Photoelectric Effect

 A metal contains many mobile electrons. It is relatively easy to excite these electrons, and if they are excited with enough energy, they can leave the metal. When such an excitation is made with light, the ejected electrons are known as photoelectrons and this effect is known as the photoelectric effect. It has been observed that in order for this to happen, the frequency (f) of the light must exceed some minimal threshold (f₀), or equivalently, the light wavelength (λ), which is related to the frequency f by:

(with c ≈ 3x10⁸ m/s being the speed of light) needs to be below some threshold (λ₀), that is, f > f₀ (λ < λ₀). Otherwise, if f < f₀ (λ > λ₀), no photoelectrons will be emitted even with intense light illumination.

Albert Einstein was able to explain these observations using the concept of photons, the quanta of light. Light consists of many of such particle-like photons, and each photon has energy:

with h ≈ 6.63x10^-34 Js, called Planck’s constant, which relates the light frequency to photon energy.

The microscopic process of the photoelectric effect is that an individual photon is absorbed by the metal and its energy is used to excite an electron. The electron will be emitted from the metal if the photon energy,

where W is known as the “work function” and represents the minimal energy needed to liberate the electron from the metal. If,

even if the light is intense (meaning it contains a large number of photons) and even if the light is shone for a long time, no photoelectrons will be produced since the individual photons do not have sufficient energy to liberate electrons.

Einstein’s explanation of the photoelectric effect was historically significant as it provided key support for the theory of photons (quanta of light), which shows that light can behave as particles in addition to as electromagnetic waves, and possess the dual particle-wave nature.

For example, zinc (Zn) metal to be used in this experiment has a work function of W ≈ 4.3 eV (with 1 eV ≈ 1.6x10^-19 J). This means that the threshold frequency for the photoelectric effect for Zn will be:

corresponding to a threshold wavelength,

In order to produce photoelectrons out of Zn, light must have a frequency exceeding f0 ≈ 1015 Hz, or a wavelength below λ₀ ≈ 300 nm. Such a short wavelength corresponds to UV (since the visible light has a wavelength exceeding ~ 400 nm, which corresponds to violet color).

Since an electron carries a negative charge, the photoelectric effect will remove negative charges from a metal (effectively adding positive charges to it). If the metal is originally negatively charged, this will make it less charged. If the metal is originally positively charged, this will make it more charged. Such effects will be studied in this experiment.

For steps 2.1-2.4, the electroscope remains charged (needle remain deflected) for both the regular lamp and UV light illumination (Figure 2b and 2c), indicating that the zinc plate remains positively charged.This is because the charged zinc plate (which has already lost some electrons in the first place to become positively charged) further losessome photoelectrons by the UV light to make it further positively charged. In thiscase, itmay be noticeablethat the needle of the electroscope deflects a bit further in Figure 2c.The regular visible light doesnot change the positive charges on the zinc plate and the electroscope remains charged as well (Figure 2b).

For steps 3.1-3.5, when the zinc plate is negatively charged, it can be observed that the regular lamp light again has no effect on the electroscope (Figure 3b), while the UV light causes the needle of the electroscope to collapse and return to the uncharged position with no deflection, Figure 3c. This is because only the UV light photons have enough energy (above the workfunction of zinc) to eject photoelectrons, thus to discharge the zinc that has been previously charged to be negative (with excess electrons).

In this experiment, we haveused an electroscope to show that UV light can discharge a negatively charged zinc metal through the photoelectric effect.In contrast, a positively charged zinc sample (which has already lost some electrons) will not be discharged, nor will a visible light (which cannot cause the photoelectric effect) discharge either negatively or positively charged zinc.

The photoelectric effect played important roles in the development of quantum physics in the 20^th century as it provided experimental evidence that light is made of particles that we call photons andcarry quanta of the light energy proportional to light frequency.

Practically, the photoelectric effect has also been used to make various optoelectronic devices, such as photosensitive electrical switches-where the blocking or unblocking of a light beam shining on a metal turns off or on an electrical current due to the absence or presence of photoelectrons.This is commonly used in many mechanical-position sensors (for example opening or closing of a door that unblocks or blocks a light beam).