Physics II Laboratory Manual
Updated: July,
January
2017
2018
Siliguri Institute of Technology
nd
B.Tech 21st
Year 1st
3rd Sem & 2nd
4th Sem
Department of CSE/IT/ECE/EE/CIVIL
Paper Name
Paper Code
: Physics II Laboratory
rd
: BS-PH191
PH391 (for
(for31stsem)
sem)
th
PH491
(for
sem)
BS-PH291
(for42nd
sem)
rd
Experiment No
: Quantum
PH391 -Physics
5 (for 3(02)sem)
PH491 - 5 (for 4th sem)
Experiment Name: Determination of Planck’s constant using photocell.
Purpose of the experiment:
To know about plank constant experimental setup & to determine the plank constant of a photocell for
different colour ray.
Basic Theory:
We know that all molecules are made of atoms which in turn contain nuclei and electrons. The
equations that govern the motions of electrons and of nuclei are not the familiar Newton equations, but
are the Schrodinger equations. When scientists first studied the behavior of electrons and nuclei they
tried to interpret the experimental findings in terms of classical Newtonian motions, but such attempts
eventually failed. They found
that such small light particles behaved in a way that simply is not consistent with the Newtonian
equations.
Kirchhoff’s Law: Emissive Power and Absorptive Power:
For a body of any arbitrary material emitting and absorbing thermal electromagnetic radiation at every
wavelength in thermodynamic equilibrium, the ratio of its emissive power to its dimensionless
coefficient of absorption is equal to a universal function only of radiative wavelength and temperature.
That universal function describes the perfect black-body emissive power.
Here, the dimensionless coefficient of absorption (or the absorptivity) is the fraction of incident light
(power) that is absorbed by the body when it is radiating and absorbing in thermodynamic equilibrium.
In slightly different terms, the emissive power of an arbitrary opaque body of fixed size and shape at a
definite temperature can be described by a dimensionless ratio, sometimes called the emissivity, the
ratio of the emissive power of the body to the emissive power of a black body of the same size and
shape at the same fixed temperature.
With this definition, Kirchhoff's law states:
For an arbitrary body emitting and absorbing thermal radiation in thermodynamic equilibrium, the
emissivity is equal to the absorptivity.
Black Body:
A black body is an ideal body which allows the whole of the incident radiation to pass into itself
(without reflecting the energy) and absorbs within itself this whole incident radiation (without passing
on the energy). The property is valid for radiation corresponding to all wavelengths and to all
wavelengths and to all angles of incidence. Thus blackbody is an ideal absorber of incident radiation.
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Physics II Laboratory Manual
Updated: July,
January
2017
2018
When black body is heated, it emits thermal radiations of different wavelengths or frequency (Fig:1).
From classical concepts Wien derived a law of energy distribution of blace body spectrum, which was
correct only for low temperature and shorter wavelength region. The total radiation spectrum could not
be explained on the basis of classical physics.
Fig:1
Planck’s Law: In 1900, Max Planck examined the entire situation critically and came forward with a
bold new postulate regarding the nature of the linear harmonic oscillator which remains in a state of
equilibrium with the radiant energy within the cavity of the black body and put forward a theory known
as Planck’s quantum theory, based on the following hypothesis:
•
•
•
The radiant energy which is emitted or absorbed by the black body is not continuous but
discontinuous in the form of small discrete packets of energy; each such packet of energy is
called a 'quantum'. In case of light, the quantum of energy is called a 'photon'.
The energy of each quantum is directly proportional to the frequency (γ) of the radiation, i.e.
∝ and is equal to hγ i.e = ℎ = . Where, h is Planck's constant.
The energies emitted or absorbed by the black body are discrete i.e. the energies are integral
multiple of a finite quantum of energies
=
i.e the energies are E = nhγ where n is an
integer.
Obviously the energy of the lowest state (the ground state) of the oscillator will be zero. It was also
assumed by Planck that the change in the emission of the radiant energy can also take place by a
discrete amount ℎ. The value of Planck constant is given by—
h = 6.626 × 10-34 Joule-second.
It is universal constant and it has crucial role to play in all quantum phenomena.
According to Planck’s law, the spectral radiation of a body gives the amount of energy it gives off as
radiation at different frequencies. It is measured in terms of the power emitted per unit area of the body,
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Physics II Laboratory Manual
Updated: January
2017
July, 2018
per unit solid angle that the radiation is measured over, per unit frequency and at any temperature T is
expressed as,
In terms of frequency,
d =
In terms of wavelength,
λdλ =
dν
dλ
λ! " #
Where, λ = wave length, ν = frequency, λ= spectral radiation within a wavelength range of λ and
$ + &$, ν= spectral radiation within a frequency range of ν and ν+&, k= Boltzmann constant, c=
velocity of light.
