MODERN PHYSICS

1. QUANTA
QUANTA
BILAN, CRISELDA
PALMA, REVEN JADE

2. All hot bodies radiate. An ideal radiator is called a black body
and the spectrum of radiator from a black body was well
known to 19th-century physicist.
BLACK - BODY RADIATION
BLACK - BODY RADIATION
The problem was to derive the spectrum from mechanics and
electromagnetism. Until 1899, no one had managed to do this,
and that was not for want of trying! The obstacle they had
encountered became known as the ultraviolet catastrophe

3. In 1900 Max Planck, a German physicist, came up
with a 'desperate remedy. He showed that an
accurate equation for the spectrum could be
derived as long as one new assumption was added
to those of classical physics. He assumed that the
oscillators that emit radiation can only have
discrete energies. Each oscillator can have zero
energy or some multiple of a fixed amount
(quantum) which depends on the frequency f of
oscillation according to the formula.

4. E-nhf
n is an integer, 0, 1, 2, and his a new constant,
now known as the
Planck constant:
h6.626 x 10MJs
FORMULA
FORMULA

5. Thermal energy is randomly
distributed, so the chance that
high-frequency oscillators will get
enough energy to start vibrating
(at least f) is much smaller than
for the lower frequency
oscillators.
OBJECTIVES
OBJECTIVES
How does this fix the ultraviolet
catastrophe? The shorter wavelengths
correspond to higher frequencies, so
the oscillators responsible for
radiation in this part of the spectrum
need a lot more energy to get into even
the first vibration state than those
emitting radiation at a longer
wavelength (lower frequency).

6. The result is that if energy is
quantized in this way the
high- frequency oscillators
are 'switched off and the
intensity of the spectrum at
high frequencies drops down
rapidly to zero exactly as
observed. (In classical physics
all oscillation frequencies
would have been excited, and
the cumulative effect was the
ultraviolet catastrophe.)
PROCESS
PROCESS
Planck and other physicists were
uneasy about this new idea, but
there seemed to be no other way
to explain the black-body
spectrum. The inescapable
conclusion was that
Electromagnetic radiation
is emitted in discrete
energy packets or quanta

7. Another problem that arose late in the nineteenth century
concerned the way light falling on some metal surfaces could
eject electrons from them. This is called the photoelectric
effect. According to wave theory, light energy is spread
evenly across the wavefront, so electrons should be emitted
only if enough energy is delivered close to an electron on the
surface. Also, the ejection should depend only on the
intensity of the incident light, and not on its frequency.
Neither of these expectations was borne out in practice.
Experiments led to these "laws of photoelectricity.
THE PHOTOELECTRIC EFFECT
THE PHOTOELECTRIC EFFECT

8. For any metal, electrons are only emitted if the frequency of the incident
light is above some threshold value fo. (So weak ultraviolet can emit
electrons from zinc, whereas very intense infrared cannot, even though
it is delivering far more energy per second to each unit of the zinc
surface.)
The threshold frequency depends on the metal and is usually lower for more
reactive elements (so electrons are emitted from potassium more readily than
from zinc, and from zinc more readily than from copper).
The maximum kinetic energy of the ejected electrons depends only on the
frequency of the incident radiation and is proportional to the difference
between the light frequency and the threshold frequency:
KEmax (f - fo).

9. Einstein, who was aware of Planck's work, tackled
the photoelectric effect in 1905. He saw that all the
experimental laws could be explained if it was
assumed that atoms can only absorb light energy in
discrete 'energy packets' or quanta, and that the
size of one quantum is proportional to the
frequency of the light and given by
E-hf

10. These quanta became known as photons, and Einstein won the 1921
Nobel Prize for Physics for this work. Photons solve all of the problems
with which wave theory had difficulty
Photons are indivisible, so each photon gives all its energy to one electron.
If there is a minimum or threshold energy required to eject electrons
from a particular metal surface, then there will be a minimum photon
energy that can do this. Photon energy is proportional to frequency, so
electrons are only ejected with light above a certain threshold frequency.
Increasing the intensity of light does not affect the energy of individual
photons, only the number arriving per second

11. The minimum energy required to free an electron
from the surface depends on the metal, so the
threshold frequency changes from one metal to
another. Reactive metals lose electrons easily, so less
energy is required and their threshold frequency is
lower:

12. If the light frequency is only just above the threshold
frequency, the photon energy is only just sufficient to eject
electrons, so there is little left over for kinetic energy. The
maximum kinetic energy of ejected electrons can be no
greater than the difference between the photon energy and
the threshold energy. This is directly proportional to the
difference between light frequency and threshold frequency.

13. THANK
YOU
THANK
YOU