in one place, and the gap between the two pieces of glass increases farther away from the places where the glass pieces touch.

Light then shines down onto these pieces of glass as in the illustration below:

Newton rings

(copied from in case the web site goes away).

For simplicity, consider light of only one color. It may bounce off of the bottom surface of the curved glass or off of the top surface of the flat glass. The two components of reflected light then add together when coming back to the observerís eye. These may add constructively, with wave crests lining up, strengthening the light signal, or destructively, by canceling each other out. Whether this interference is constructive or destructive depends on how much extra distance the light must travel to bounce off of the flat piece of glass, compared to that in the curved piece of glass, expressed in terms of the wavelength. That is, it depends on how many wavelengths of the light "fit" in that little space between the curved and flat glass pieces. (Light has a small wavelength, which is why the gap should be small to see this the best).

If you do this with different colors of light, the fringes will be in different places. Blue light will make fringes more closely spaced than red light because it has a smaller wavelength. If you shine white light on it, you will see all the different colored fringes added together. Usually this makes a white mess, but very close to the contact point, you will see colored fringes. Here are some pictures of all of this from Carnegie Mellon University:

Demo 326A


Demo 326B


By the way, hereís a peculiar historical note. Although we see Newtonís rings as clear evidence of the wave nature of light, Newton didnít. He thought light consisted of little particles. To account for observations like the rings he had a story about the particles having 'alternating fits of easy and hard refractability'. (The quote is from memory, not exact.) / Mike

(published on 10/22/2007)

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