Photoelectric Effect:
The black body radiation experiment launches the research of quantization model. Through several
famous experiments, consolidates the importance of quantum mechanics. Among them, photoelectric
effect is one of the most famous experiments.
If a light strikes the surface of the metal and the wavelength of the light is longer than a specified
amount, there would be no electron emitted from the surface of the metal to induce the photo current
whatever the strength of the light is. This is the phenomenon of photoelectric effect that the classical
wave model can't explain. Einstein announced the theory of photoelectric effect in 19th century, that is,
explained the photoelectric effect in terms of the quantum model of light.
Assume there is a light wave with frequency ν. Each photon has energy E= hν, and the total energy of
the wave light is E total = nhν. Where n is the number of the photons. If the intensity of the light is
higher, it means the number of the photons is larger. When the energy of the photon is higher enough,
light strikes the surface of the metal, and then the photon would collide with the electrons in the atom,
so that the electrons would emit from the surface into the air. The kinetic energy of the emitted electron
is equal to the energy of the photon minus the electron attracted energy(φ) from the surface of the
metal. This energy φ is known as work function.
At the same time, we can imagine that the metal slab as the emitter in a vacuum tube, so that the
emitted electrons have kinetic energy ' () ' . Then the electrons can move towards the collector and
form the current, known as photo-current. However, when we add a voltage between the emitter and
the collector of the vacuum tube, the voltage would form an electric field between the emitter and the
collector, stop the electrons from moving towards collectors and decrease the photo-current. Therefore,
if the energy that the electrons received from the electric field is equal to the maximum kinetic energy
of the emitted electrons, the electrons can't arrive at the collectors, and then the photo-current would
become zero. At this time if the reverse voltage V0 then the potential eV0 is known as stopping
potential.
In a word, the energy of the photon = hν
the work function = φ
the stopping potential = eV0
the kinetic energy of the electron= ' () '
the charge of the electron= e
Hence, the variables have the relationship:
() ' = eV0 = hν −φ
'
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Physics II Laboratory Manual
Updated: July,
January
2017
2018
In this experiment, the energy of emitted photon can be measured by simple retardation potential
technique. We measure the retardation potential at which the photo current stops i.e we obtain the
stopping potential. Hence in this case, eV0 = hν
If L is the luminous intensity of an electric lamp and E is the luminescence (intensity of illumination) at
point “r” from it, then according to inverse square law, =
*
+,
If this light is allowed to face on the cathode of a photo-electric cell, then the photo-electric current (I)
would be proportional to E, i.e, =
Hence a graph between
+,
*
+,
= -. .
and I is a straight line, which verify the inverse square law of radiation.
Apparatus used:
•
The cover of the receiving dark-box-1
•
Drawtube– 2
•
Light source- 3
•
Guide (used to change the distance of the light source from the phototube)- 4
•
Scale- 5
•
Digital meter(displays the voltage and Photocurrent)- 6
•
Current multiplier- 7
•
Light intensity adjuster- 8
•
Accelerate voltage adjuster- 9
•
Voltage direction switch- 10
•
Power indicator- 11
•
•
Display mode switch (used to change display from voltage to photocurrent)- 12
Power switch- 13
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Physics II Laboratory Manual
Updated: July,
January
2017
2018
•
•
Colour filters (can be inserted in the drawtube to change the wavelength of the light)- 14
Vacuum phototube- 15
•
Focus lens- 16
•
Dark-box (box of the vacuum phototube)- 17
Procedure:
•
•
•
•
•
•
Insert the red colour filter (635 nm), set light intensity switch (12) at strong light, voltage
direction switch (14) at negative, display mode switch (10) at current display.
Adjust to de-accelerating voltage to 0 V and set current range selector (4) at X 0.001. Increase the
de-accelerating to decrease the photo current to zero. Take down the de-accelerating voltage (Vs)
corresponding to zero current of 635 nm wavelength. Get the Vs of other wavelengths, in the
same way.
The connection would be same as before except a positive voltage would be applied to the anode
with respect to cathode.
Place a filter in front of the photoelectric cell.
Keeping the voltage constant and position of photocell fixed increase the distance of lamp from
photo-cell in small steps. In each case note the position of the lamp r on the optical bench and the
current I.
