Transcript PHOTOELECTRIC EFFECT - Yola

PHOTOELECTRIC EFFECT
The particle nature of light
Recall…
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Light (energy) has a wave-nature (exhibits diffraction and interference effects)
Matter has a particle nature (has mass and occupies space)
Matter has a wave nature (Electron beam undergoes diffraction / interference)
Light (energy) has a particle nature
Note: When we refer to light we are talking about “EM – radiation”. Light is not
only the visible part, but also the invisible (radio to IR, visible, UV to Gamma)
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History
- Heinrich Hertz made the first notable discovery that pointed towards light
having a particle nature (Certain metals discharge electrons when light is
incident upon them – photoelectric effect)
- Confirmation of this hypothesis by Einstein, when he explained the
photoelectric effect using the idea of light having a particle nature
THE PHOTELECTRIC EFFECT
Consider the following setup…
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The electroscope is charged negatively
The experiment is repeated using an ultraviolet lamp, a 40W and 200W bulb
The electroscope is positively charged, and tested with an ultraviolet light
A sheet of glass is placed over the electroscope and tested with UV light
Observations
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UV light causes the negatively charged electroscope to discharge, causing the
leaves of the electroscope to collapse
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When visible light (40 and 200W bulb) is used, the electroscope retains its
charge
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When the electroscope is positively charged, no visible change occurs
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When a glass plate is inserted between the UV light and electroscope, no
visible change can be observed (UV cannot move through glass)
Conclusions
• The fact that ONLY the negatively charged electroscope discharges,
indicates that electrons are removed from the metal
• White light is not able to liberate electrons (whether it is 40W or 200W)
indicating that it is not the intensity of the light that determines whether
electrons are liberated or not
• Ultraviolet light is able to liberate electron, while visible light cannot,
thus it would seem that the higher the frequency of the light, the greater
the probability of electrons being liberated
• The phenomenon of the liberation of electrons from a metal surface by
radiation is known as the “PHOTO-ELECTRIC effect”. The electrons that
are liberated are called photo-electrons
Threshold frequency
“Each specific metal has a minimum frequency called the threshold frequency
for which electrons will be released from the metal.
The frequency of the incident light must be equal to or greater than the threshold
frequency before electrons can be released”
Einstein explains the photo electric effect
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When a photon (energy ‘packet’) of light strikes an electron in the metal
surface, all the energy of the photon (hf) is transferred to the electron. After
which the photon ceases to exist
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The metal exerts an attractive force on the electron. The electron requires
energy to break free from this attraction. The minimum energy required is known as
the “WORK FUNCTION (W0)” of the metal
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If hf < W0, the energy of the photon will not cause the emission of an electron, but
will rather be taken into the metal increasing its internal energy
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If hf = W0, the energy of the photon is just enough to cause emission of an
electron. This photon has the minimum required frequency f0 (called the
threshold frequency) to release an electron
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If hf > W0, then the electron receives more energy than it requires to break
away from the surface of the metal. The remainder of the energy is used by
the electron to move away from the surface as kinetic energy.
E  hf  W0  1 m v2
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This implies that the kinetic energy of the electrons depend on the FREQUENCY
of the light used!
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The intensity of the light will increase the number of photons striking the
metal per unit time, but the kinetic energy of the electrons will be the
same!
Note:
1) Einstein’s explanation was later verified by experimentation (milikan)
and he received a noble prize
2) The photoelectric effect was of great significance in that it…
a) Established quantum theory
b) Provided an illustration of the particle nature of light
http://www.lon-capa.org/~mmp/kap28/PhotoEffect/photo.htm
Application – Photocell
Consider the diagram of the photoelectric cell shown below…
• Inside the cell is a substance that has a low threshold frequency (eg. Selenium)
• When light is incident on the selenium, photoelectrons are emitted
• Because selenium is losing negative charges, it becomes positively charged
• These electrons are absorbed by a metal on the other side (becomes negative)
• This results in a potential difference between the negative electrode and the
positive electrode and therefore, a current!
Note: Photocells are used in…
a) Light meters of cameras
b) Power source for satellites
c) Pocket calculators
Note: Read section on “dual nature of light”, pg 241