Photoelectric Effect

photoelectric effect

blue light - electrons emmited

red light - none

Increase the amplitude of light, the light is brighter. But, does brighter light give off more photo-electrons (photons)?

No.

This cannot be explained if light is a wave.

So, Max Planck suggested so-called quanta which could be seen as “lumps” of energy.

If the quanta is too small, the energy is not high enough for elctrons to escape.

TODO Sketch of Quanta

Planck’s equation

Max Planck realised that the energy of light emitted is proportional to the frequency of the light. $E \propto f$ (or $E \sim f$)

$$E = h \cdot f$$

where E is the energy, h is Planck’s constant ($h \approx 6.62 \cdot 10^{-34}$ Js) and f is the frequency of the light wave.

Note: the left-hand side is a property of a particle and the right-hand side is a property of a wave. This means that for light both ideas need to be used.

Remember also that a photon has no mass and travels at the speed of light.

So, $$c = \lambda \cdot f$$ or

$$f = \frac{c}{\lambda}$$

The energy of light photons varies between 1.7 eV and 3.2 eV .

TODO - links to experiments to determine the value of h.

the electronvolt eV

It is the kinetic energy gained by a single electron by accelerating through an electric potential difference of one volt in a vacuum.

After the revision of SI units in 2019, 1 eV is equivalent to $1.602176634 \times 10^{- 19}$ J.