1. Dualisme Cahaya Sebagai Gelombang dan Partikel
Wave Properties
Light intensity = 𝐼∝ 𝐸 2
Particle Properties
Light intensity: 𝐼=𝑁ℎ𝑓
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2. Quantum Theory of Light
Under a constant frequency, the smallest energy unit of light is quantized.
For one photon
𝐸=ℎ𝑓= ℎ𝑐 𝜆
For N photon
𝐸=𝑁ℎ𝑓
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3. Spectrum of electromagnetic radiation
Free electrons in resonance
Nucleus
Transition of bound electrons in atoms
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4. 1.3.2 Photoelectric effect
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5. Photoelectric effect applet Photoelectric effect applet
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6. Observations
Monochromatic light is incident to one of the electrodes made by a particular metal. The induced current called photocurrent is collected.
If V is fixed, there exists a threshold frequency o, below which there is no photocurrent.
Different electrode materials have threshold frequency o
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7. Observations
The photocurrent is constant above the threshold frequency under a constant illumination, regardless of the frequency.
The photocurrent is proportional with the intensity.
The energy of electron is proportional with the frequency.
For a particular electrode and frequency of light, a stopping voltage Vs exists. No photocurrent can be collected regardless of the intensity of light.
There is no measurable tie lag between the illumination of light and the release of photoelectrons. (10-9s)
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8. Graphical presentations o
Energy of photoelectrons is proportional to the frequency;
Existence of threshold frequency
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9. Photocurrent is proportional to light intensity
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10. Different frequency of light has different stopping voltage
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11. Stopping voltage varies with electrode material for the same frequency
increases with frequency of light for the same electrode
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12. Einstein’s interpretation
Under a constant frequency, the smallest energy unit of light is quantized.
One photon can at most release one electron, regardless of the photon energy
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13. where K is the kinetic energy of the photoelectron
Work Function, W– the minimum energy for electron to escape from the metal surface
where K is the kinetic energy of the photoelectron
Photocurrent
where  is the quantum efficiency, n number of photons striking to the electrode per second, q electron charge
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14. Where A is the area of the electrode exposed to light.
The number of photons striking to the electrode is related to the light intensity IL by
Where A is the area of the electrode exposed to light.
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15. Threshold frequency
When K = 0, there is no photoelectrons. The frequency reaches the threshold frequency, i.e.
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16. Stopping voltage The reverse biasing voltage: where the photoelectrode
is positively biased and the other negatively biased.
When the biasing voltage increases from zero, the photocurrent decreases. At V  VS, IP = 0
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17. Stopping voltage
The physical meaning is that the potential difference barrier is decelerating the electron, and finally consumes all its kinetic energy. Thus, the stopping voltage gives us a tool to determine the kinetic energy,
mgh
qVS K
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18. Stopping voltage against frequency
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19. Hitung energi kinetik elektron yang dilepas oleh elektroda tersebut.
Example:
Cahaya ultraviolet dengan panjang gelombang 350 nm dan intensitas 1 W/ m 2 mengenai permukaan elektroda yang “work function” nya 2.2 eV.
Hitung energi kinetik elektron yang dilepas oleh elektroda tersebut.
Jika 1% dari photon yang datang diubah menjadi fotoelektron, berapa jumlah elektron yang dilepas per detik jika luas permukaan 1 c m 2
Diketahui konstanta Planck =6.626x 10 −34 Js= 4.136x 10 −15 eVs
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20. Failure of classical theory
In 1902, Philipp Eduard Anton von Lenard observed that the energy of the emitted electrons increased with the frequency of the light. This was at odds with James Clerk Maxwell's wave theory of light, which predicted that the energy would be proportional to the intensity of the radiation.
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21. Within the experimental accuracy, (about 10-9 second), there is no time delay for the emission of photoelectrons. In terms of wave theory, the energy is uniformly distributed across its wavefront. A period of time is required to accumulate enough energy for the release of electrons.