The experiment may be repeated with other filters.
Precautions:
•
•
Clean the colour filter, lens etc, before performing the experiment.
After finishing the experiment remember to switch off power
Observation:
1) Determination of stopping voltage for different wavelengths:
Sl.
No.
Filters
Wavelength in
nm
1
Red
635
2
Yellow I
585
3
Yellow II
540
4
Green
500
5
Blue
460
ν (sec-1 X 1014)
Stopping Voltage (Vs)
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Physics II Laboratory Manual
Updated: January
2017
July, 2018
2) Verification of inverse square law:
Filter Colour = ………………..
Filter wavelength= …………………. nm
Anode voltage= ……………………V
Sl. No.
Distance between lamp and
photo-cell (r) in cm
1
18
2
20
3
22
4
24
5
26
6
28
7
30
8
32
1
× 103 4('
0'
Current (I) in µA
3) Calculation:
• Draw a graph of frequency(ν) Vs. Stopping Voltage (Vs)
∆6
•
Find the slope= ∆7
•
Find the value of h using the formula eVs = hν i.e, h=e x slope.
•
Draw a graph between
+,
and current I and prove the inverse square law of radiation.
4) Error:
(:;<=><?> @<ABC~ EFGC?@C> H<ABC)
Percentage error = 8
:;<=><?> @<ABC
8 × 100%
Sample Questions & Answers
1. What wavelengths are present in blackbody radiation?
• All wavelengths from zero to infinity.
2. Write down wein’s displacement law.
• Wien's displacement law states that the black body radiation curve for different temperatures
peaks at a wavelength inversely proportional to the temperature. The shift of that peak is a
direct consequence of the Planck radiation law which describes the spectral brightness of black
body radiation as a function of wavelength at any given temperature. However it had been
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Physics II Laboratory Manual
Updated: January
2017
July, 2018
discovered by Wilhelm Wien several years before Max Planck developed that more general
equation, and describes the entire shift of the spectrum of black body radiation toward shorter
wavelengths as temperature increases. Formally, Wien's displacement law states that the
spectral radiance of black body radiation per unit wavelength, peaks at the wavelength λmax
given by: λmaxT=b(Constant) where T is the absolute temperature in kelvin. b is a constant of
proportionality called Wien's displacement constant, equal to 2.897×10−3 mK,
3. What is a Black Body?
• A Body which neither reflects, transmits but absorbs the whole of the incident radiation is
called the black body.
4. Give Examples of Black Body.
• A Lamp – Blackened surface approximately behaves like a Black Body. There are some
specially designed black bodies like Ferry’s Black Body and Wien’s Black Body.
5. Is the intensity of the energy radiated by a Black Body uniformly distributed over the whole range
of its wavelength?
• No, According to Wien’s Law, the amount of energy dE contained in the spectral region
between $ + &$ emitted by a black body at absolute temperature T is given by
& = K$L M N⁄O &$ where A and a are constants.
6. Can you name some devices which can detect thermal radiation?
• Thermopiles, Bolometer, Boy’s radiometer
7. Define Photoelectric effect?
• When light falls on metal surface, an electron is emitted from a metal if the energy of the
photon is greater than the work function of the metal.
8. What is Reverse Photoelectric effect?
• If an electron of sufficient voltage is passed across a material then a photon is emitted whose
energy is equivalent to the work function of that material. The voltage at which this effect
observed is the ‘turn on voltage’. In case of LED reverse photoelectric effect works.
9. Can we observe reverse photoelectric with Metal surface?
• This effect is not normally observed in metals and other typical substances because the photons
emitted are usually outside the range of visible light, usually somewhere in the infrared Range.
10. What is the full form of LED?
• Full form of LED is Light Emitting Diode.
11. What is the standard value of h.
• h= 6.6x10-34 Joule / s
12. What is LED?
• A light-emitting diode (LED) is a semiconductor device that emits visible light when an electric
current passes through it.
13. Who found the fundamental theory ?
• In 1905, Einstein found fundamental theory which proposes that light is composed of photons
having energy quanta.
14. Who proposed discrete behavior for an object of subatomic dimensions?
• In 1990, Max Planck proposed.
15. What is light emitting diode?
• A light-emitting diode (LED) is a two-lead semiconductor light source. It is a p–n junction diode,
which emits light when activated.[4] When a suitable voltage is applied to the leads, electrons are
able to recombine with electron holes within the device, releasing energy in the form of photons.
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