Photoelectric EffectPhotoelectric Effect

(How Einstein really became (How Einstein really became famous!)famous!)

Photoelectric EffectPhotoelectric Effect

Metal Foil

Photoelectric EffectPhotoelectric Effect

Metal Foil

Photoelectric EffectPhotoelectric Effect

►As blue light strikes the metal foil, the As blue light strikes the metal foil, the foil emits electrons.foil emits electrons.

Photoelectric EffectPhotoelectric Effect

Photoelectric EffectPhotoelectric Effect

►When red light hits the metal foil, the When red light hits the metal foil, the foil does not emit electrons.foil does not emit electrons.

►Blue light has more energy than red Blue light has more energy than red light.light.

►How could we get more energy into How could we get more energy into the red light?the red light?

►Try increasing the brightness.Try increasing the brightness.

Photoelectric EffectPhotoelectric Effect

Photoelectric EffectPhotoelectric Effect

►Well, that didn’t work!Well, that didn’t work!►Maybe its still not bright enough.Maybe its still not bright enough.

Photoelectric EffectPhotoelectric Effect

Photoelectric EffectPhotoelectric Effect

►Still not working.Still not working.►What happens with brighter blue light?What happens with brighter blue light?

Photoelectric EffectPhotoelectric Effect

Photoelectric EffectPhotoelectric Effect

►More blue light means more electrons More blue light means more electrons emitted, but that doesn’t work with emitted, but that doesn’t work with red.red.

Photoelectric EffectPhotoelectric Effect

Photoelectric EffectPhotoelectric Effect

►Wave theory cannot explain these Wave theory cannot explain these phenomena, as the energy depends on phenomena, as the energy depends on the intensity (brightness)the intensity (brightness)

►According to wave theory bright red According to wave theory bright red light should work!light should work!

►BUT IT DOESN’T!BUT IT DOESN’T!

Photoelectric EffectPhotoelectric Effect

►Einstein said that light travels in tiny Einstein said that light travels in tiny packets called packets called quanta.quanta.

►The energyThe energy of each quanta is given by of each quanta is given by its frequencyits frequency

E=hfE=hfEnergyEnergy

Planck’s constantPlanck’s constant

frequencyfrequency

Photoelectric EffectPhotoelectric Effect

►Each metal has a minimum energy Each metal has a minimum energy needed for an electron to be emitted.needed for an electron to be emitted.

►This is known as the This is known as the work function, work function, W.W.

►So, for an electron to be emitted, the So, for an electron to be emitted, the energy of the photon,energy of the photon, hf hf, must be , must be greater than the work function, greater than the work function, WW..

►The excess energy is the The excess energy is the kinetic energy, kinetic energy, EE of the emitted electron. of the emitted electron.

Photoelectric EffectPhotoelectric Effect

EINSTEIN’S PHOTOELECTRIC EQUATION:-EINSTEIN’S PHOTOELECTRIC EQUATION:-

E= hf-W

Photoelectric EffectPhotoelectric Effect

►For a further demonstration try this For a further demonstration try this website:-website:-The Photoelectric EffectThe Photoelectric Effect

Try to find the work functions ofTry to find the work functions of :- :-

AluminiumAluminiumSilverSilverCopperCopperSodiumSodiumPlatinumPlatinum