Lesson 10, Topic 2
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Einstein’s Equation

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Einstein proposed that light waves consist of packets of energy called quanta or photons and each photon has an energy, hf, h is Plank’s constant (6.6 x 10-34Js) and f is the frequency of the light i.e E = hf = $$\frac{hc}{\lambda}$$

The incident light must produce enough energy for electrons to overcome the electrostatic force of the metal. The workdone by an incident light to remove electrons from a metal is called work function (w)

Work function is the maximum energy needed to liberate an electron from a metal surface.

Using Einstein relation

W0 = Work function

hf = W0 + $$\frac{1}{2} \scriptsize mv^2$$

hf = hf0 + $$\frac{1}{2} \scriptsize mv^2$$

mv2 = hf – W0

∴hf = hf0 + E

E = $$\frac{1}{2} \scriptsize mv^2$$ – Kinetic energy of the photoelectrons.

E = hf – hf0

This is known as Einstein’s photoelectric equation.

E = $$\frac{1}{2} \scriptsize mv^2$$= eV

$$\frac{1}{2} \scriptsize mv^2$$ = eV

eV is electron volt.

∴ E = eV = hf – hf0

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