A zinc plate attached to an electroscope is given a negative charge. This moves the leaves of the electroscope away from each other. Visible light is shown on the plate with no result, but when light from an ultraviolet source is shone on the plate, the plate discharges and the leaves of the electroscope move back together.
This demonstration is usually used when the concept of the quantum is being introduced since it contrasts the energy of the photons of a light source with the intensity of the light. Video projection greatly improves the visibility of this demo. This demo frequently does not work well if the air is humid because the electroscope discharges spontaneously before being irradiated.
When a negative charge is placed on a zinc plate, one would assume that some zinc anions were being formed. The electron configuration of the zinc anion is: [Ar]: 3d10, 4s2, 4p1 It is reasonable, when one considers that the "extra" electron is by itself in the p subshell, that the energy required to remove it might be very small. In fact, this energy would be the inverse of the electron affinity of zinc, which is so small that I wasn't able to find it listed in any of the sources that I examined. (This is probably why zinc is used for this demo.) In layman's terms (or freshman chemistry terms), the photons of the light need to be of sufficient energy (i.e., high enough frequency) to "knock" the extra electron loose. In other words, each photon needs to be capable of imparting enough energy to the highest energy electron to escape from the zinc atom. The frequency of visible light is too low. Its photons do not have enough energy. However, the energy of photons of high frequency ultraviolet light is sufficient to strip the zinc atom of its extra electron.
Pre-quantum Maxwellian theory would have predicted that one could have supplied sufficient energy to discharge the plate simply by increasing the intensity of the visible light. You can debunk this by holding the spotlight very close to the plate - nothing happens. On the other hand, if you decrease the intensity of the UV light by moving it further away from the plate, it continues to discharge, only at a slower rate.
- bakelite rod
- rabbit pelt
- electroscope with a zinc plate attached to the terminal
- UV lamp
- power strip
Rub the rod vigorously with the rabbit pelt for about 15 seconds and draw it across the edge of the zinc plate. The leaves of the electroscope should move apart slightly. You may have to repeat this process two or three times to get maximum separation of the leaves.Shine the spotlight on the plate. Nothing should happen. The UV lamp has two power buttons on it. One is for low frequency UV and the other is for high frequency UV light. Shine the low frequency UV light on the zinc plate. The energy of the low frequency photons is usually insufficient to discharge the plate. Shine the high frequency UV light on the zinc plate. The plate discharges and the leaves of the electroscope move together.
Don't let the UV light shine into anyone's eyes, including your own. This is a powerful UV source. Be particulularly cautious of the short wavelength UV.
The electroscope, the bakelite rod, and the rabbit pelt are pbtained from the physics demo room.