Quantum Physics

what is a photon

a quantum (packet) of electromagnetic energy.

define the electron volt

THE Energy transferred to or from an electron when it moves through a potential difference of 1V

what does the term photoelectron mean

Emitted electrons

Describe how a gold leaf electroscope can be used to demonstrate the photoelectric effect

Briefly touching the top plate with the negative electrode from a high-voltage power supply will charge the electroscope.

Excess electrons are deposited onto the plate and stem of the electroscope. Any charge developed on the plate at the top of the electroscope spreads to the stem and the gold leaf. As both the stem and gold leaf have the same charge, they repel each other, and the leaf lifts away from the stem (Figure 3). If a clean piece of zinc is placed on top of a negatively charged and UV radiation shines onto the zinc surface , then the gold leaf electroscope has gradually lost its negative charge because the incident radiation ( in this case UV) has caused the free electrons to be emitted from the zinc. These electrons are known as photoelectrons

What are the key observations of the photoelectric effect Experiment

1)Photoelectrons were emitted only if the incident radiation was above a certain frequency (called the threshold frequency fo) for each metal. No matter how intense the incident radiation (how bright the light), not a single electron would be emitted if the frequency was less than the threshold frequency.

2 )If the incident radiation was above the threshold frequency, emission of photoelectrons was instantaneous.

3 )If the incident radiation was above the threshold frequency, increasing the intensity of the radiation did not increase the maximum kinetic energy of the photoelectrons. Instead more electrons were emitted. The only way to increase the maximum kinetic energy was to increase the frequency of the incident radiation.

Explain why the observations of the photoelectric experiment can not be explained by the wave model of light

These observations could not be explained using wave model of electromagnetic radiation. For example, if the threshold frequency for a particular metal is in the green part of the visible spectrum, bright red light does not cause emission, yet very dim blue light would. This does not fit with the wave model, in which the rate of energy transferred by the radiation is dependent on its intensity (brightness). The more intense the radiation, the more energy is transferred to the metal per second, and bright red light transfers more energy per second than dim blue light. Clearly a new model for electromagnetic radiation was needed to explain the observations.

What model can be used to explain the observations of the photoelectric effect

The photon model

How does the photon model explain the observation that photoelectrons are only emitted when the incident EM radiation is above the threshold frequency

EM radiation as a stream of photons rather than continuous waves. He suggested that each electron in the surface of the metal must require a certain amount of energy in order to escape from the metal, and that each photon could transfer its exact energy to one surface electron in a one-to-one interaction.

How does the photon model explain why increasing the frequency of radiation increases photoelectron KE, but increasing intensity does not?

As the energy of the photon is dependent on its frequency (E = hf),

if the frequency of the photon is too low, the intensity of the light - that is, the number of photons per second - does not matter,

How does the photon model explain why there is no time delay in photoelectron emission

As long as the incident radiation has frequency greater than, or equal to, the threshold frequency, as soon as photons hit the surface of the metal, photoelectrons are emitted. Electrons cannot accumulate energy from multiple photons. Only one-to-one interactions are possible between photons and electrons.

Define the work function

This is the minimum energy required to free an electron from the surface of the metal.

Measure in joules

Define the threshold frequency

Minimum frequency of radiation required to release an electron from the metal surface.

Measured in Hz

Explain why increasing intensity increase photoelectron emission if the frequency of the EM radiation is above the threshold frequency

Increasing the intensity of the radiation means more photons per second hit the metal surface. As each photon interacts one-to-one with a single surface electron, as long as the radiation has frequency above the threshold frequency for the metal, more photons per second means a greater rate of photoelectrons emitted from the metal.

The rate of emission of photoelectrons is directly proportional to the intensity of the incident radiation. Double the intensity and you double the number of photons per second, leading to a doubling in the number of electrons emitted from the metal per second.

Einsteins photoelectric effect equation is a statement of conservation of what?

energy

Why do some electrons require more energy than the work function to be liberated from the surface

Some electrons in the surface of the metal are closer to the positive metal ions than others. Their relative positions affect how much energy is required to free them. The work function is the minimum energy required to free an electron from the metal - most electrons need a little more energy than the work function to free them.

Why is it maximum KE in Einsteins equation

An electron that requires the minimum amount of energy to free it (the work function of the metal) would have the most energy left over from the incident photon. Only a few of the emitted photoelectrons have this maximum kinetic energy - most have a little bit less, and so travel a little slower.

What is the relationship between threshold frequency and work function

If a photon strikes the surface of the metal at the threshold frequency fo for the metal then it will only have enough energy to free a surface electron, with none left over to be transferred into kinetic energy of the electron.

In this case Einstein's photoelectric effect equation becomes

hfo = ф+ 0 or simply hf = ф

What experiment provides evidence for the particulate nature of light

the photoelectric experiment

What is meant by the phrase wave particle duality

A theory that states that matter has both particle and wave properties and also electromagnetic radiation has wave and particulate (proton) nature

why do we rarely see wave properties in larger particles

as particles become larger their wave properties become harder to observe. The mass of individual protons is much greater than electrons, so at the same speed their momentum is significantly greater and therefore their wavelength is much smaller, and much harder to observe.

Why do we not normally observe the wave nature of electrons

We do not normally notice the wave nature of electrons because we need a tiny gap in order to observe electrons diffracting. For diffraction to occur the size of the gap through which the electrons pass must be similar to their wavelength.

Describe an experiment that can be used to demonstrate the wave (and particle) nature of electrons

If an electron gun fires electrons at a thin piece of polycrystalline graphite, which has carbon atoms arranged in many different layers, the electrons pass between the individual carbon atoms in thei graphite. The gap between the atoms is so small that it is similar to the wavelength of the electrons and so the electrons diffract, as waves and form a diffraction pattern seen on the end of the tube

This experiment beautifully demonstrates both the particle and wave nature of electrons. They are behaving as particles when they are accelerated by the high potential difference, they behave as waves when they diffract and then they behave as particles as gain as they hit the screen with discrete impacts