Photons and the photoelectric effect IB DP Physics Study Notes - 2025 Syllabus
Photons and the photoelectric effect IB DP Physics Study Notes
Photons and the photoelectric effect IB DP Physics Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Physics syllabus with Students should understand
that energy is released in spontaneous and neutron-induced fission
the role of chain reactions in nuclear fission reactions
the role of control rods, moderators, heat exchangers and shielding in a nuclear power plant
the properties of the products of nuclear fission and their management.
Standard level and higher level: 4 hours
Additional higher level: There is no additional higher level content.
- IB DP Physics 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Physics 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Physics 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Physics 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
The quantum nature of radiation
∙Back in the very early 1900s physicists thought that within a few years everything having to do with physics would be discovered and the “book of physics” would be complete.
∙This “book of physics” has come to be known as classical physics and consists of particles and mechanics on the one hand, and wave theory on the other.
∙Two men who spearheaded the physics revolution which we now call modern physics were Max Planck and Albert Einstein.
∙To understand Planck’s contribution to modern physics we revisit black- body radiation and its characteristic curves:
∙Recall Wien’s displacement law which gives the relationship between the wavelength and intensity for different temperatures.
FYI
∙Note that the intensity becomes zero for very long and very short wavelengths of light.
∙Blackbody radiation gave Planck the first inkling that things were not as they should be.
∙As far as classical wave theory goes, thermal radiation is caused by electric charge acceleration near the surface of an object.
FYI
∙Recall that moving electric charges produce magnetic fields. Accelerated electric charges produce electromagnetic radiation, including visible light.
∙According to classical wave theory, the intensity vs. wavelength curve should look like the dashed line:
∙For long wavelengths the predicted and observed curves match up well.
∙But for small wavelengths, classical theory fails.
FYI
∙The failure of classical wave theory with experimental observation of blackbody radiation was called the ultraviolet catastrophe.
∙In 1900, the UV catastrophe led German physicist Max Planck to reexamine blackbody radiation.
∙Planck discovered that the failure of classical theory was in assuming that energy could take on any value (in other words, that it was continuous).
∙Planck hypothesized that if thermal oscillators could only vibrate at specific frequencies delivering packets of energy he called quanta, then the ultraviolet catastrophe was resolved.
EXAMPLE:
Using Planck’s hypothesis, show that the energy \( E \) of a single quantum with frequency \( f \) is given by
\[ E = \frac{hc}{\lambda}. \]
Find the energy contained in a single quantum of light having a wavelength of \( 500 \, \text{nm}. \)
SOLUTION:
From classical wave theory, \( v = \lambda f \).
But for light, \( v = c \), the speed of light.
Thus \( f = \frac{c}{\lambda} \) and we have \( E = hf = \frac{hc}{\lambda} \).
\[
E = \frac{hc}{\lambda}
\]
For light having a wavelength of \( 500 \, \text{nm} \), we have:
\[
E = \frac{hc}{\lambda} = \frac{(6.63 \times 10^{-34})(3.00 \times 10^8)}{500 \times 10^{-9}} = 3.98 \times 10^{-19} \, \text{J}.
\]
∙According to Planck’s hypothesis, thermal oscillators can only absorb or emit light in chunks which are whole-number multiples of E.
∙Max Planck received the Nobel Prize in 1918 for his quantum hypothesis, which was used successfully to unravel other problems that could not be explained classically.
∙The world could no longer be viewed as a continuous entity – rather, it was seen to be grainy.
FYI
∙The Nobel Prize amount for 2017 was 1.1 million USD at the time of its announcement.
The photoelectric effect
∙In the early 1900s Albert Einstein conducted experiments in which he irradiated photosensitive metals with light of different frequencies and intensities.
∙In 1905 he published a paper on the photoelectric effect, in which he postulated that energy quantization is also a fundamental property of electromagnetic waves (including visible light and heat).
∙He called the energy packet a photon, and postulated that light acted like a particle as well as a wave.
