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THE IB SAYS
THE IB SAYS
Essential idea: The microscopic quantum world offers a range of phenomena, the interpretation and explanation of which require new ideas and concepts not found in the classical world.
Understandings: Photons; the photoelectric effect
Applications and skills: Discussing the photoelectric effect experiment and explaining which features of the experiment cannot be explained by the classical wave theory of light; solving photoelectric problems both graphically and algebraically
Guidance: -
International-mindedness: -
Utilization: -
Data Booklet: E = hf; E(max) = hf - φ
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My vodcasts
The science video
The question video
The first use of E= hf
The first use of E= hf
By the early twentieth century the different intensities of EM radiation emitted at different wavelengths by objects of different could be measured, but the then current best theory, the Rayleigh-jeans law, failed miserably to make accurate predictions, particularly for emissions in the ultraviolet.
A more accurate prediction, developed by Max Planck in 1900, already existed, however it made the very unusual assumption that the light energy was quantized, i.e. it only exists in packets whose energy was proportional to their frequency, E = hf. This was the first prediction of photons but was note widely accepted at first.
The photoelectric effect
The photoelectric effect
During the 19th century physicists had noticed the certain metals produced emissions (later revealed to be electrons) when illuminated by ultraviolet light. However they lacked a theory that was able to accurately predict and explain two problems: why did this instantaneous ionisation effect only took place when a metal was exposed to a certain or higher frequency of light (the threshold frequency), and why, below that frequency, there was no emission no matter how intense the illumination was.
In 1905 Albert Einstein proposed a solution that used Max Plank's idea of quantised packets of light energy, thus providing strong experimental support for Planck's theory and earning the Nobel Prize for Physics in 1921.
What Einstein realised was the light existed in these photon packets, whose energy was related to the frequency of light by Max Planck's formula, E = hf, and that each photon interacts with just one electron. Each electron is held to the surface of the metal and requires a certain amount of energy, called the work function, to be released.
If the photon does not have at least the threshold frequency then it does not have enough energy to liberate an electron. Increasing the intensity of light increases the number of photons, but it doesn't matter how many photons you have if they don't have enough energy.
If the photon DOES have enough energy then the electron (known as a photoelectron) is emitted from the surface. Any excess energy is passed on to the photoelectron as kinetic energy.
Millikan's experiment
Millikan's experiment
In 1912-1916 Robert Millikan conducted a series of experiments in which he used Einstein's theory to make an experimental determination of Planck's constant, h.
He placed the metal plate in a vacuum and added a collector plate and a power supply. By placing an electric field between the emitter and the collector he was able to either accelerate or slow-down the emitted photoelectrons. When photoelectrons were created he could determine the potential difference required to stop the photoelectrons (the stopping potential).
By graphing the stopping potential need for different frequencies of light Millikan was able to calculate the value of Plank's constant.
Wave-particle duality
Wave-particle duality
There is significant evidence that light is a wave. Everything in topic 4 (Waves) and Topic 9 (Wave phenomena) indicates this, including such concepts as superposition, interference and diffraction. Yet the photoelectric effect can only be explained by light acting as a stream of particles. The solution is that light is both, and neither. Hence wave-particle duality.
Additional videos
Additional videos
Excellent crash course covering this topic
A demonstration of the photoelectric effect
Wave-particle duality and the photoelectric effect (no mathematics)
A focus on solving problems
Resources from textbooks
Resources from textbooks
Oxford Physics (2014): pages 476 - 480
Hamper HL (2014): pages 282 - 285
Relevant questions from Dot Point Physics (13 Quantum Physics and Nuclear Physics)
Relevant questions from Dot Point Physics (13 Quantum Physics and Nuclear Physics)
Pages 171 - 186
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