Chapter 51
Young’s Experiment
We now begin a study ofphysical optics, which is the study of the physical properties of light, including its
wave nature.
A key experiment in physical optics isYoung’s experiment, first performed by British physicist Thomas
Young (1773–1829). In this experiment, one allows a light source to pass through two closely-separated
slits and then be projected onto a screen. The light source should bemonochromatic(that is, of a single
wavelength) andcoherent(each wave train is many wavelengths long). In Young’s time such light sources
were very faint and difficult to work with, but today we can perform the experiment easily using alaseras a
coherent monochromatic light source.
The significance of Young’s experiment is that it demonstrates that light is awave: on performing the
experiment, you find aninterference patternof alternating light and dark bands on the screen. At any pointP
on the screen, the distance from one slit will be different from the distance from the other slit; this difference
in distances will bedsin, wheredis the separation distance between the slits, andis the angle from the
midpoint of the slits to the pointP. If the path length difference is an integral number of wavelengths, the
interference will be constructive, and abright fringewill be observed on the screen:
dsinDm .mD0;1;2;:::/ (bright fringes) (51.1)Heremis called theorderof the fringe. In between the bright fringes, one will seedark fringes:
dsinDmC1
2
.mD0;1;2;:::/ (dark fringes) (51.2)51.1 Quantum Effects
Young’s experiment may be used to demonstrate some very oddquantum mechanicaleffects. (Quantum
mechanicsis the theory of mechanics that describes particles at very small distance scales — say at the size
of an atom or smaller.) Light is — in some way we don’t entirely understand – both an electromagnetic
wave and a particle (called aphoton) at the same time. It’s possible to send light through Young’s experiment
one photon at a time, in which case you would expect the interference pattern to disappear. After all, the
interference pattern is caused by light from one slit interfering with light from the other slit, but the photon
goes through only one of the two slits. But if we do this experiment, we discover that the photons, one by
one, will build up the same interference pattern.
Now if we try to determinewhichslit the photon went through (by bouncing another photon off of it
near the slit, for example), the interference pattern disappears: the photon we used to make the determination
messes us the experiment in such a way that it destroys the interference pattern. We might try to fix this by