Title: THE PHOTOELECTRIC EFFECT
1THE PHOTOELECTRIC EFFECT
2When red light is incident on a clean metal
surface
Clean metal surface
e
e
e
e
e
e
no electrons are released, however long light is
shone onto it, however intense the light source
is.
3When UV light is incident on a clean metal
surface
UV light
Clean metal surface
e
e
e
e
e
e
e
- electrons are released instantaneously,
- however weak the light source.
4Classically this cannot be explained because
- If red light is shone onto the metal surface
- for long enough some electrons should
- gain sufficient energy to enable them to escape.
5Einstein put forward a theory
- Light energy is quantized.
- Light consists of a stream of particles called
photons. - The energy of each photon (E) depends
- on the frequency (f ) of the light.
h x
f
E
6h
is Planck's constant
red light has a smaller
frequency
Frequency increasing
than violet light
7E h x f
Photon energy
Red light photons therefore
have less energy
than violet light photons
and even less than UV photons
8ONE PHOTON
GIVES ALL ITS ENERGY
e
TO ONE ELECTRON
9A photon of red light gives an electron
insufficient energy to
enable it to escape from the surface of the metal.
Red light photon
Clean metal surface
e
e
e
e
e
e
e
surface electrons
No electrons are released from the metal surface.
10A photon of UV light gives an electron sufficient
energy to
enable it to escape from the surface of the metal.
UV photon
Clean metal surface
e
e
e
e
e
e
e
surface electrons
Electrons are released instantaneously. Each
photon releases an electron This is called
photoemission.
11BLACKBODY RADIATION
12Significance of Black Body
- The black body concept has a significant role in
thermal radiation theory and practice. - The ideal black body notion is important
in studying thermal radiation and
electromagnetic radiation transfer in all
wavelength bands. - The black body model is used as a standard with
which the absorption of real bodies is compared.
13Definition of a black body
A black body is an ideal body which allows the
entire incident radiation to pass into itself
(without reflecting the energy ) and absorbs
within itself (without transmitting the energy).
This propety is valid for radiation corresponding
to all wavelengths and to all angles of
incidence. This renders a black body an ideal
absorber of incident radiation.
14The UV Catastrophe
Theory experiment disagree wildly
Pre-1900 theory
15Plancks solution
EM energy cannot be radiated or absorbed in any
arbitrary amounts, but only in discrete quantum
amounts. The energy of a quantum depends on
frequency as Equantum h f
h 6.6 x 10-34 Js
Plancks constant
16Quanta and the UV catastrophe
Without the quantum
With the quantum
Low frequency, small quantum, Negligible effects
high frequency, large quantum, huge effects
17Comparison between Classical and Quantum
Viewpoints
18Conclusion
- As the temperature increases, the peak wavelength
emitted by the black body decreases. - As temperature increases, the total energy
emitted increases, because the total area under
the curve increases. - The curve gets infinitely close to the x-axis but
never touches it.
19Black-body Radiation
2.9 x 10-3 m T(Kelvin)
l peak
Light intensity
UV
IR
20lpeak vs Temperature
2.9 x 10-3 m T(Kelvin)
T
l peak
310 K (body temp)
2.9 x 10-3 m 310 K
9x10-6m
infrared light
5800 K (Suns surface)
visible light
2.9 x 10-3 m 5800 K
0.5x10-6m