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22. Inconsistency of Classical wave theory and experiment
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23. 1.3.1 What is blackbody?
Under thermal equilibrium, an object of a finite temperature emit radiation (supposed to be EM wave).
It is found that an object that absorbs more also emits more radiation.
A perfect absorber is an object with black surface that must of the incident energy is absorbed. It is also expected to be a perfect radiator.
Consequently, a blackbody is a perfect radiation.
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24. Phenomena for a blackbody
Temperature-dependence emission spectrum.
Reciprocal relation between wavelength at the peak intensity max and ambient temperature T
Inconsistency between observed spectrum and the prediction from classical theory (Reyleigh and Jeans)
A model suggested by Planck (quanta of light energy)
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25. Temperature dependence of emission spectrum
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26. Classic: Rayleigh – Jeans Formula
𝐸 = 𝑘 𝐵 𝑇
Total Energy in the cavity with frequency interval between v and v+dv
𝑢 𝑣 𝑑𝑣= 𝐸 𝐺 𝑣 𝑑𝑣= 8𝜋 𝑘 𝐵 𝑇 𝑐 3 𝑣 2 𝑑𝑣
Planck Radiation
𝐸 = ℎ𝑣 exp ℎ𝑣 𝑘 𝐵 𝑇 −1
𝑢 𝑣 𝑑𝑣= 𝐸 𝐺 𝑣 𝑑𝑣= 8𝜋ℎ 𝑐 3 𝑣 3 exp ℎ𝑣 𝑘 𝐵 𝑇 −1 𝑑𝑣
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27. Inconsistency of Classical wave theory and experiment
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28. Blackbody radiation – Stefan’s Law
An object having a surface temperature T will emit radiation power P which is proportional to the surface area A of the object and to the fourth power of the temperature.
s is the Setfan’s constant and is equal to 5.67 x 10-8 Wm-2K-4(Jm-2s-1K-4)
e is the emissivity of the object and is
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29. Stefan ‘s law
The total power (area below a constant temperature curve P)
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30. Wein displacement Law
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31. -Hitung panjang gelombang yang dipancarkan tubuh anda
- Berapa energi yang dipancarkan oleh tubuh anda?
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32. Blackbody radiation and ultra-violet catastrophe
as we do the integration
Blackbody radiation and ultra-violet catastrophe
From Maxwell E.M. theory we know that a dipole oscillating with frequency n will on average emits energy r(n)
r(n) = const. n2
where
is the average energy of the oscillating dipole.
Rayleigh - Jean's Law
Maxwell-Boltzman distribution gives the energy state number density N(E) at energy E as
total number of possible energy state
and
Total energy
Hence the average energy:
r(n)
Rayleigh-Jeans
Blackbody radiation
UV catastrophe n
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33. This is called UV catastrophe.
Ultra-violet catastrophe
This result is in fact well known from kinetic theory where the energy of vibrational degree of freedom
The energy of rotational / bending degree of freedom
So now we have
r(n) = Const. n2T
Rayleigh-Jeans Law describes the beginning part of the blackbody radiation correctly at low frequency (long wavelength) but it obviously wrong at high n. As n increases r(n) increases without limit, i.e. At , as we do the integration , r(n), the radiate energy goes to infinity!
This is called UV catastrophe.
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34. Planck's derivation of blackbody radiation
Rayleigh - Jean's Law:
This integration assume continuous distribution of energy, i.e. the oscillator can take up any value.
Plank made the hypothesis that the oscillator will only take up discrete energies
0, E0, 2E0, 3E0, .....etc.
the average energy is obtained from a summation instead of integration.
Let
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35. Blackbody radiation http://surendranath.tripod.com/Applets.html
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36. Conclusion
Rayleigh – Jeans ‘s derivation is only valid at long wavelengths (low frequencies) and fails at short wavelength.
Planck made an assumption: energy emitted from the radiator at a frequency , E = nh, where n and integer, h the Planck constant (energy is discretized)
Implication: classical theory predicts energy is continuous
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