Certain metals are photosensitive – meaning that when they are struck by radiant energy, they emit electrons from their surface.
∙In order for this to happen, the light must have done work on the electrons.
FYI
∙Perhaps the best-known example of an application using photosensitive metals is photocopy machine.
∙Light reflects off of a document causing a charge on the photosensitive drum in proportion to the color and intensity of the light reflected.
∙Einstein enhanced the photoelectric effect by placing a plate opposite and applying a potential difference:
∙The positive plate attracts the photo- electrons whereas the negative plate repels them.
∙From the reading on the ammeter he could determine the current of the photoelectrons.
∙If he reversed the polarity of the plates, Einstein found that he could adjust the voltage until the photocurrent stopped.
∙The top plate now repels the photoelectrons whereas the bottom plate attracts them back.
∙The ammeter now reads zero because there is no longer a photocurrent.
∙The experimental setup is shown:
∙Monochromatic light of fixed intensity is shone into the tube, creating a photocurrent Ip.
∙Note the reversed polarity of the plates and the potential divider that is used to adjust the voltage.
∙Ip remains constant for all positive p.d.’s.
∙Not until we reach a p.d. of zero, and start reversing the polarity, do we see a response:
We call the voltage –V0 at which Ip becomes zero the cutoff voltage.
∙Einstein discovered that if the intensity were increased, even though Ip increased substantially, the cutoff voltage remained V0.
FYI
∙Classical theory predicts that increased intensity should produce higher Ip.
∙But classical theory also predicts that the cutoff voltage should change when it obviously doesn’t.
Einstein also discovered that if the frequency of the light delivering the photons increased, so did the cutoff voltage.
∙Einstein noted that if the frequency of the light was low enough, no matter how intense the light no photocurrent was observed. He termed this minimum frequency needed to produce a photocurrent the cutoff frequency f0.
∙And finally, he observed that if the frequency was above f0, even if the intensity was extremely low, the photocurrent would begin immediately.
∙Einstein found that if he treated light as if it were a stream of particles instead of a wave that his theory could predict all of the observed results of the photoelectric effect.
∙The light particle (photon) has the same energy as Planck’s quantum of thermal oscillation:
∙Einstein defined a work function φ which was the minimum amount of energy needed to “knock” an electron from the metal. A photon having a frequency at least as great as the cutoff frequency f0 was needed.
∙If an electron was freed by the incoming photon having energy E = hf, and if it had more energy than the work function, the electron would have a maximum kinetic energy in the amount of
∙Putting it all together into a single formula:
The wave nature of matter
∙The last section described how light, which in classical physics is a wave, was discovered to have particle-like properties.
∙Recall that a photon is a discrete packet or quantum of energy (like a particle) having an associated frequency (like a wave). Thus
is just a statement of the wave-particle duality of light.
∙Because of the remarkable symmetries observed in nature, in 1924 the French physicist Louis de Broglie proposed that just as light exhibited a wave-particle duality, so should matter.
The de Broglie hypothesis is given in the statement:
“Any particle having a momentum \( p \) will have a wave associated with it having a wavelength \( \lambda \) of \( \frac{h}{p}.”
In formulaic form, we have:
With de Broglie’s hypothesis, the particle-wave duality of matter was established.
FYI: At first, de Broglie’s hypothesis was shunned by the status quo. Then it began to yield fruitful results…
The de Broglie hypothesis
∙Recall that diffraction of a wave will occur if the aperture of a hole is comparable to the wavelength of the incident wave.
∙In 1924 Davisson and Germer performed an experiment which showed that a stream of electrons in fact exhibit wave properties according to the de Broglie hypothesis.
FYI
∙For small apertures crystals can be used. Crystalline nickel has lattice plane separation of 0.215 nm.
The de Broglie hypothesis
∙By varying the voltage and hence the velocity (and hence the de Broglie wavelength) their data showed that diffraction of an electron beam actually occurred in accordance with de Broglie!