From these results Thomas Young
concluded that light behaved like a wave and that Newton’s particle
model of light could not explain these observations. Even though Newton
was an intellectual giant essentially everyone agreed with Young. To see
why, we need to complete a couple of short experiments. The first
experiment involves a version of the two-slit experiment of Young. We
will use something that we know is a wave. It’s a wave drawn on a piece
of plastic. we need to find two fixed points where they will represent
the double slit set up. To do that find a box like object (some ideas
can be your napkin box, cellphone or cellphone cover, etc...). I find it
easier to work with the napkin box just because it can stand stable by
itself.
Take the end of each wave that says tape here and tape it such that it is aligned with the box vertical edge, this will hold that end fixed and will serve as the two slits. Tape the box with waves on a piece of paper to minimize the box moving, it is also recommended to trace the box edge (full length) which contains the waves and if you like you can draw small slits at each end as shown in Figure 3-2(A). Taping the box and tracing the edge are just to help you to get your box to where it was in case it moved while doing your experiment. Hold the waves together, such that the peak to peak coincide with each other at the middle distance between between the two slits as shown in Fig 3-2 (A). If you taped the waves correctly from the marked end then you will have maximum addition of these two waves ( either two positive peaks meet or two negative peakes meet) in this case you will have constructive interference. Fig 3-2(b) show constructive interference resulting in addition of two positive peaks. Where as Figure 3-2 (C) shows a positive peak meet with a negative peak so they cancel each other (destructive interference). Mark with an X the locations where the addition of the two waves is a maximum (constructive interference). Place zeros where the two waves cancel each other (destructive interference).
Fig 3-2: (A) The waves hold on a napkin box and pulled gently to meet at almost equal distance between the two slits.(B) closer look at the waves from (A) which corresponds to
constructive interference this is the starting point(mark x at such locations) (C) represent a destructive interference where the two waves cancel each other(mark 0 at such locations)
Take the end of each wave that says tape here and tape it such that it is aligned with the box vertical edge, this will hold that end fixed and will serve as the two slits. Tape the box with waves on a piece of paper to minimize the box moving, it is also recommended to trace the box edge (full length) which contains the waves and if you like you can draw small slits at each end as shown in Figure 3-2(A). Taping the box and tracing the edge are just to help you to get your box to where it was in case it moved while doing your experiment. Hold the waves together, such that the peak to peak coincide with each other at the middle distance between between the two slits as shown in Fig 3-2 (A). If you taped the waves correctly from the marked end then you will have maximum addition of these two waves ( either two positive peaks meet or two negative peakes meet) in this case you will have constructive interference. Fig 3-2(b) show constructive interference resulting in addition of two positive peaks. Where as Figure 3-2 (C) shows a positive peak meet with a negative peak so they cancel each other (destructive interference). Mark with an X the locations where the addition of the two waves is a maximum (constructive interference). Place zeros where the two waves cancel each other (destructive interference).
Fig 3-2: (A) The waves hold on a napkin box and pulled gently to meet at almost equal distance between the two slits.(B) closer look at the waves from (A) which corresponds to
constructive interference this is the starting point(mark x at such locations) (C) represent a destructive interference where the two waves cancel each other(mark 0 at such locations)
? How does the distance between zeros depend on
- wavelength?
- amplitude?
we will use this information soon.
Try a similar experiment with some particles. To simulate objects similar to Newton’s particles of light you will use BBs. Place a blank piece of paper in a shallow pan. Over this paper place a sheet of carbon paper with the carbon side down.
To simulate the two slit experiment place the BBs in a small box with two holes in it. Over the carbon paper take your fingers off the holes and let the BBs drop. A carbon mark will appear where the BBs land. [Hint: A much better pattern appears if the carbon paper is placed on a hard surface that is angled so that when the BB’s bounce they don’t land back on the carbon paper.]
Describe the pattern of particle locations.
? You have now completed three two-slit experiments. One each with light, waves and particles. Use the results of these experiments to describe why Young concluded that light behaved as a wave.
As you see with these experiments, interference — the pattern of light and dark for light — only occurs for waves. Particles can’t do it.