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## 12.4 Chapter summary (ESCQW)

Presentation: 27ZN

• The photoelectric effect is the process whereby an electron is emitted by a substance when light shines on it.

• A substance has a work function which is the minimum energy needed to emit an electron from the metal. The frequency of light whose photons correspond exactly to the work function is known as the cut-off frequency.

\begin{align*} E & = W_{0}+{E}_{k\max} \\ {E}_{k\max}& = hf-W_{0} \end{align*}
• The number of electrons ejected increases with the intensity of the incident light.

• The photoelectric effect illustrates the particle nature of light and establishes the quantum theory.

• Emission spectra are formed when certain frquencies of light are emitted by a gas, as a result of electrons in the atmoms dropping from higher to lower energy levels. The pattern of the spectra is charactersistic of the specific gas.

• Absorption spectra are formed when certain frequencies of light are absorbed by a material. These photons are absorbed when their energy is exactly the correct amount to raise an electron from one energy level to another.

 Physical Quantities Quantity Unit name Unit symbol Energy ($$E$$) joule $$\text{J}$$ Work function ($$W_0$$) joule $$\text{J}$$ Frequency ($$f$$) hertz $$\text{Hz}$$ Wavelength ($$\lambda$$) metre $$\text{m}$$

Table 12.2: Units used in optical phenomena and properties of matter.