1

EM wave spectrum

Low photon energy

High f Low f

Long short

High photon energy

1 photon has Photon Energy = hf = h c/

n photon has Photon Energy = n hf

A

1

t

nhf

At

E

A

PI

total

Intensity of Light

(1)

(2)

More photons, higher Intensity

UVX-raygamma-ray IRMicrowave

Radio wave

Visible ray

2

Photo-electronic Effect Let a story to explain this effect: How “Photon” Rescues Miss “Electron”

__ _ _ _

Electrons are trapped inside the metal-X

No Electron is released !!!

metal-X Cage

Yellow Photons

3

One Green Photon releases one “Electron”, but vel=0

0hf

Electrons are trapped inside the metal-X

_ _ _ _

_

V=0 Electron is motionless

Photon energy E just has sufficient energy to release Electron, but no extra energy for Electron so K.E=0

Frequency of green is called threshold frequency f0

Green Photon

4

frequency of Green Photon = f0 for metal x

Incident Photon Energy EGreen = hf0 + 0 where K.E= 0

Rewritten as : hfGreen = hf0

fGreen= f0 i.e. fGreen is the “threshold frequency f0” of metal X

hf0 = is called work function

Work function () is the amount of energy to release electron for this metal-x

5

One Violet Photon gives K.E to released “Electron”

Electrons are trapped inside the metal-X

_ _ _ _

Electron gains K.E

She flies

Violet Photon

Blue, violet or photon with higher f

6

Higher frequency photon f>f0, gives K.E

max

max0

max0

E.Khf

E.Khfhf

E.KhfE

Incident photon energy (E = hf ) is higher than work function

Or f>f0

The expression can be written as

7

Conclusion

1. Electron to be released is dependent on frequency or wavelength of Photon NOT the number of photon (or intensity).

2. The min. frequency of photon to be released the electron is called threshold frequency (f0) which does not give K.E to the electron.

i.e in this example, green is the threshold frequency for metal X

3. Higher frequency of 1 photon as f>f0, will give release 1 electron with K.E

incident light: E = hf0+K.E

Rewrite hf = +K.E where =hf0= work function