29.2 The Photoelectric Effect

It is possible to determine the type of gas and its temperature.

Atomic spectrum is an important analytical tool.
One of the most important characteristics is that they are not in a straight line.
We mean that only certain frequencies are emitted.

A line spectrum is what this is.

The electrons in the emitting atoms and molecules have their frequencies and energy quantized.
This is discussed later in the chapter.

When an electrical discharge is passed through a substance, its atoms and molecules absorb energy.

The emission's nature implies that the energy states of the atoms are quantized.
Before it was understood why they are quantized, atomic spectrums were used as analytical tools.

It was a big puzzle that atomic spectrums are quantized.

The best minds of 19th-century science didn't explain why this might be.
Not until the second decade of the 20th century did an answer based on quantum mechanics emerge.

The effect of the studies is due to individual atoms and molecule, not a classical body of gas.

Shoot light at the atom.

The model predicts the experimental results.

Light can remove electrons from materials.

Light meters that adjust the automatic iris on various types of cameras are a common use of the photoelectric effect.
You probably have seen solar cells on a roof top or on a roadside sign, but in a similar way.
The photoelectric effect can be used to convert light into electricity.

Light can be seen falling on a metal plate in this evacuated tube.

The collector wire is used to collect the electrons that are ejected by the light.
The energy of the ejected electrons can be determined by adjusting the retarding voltage between the collector wire and plate.
No electrons will reach the wire if it is sufficiently negative.

This effect has been known for more than a century and can be studied using a device such as that shown in figure, which shows an evacuated tube with a metal plate and a collector wire that are connected by a variable voltage source, with the collector more negative than the plate.

electrons may be ejected when light strikes the plate in the evacuated tube.
Some electrons will be collected on the wire if the energy in the electrons is greater than the potential difference between the plate and the wire.
The electron energy in eV can be measured by adjusting the retarding voltage between the wire and the plate.
The energy in eV is equal to the voltage that stops the electrons from reaching the wire.
The electrons have an energy of 3.00 eV.
The number of electrons ejected can be determined by the current between the wire and plate.
This device can be used as a light meter if it has enough light.

Albert Einstein deduced from the photoelectric effect.

Einstein realized that there were several characteristics of the photoelectric effect that could only be explained by the quantization of the radiation.

The energy of a photon of frequencies is Planck's constant.

This revolutionary idea is quite different from the other one.
The quantization of radiation is what it is.

Their energy is absorbed and released into the air.

This is consistent with the quantization of energy levels in blackbody oscillators, since they increase and decrease their energy in steps.
We don't observe this with our eyes because we don't know how many individual photons are in common light sources.
There is a discussion in the next section of the text about the characteristics and implications of photons.
Einstein used the photon concept to explain the photoelectric effect.

A wave of frequencies is composed of quanta of radiation.

The higher the intensity, the higher the number of photons per unit area.

The flashlight emits many different types of light waves.

The properties of the photoelectric effect are discussed.

All of these properties are consistent with the idea that individual electrons in a material absorb the photon's energy.
Some of the properties are not consistent with the idea of a simple wave.
Let's take a look at what happens when all of the photons have the same energy.

There is a threshold Frequency for the EM radiation below which no electrons are ejected, regardless of intensity.

No electrons will be ejected if the photon energy is too small.
If the wave was simple, enough energy could be obtained by increasing the intensity.

electrons are ejected from a material without delay.

The electron is ejected when an individual photon is absorbed by an individual electron.
It would take several minutes for enough energy to be deposited to the metal surface to cause an electron to be ejected.

The number of electrons ejected per unit time is determined by the intensity of the radiation.

High-intensity EM radiation consists of large numbers of photons per unit area with all of them having the same characteristic energy.

The maximum energy of ejected electrons is not dependent on the intensity of the EM radiation.

Increased intensity results in more electrons of the same energy being ejected.

The higher the intensity of the radiation, the higher the electrons would be ejected.

The photon energy of an ejected electron is the same as the binding energy of the electron in the material.

A photon can give all of its energy to an electron.
The electron is broken away from the material by the photon's energy.
The ejected electron's energy goes into the remainder.

The properties of the photoelectric effect are explained in this equation.

An individual photon of EM radiation interacts with an individual electron, providing enough energy, BE, to break it away, with the remainder going to kinetic energy.
Where is the threshold frequency for the material?

There is a graph of the energy of an electron that is ejected.

The individual photon interacting with an individual electron has insufficient energy to break it away, so no electrons are ejected.
Above the threshold energy increases linearly.
The data can be used to determine the constant.
The first successful explanation of the data was given by Einstein.

The beginnings of quantum mechanics can be traced back to Einstein's idea of quantized radiation.

The explanation of the photoelectric effect might imply that it is a general concept.
The characteristics of EM radiation are consistent with this fact and can be modeled in the form of photons.
The same year Einstein published his first paper on special relativity, he planted an important seed for quantum mechanics.
The basis for the prize was his explanation of the photoelectric effect.
Although his other contributions to theoretical physics were also noted in that award, special and general relativity were not fully recognized in spite of having been partially verified by an experiment by 1921.
Although hero-worshipped, this great man never received a prize for his most famous work.

The energy of a photon is given by.

Once the photon's energy is calculated, it is easy to find the ejected electron's maximum energy since BE is given.

The equation can be used to find the ejected electron's energy.

Humans are more sensitive to energies on the order of joules than a single photon of violet light would be.

We can see that the photon has enough energy to affect atoms and Molecules.
A DNA molecule can be broken with 1 eV of energy, so that the UV photon in this example could have biological effects.
The ejected electron has a low energy and would not travel far unless in a vacuum.
A retarding potential of 0.26 eV would stop the electron.
The formula would give a negative energy if the photon wavelength was longer and the energy was less.

This simply means that the 420-nm photons with their 2.96-eV energy are not much above the threshold.

You can show the threshold wavelength to yourself.
If calcium metal is used in a light meter, it will be less sensitive to blue light than it is to the other way around.
The light meter would not be affected by red light.

You can see how light knocks electrons off a metal target.