UNIT OVERVIEW

When light shines on certain metals, electrons can be ejected from the surface. This is called the photoelectric effect. It happens not because the light heats the metal, but because light carries energy in tiny packets called photons.

In the early 1900s, scientists were puzzled by their experiments. They expected brighter light to give electrons more energy, but the results showed something different. In 1905, Albert Einstein explained this by proposing that light behaves like a stream of particles, each carrying a specific amount of energy. This work earned him the Nobel Prize in Physics in 1921.

In this unit, you will learn:

• Why classical wave theory fails to explain the photoelectric effect
• How Einstein's photon model explains all the experimental observations
• The meaning of work function, threshold frequency, and photon energy
• How to calculate photon energy using $E = hf$
• The relationship between light frequency and kinetic energy of emitted electrons
• How emission and absorption spectra prove that energy is quantised
• Real-world applications of the photoelectric effect