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Photoelectric Effect in Our Daily Lives
Photoelectric Effect in Our Daily Lives
Quantum mechanics was developed in the early twentieth century to explain experimental observations that could not be explained by classical physics.
Ultraviolet catastrophe
In the 1890s, with the recent invention of the light bulb, German engineering companies recognised its potential. However the physics behind the concept of transforming electricity into light was not well known, especially the connection between the temperature of the bulb and its colour. The video clip explains the problem and how the German physicist Max Planck addressed it.
The black line on the graph below shows the relationship between intensity and wavelength of a 5000 K black body as predicted from classical physics theory. The blue line shows the experimental data for the same temperature - with the absence of the lower, ultra-violet wavelengths. This was the 'ultra-violet catastrophe'.
Planck discovered that he could solve the ultra-violet problem by making the assumption that the emission of raditaion from a black body could only happen in jumps. The jumps would be multiples of an amount of energy that could be calculated from the frequency of the light.
The Planck relationship between frequency of light f and its energy E can be written as,
E=hf
Where h is known as Plank’s constant.
The value of h is 6.63 x 10-34 J s. (In the data booklet)
The term quantum was introduced for these energy values. Initially Planck didn't believe that radiation was actually emitted in bundles of energy, but rather that his equation was simply a calculation of convenience.
The photoelectric effect
In 1887 Heinrich Hertz observed that a spark passed between two plates more often if the plates were illuminated with ultraviolet light. Later, experiments by Wilhelm Hallwachs and Philipp Lenard gave unexpected observations that are now known as the photoelectric effect.
Now observe the photoelectric effect for yourself. First watch the video clip below which explains how to carry out the demonstration. Pay particular attention to the instructions about how to charge the plate of the electroscope by induction.
The apparatus in our physics lab is shown in the diagrams below. First look at the effect of visible light shining on the zinc plate.
Then observe the effect of UV light shining on the plate.
The general instructions are:
1. Clean the zinc plate.
2. Charge the electroscope negatively as shown in the video clip using an acetate rod.
3. Shine visible light from a lamp on the zinc plate.
4. Shine UV light from a mercury vapor lamp on the zinc plate.
In the gif below I switch on the UV lamp next to the zinc plate and the needle of the electroscope can be seen to fall immediately.
Light as a wave
Since the early 1800s it had been accepted that light was a wave. The diffraction and interference of light through two slits as demonstrated by Thomas Young was evidence that light was indeed a wave. You have witnessed the same evidence in the activities with lasers when interference patterns were observed when the laser light passed through slits and gratings. like the images below.
Light as a particle - quanta
The observations of the photoelectric effect however did not fit with the idea of light being a wave. Which prompted 2 questions:
Why is there no emission of electrons from the surface of the metal when low frequency, but high intensity light is used e.g. very bright red light?
Why is there an increase in the speed of the emitted electron with frequency but not with intensity. Increasing the intensity only produces more emitted electrons?
These observations were unexpected because energy should have been able to be absorbed continuously from a wave. An increase in the intensity of a wave also means an increase in amplitude and hence a larger energy. The puzzle was why energy is not absorbed continuously from such a wave, e.g. any electromagnetic radiation, in a cumulative manner. It should just take more time for energy to be absorbed and for the electron to be emitted but this is not what is observed.
A solution to this puzzle was offered in 1905 when Albert Einstein published a paper on the photoelectric effect entitled “On a Heuristic Viewpoint Concerning the Production and Transformation of Light”. He received the Nobel Prize for Physics for this work in 1921.
Photons
Einstein proposed that electromagnetic radiation is emitted and absorbed in small packets. (The word ‘photon’ was unintentionally introduced by the physical chemist Gilbert Lewis in 1926). The energy of each photon is given by the equation Planck discovered
E = hf
This proposal also explained why the number of electrons emitted depended on the intensity (i.e. number of photons) of the electromagnetic radiation and why the velocity of the emitted electrons depended on the frequency. A summary of this effect is shown in the gif.
PhET Simulation
PhET Simulation
Set the metal to sodium – the most reactive of the metals given.
Set the intensity of the radiation to a set value e.g. 100%.
Adjust the wavelength of the radiation until a current begins to flow in the circuit. This will happen somewhere in the blue part of the visible spectrum. The ejected electrons now have kinetic energy.
Then apply a voltage to counteract the current. When the current reading is zero then the voltage is known as the stopping voltage. In the example shown the stopping voltage is 0.40 V.
Continue to change the wavelength of the radiation from visible into ultra violet and record the stopping voltage for each wavelength.
At the stopping voltage the electric potential energy eV is equal to the maximum kinetic energy Emax of the photoelectrons, eV = Emax.
Calculate the maximum kinetic energy of the electrons from the product of the stopping voltage and the charge of an electron.
Calculate the frequency of the radiation using the speed of light in a vacuum divided by the wavelength in metres.
Plot a graph of maximum kinetic energy against frequency.
The gradient of the straight line graph is Planck’s constant and the x-intercept is the work function of sodium.
Photoelectric Theory Recap
Photoelectric Theory Recap
The photoelectric effect was discovered in 1887 when Heinrich Hertz discovered that electrodes emitted sparks more effectively when ultraviolet light was shone on them. We now know that the particles are electrons, and that ultraviolet light of sufficiently high frequency (which varies from element to element) causes the electrons to be emitted from the surface of the element:
The photoelectric effect requires light with a sufficiently high frequency, because the frequency of the light is related to the amount of energy it carries. The energy of the photons needs to be above a certain threshold frequency in order to have enough energy to ionize the atom.
For example, a minimum frequency of 10.4x10^14 Hz is needed to dislodge electrons from a zinc atom:
The maximum kinetic energy of the emitted electron is equal to Planck’s constant times the difference between the frequency of incident light (f) and the minimum threshold frequency of the element (f_0):
The quantity (h f_0) is called the “work function” of the atom, and is denoted by the variable φ. Thus the kinetic energy equation can be rewritten as:
Values of the work function for different elements range from about 2.3–6 eV. (1 eV=1.6x10^-19 J)
In 1905, Albert Einstein published a paper explaining that the photoelectric effect was evidence that energy from light was carried in discrete, quantized packets. This discovery, for which Einstein was awarded the Nobel prize in physics in 1921, led to the birth of the field of quantum physics.
Putting all the Pieces together:
Putting all the Pieces together:
Photoelectric Resources/Problem Set:
Photoelectric Resources/Problem Set:
Topic_12_-_Activity_1_-_Notes.pdfAdapted from Mr. Gordon McNeil, ISD
Make a copy HERE.
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