interference experiment of electrons

interference experiment of electrons

Our strategy in the double-slit experiment is to send electrons through a double-slit diaphragm and see how the intensity measured by the detector differs from the interference pattern formed by light. To implement this strategy, we must make a few modifications in the apparatus Young used when he performed this experiment with light. First we replace the light source with an electron gun-a device that produces a (nearly monoenergetic) beam of electrons of energy E. A heated tungsten wire, for example, produces a stream of electrons that we can accelerate to the desired velocity. Second, we replace the photographic plate with an electron detector: a device that counts the number of electrons that arrive in each square meter of unit area per sec. (Like the photographic plate used in Young's experiment, our electron detector measures the rate at which energy arrives at each point on the detector.) A screen covered with phosphor will do; when an electron arrives at the screen, it produces a spot.

What would we expect to see at the detector if the electrons were particles, subject to the same physical laws as, say, marbles? Imagine for a moment that we block one slit-say, the lower slit in Fig. 2.3-so that all electrons must come through the other, open slit. Most electrons that make it through the diaphragm will go straight through this slit, "piling up" at the detector directly opposite it. We therefore expect to see a maximum in the measured intensity opposite the upper slit. But some particles will scatter from the edges of this slit, so we expect some amount of spread in the pattern. A reasonable guess for the intensity for a beam of particles passing through this apparatus with only the upper slit open is the curve  sketched in Fig. 2.6a. The curve  should be obtained if only the lower slit is open.

What should happen when both slits are open? Well, if the electrons are indeed particles, then the measured intensity should be simply . This rather featureless curve is sketched in Fig. 2.6b. (Were you to scale the apparatus to macroscopic size and send through it a uniform beam of marbles-with, of course, a suitable detector this is what you would see.) But what we actually see when we run the experiment with electrons is altogether different.

The measured intensities in Fig. 2.7 clearly exhibit bands of alternating high and low intensity: an interference pattern, like the one formed by light (see Fig. 2.4). This observation seems to imply that the electrons are diffracted by the slits at the diaphragm and then interfere in the region between the diaphragm and the detector. We can even fit the measured intensity of the scattered electrons to the double-slit function  of Eq. (2.6) provided we assign to each electron (of mass m and energy E) a wavelength

But to a classical physicist, steeped in the idea that electrons are particles, Eq. (2.9) is nonsense!

The source:

Michael A. Morrison - Understanding Quantum Physics.

By. Fady Tarek