electrons and photons grade 12

1. Electrons & Photons

3. Photoelectric effect
(Hertz & Lenard’s Observations)
• Discovered in 1887 by Heinrich Hertz.
• The phenomenon of emission of electrons by
certain substance (metal), when it is exposed
to radiations of suitable frequencies is called
as photoelectric effect and emitted electrons
are called photoelectrons.

4. Experimental study of photoelectric
effect

5. Effect of frequency on photoelectric
current
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 0.5 1 1.5 2 2.5 3
PHOTOELECTRICCURRENT
FREQUENCY

6. The minimum frequency of
incident radiation for which
photoelectrons are just
emitted from
photosensitive material is
called threshold frequency.
It is denoted by V0
Wavelength corresponding
to threshold frequency V0
is known as threshold
wavelength.

7. Effect of intensity on photoelectric
current
The photoelectric current is
directly proportional to the
number of photoelectrons
emitted per second.
Hence ,
Photoelectrons
emitted per second α Intensity

8. Effect of potential difference on
photoelectric current

9. Characteristics of photoelectric effect
• Below cutoff frequency of incident radiation also known as
threshold frequency V0 no emission of photoelectrons takes
place. V0 is different for different metals.
• Photoelectric current is directly proportional to the intensity
of incident light .
• Above the threshold frequency V0 the maximum K.E of the
emitted photoelectrons increases linearly with the frequency
of the incident radiation, but is independent of intensity of
incident light.
• The emission of photoelectrons is an instantaneous process.
• No time lag between the irradiation of the metal surface and
emission of photoelectrons.

10. Einstein’s photoelectric equation:
• Einstein extended Planck’s idea in 1905 by deriving an
equation . He made two assumptions:
A radiation of frequency v
consist of discrete quanta
or photons.
E = hv
Where h is Planck’s
constant.
Photons move with speed of
light.
When radiation of
frequency v is incident on
photosensitive surface there
are collisions between
photons & electrons.
Entire energy of photon is
transferred to electron
without time lag.

11. • The absorbed energy hv by electron is used in
two ways

12. • ev = hv-Φ
• We can now explain all characteristics of
photoelectric effect with the help of Einsteins
photoelectric equation
• If frequency of incident radiation is decreased ,the
kinetic energy of photoelectrons also decreases and
finally becomes zero for particular frequency say vo
• v= vo then K.E = ½ mv2 =0
• 0 =
• ½ mv2 =h (v- vo)
• If v > vo ,photoelectrons are emitted with some velocity
• If v<vo ,no photoelectrons are emitted.
• If v= vo , photoelectrons are emitted with K.E= 0
hv-Φ

13. Photoelectric cell

14. APPLICATIONS OF PHOTOELECTRIC CELL
• EXPOSURE METER
• BURGLAR ALARM
• SOUND REPRODUCTION FROM
MOTION PICTURES

15. THANK YOU