The Photoelectric Effect

What happens when light strikes the surface of a metal? The Photoelectric Effect can help to explain that. 

Examining the Photoelectric Effect

Concept: Understanding the Photoelectric Effect

Video Transcript

We're going to take a look what's called the photoelectric effect. We all know who Albert Einstein is. We know his theory of relativity, E equals mc2. We need to realize that he never won a Nobel Prize for that theory. What he won it for was the photoelectric effect.
We said that Planck and Albert Einstein both theorized that light was made up of what they called small packets of energy. It wasn't until later on that we moved away these packets of energy or quantums of energy to the more modern term photon. We're going to say a single particle of this quantized packet of electronic energy was later named a photon. Photon is just a light particle.
According to Einstein's photoelectric effect, when photons with enough energy strike the surface of a metal, they'll be able to knock electrons off of the surface of the metal. In that way, we use the word emitting. The metal is emitting electrons. Electrons are coming off the surface of the metal.
We're going to say the energy that's required to do this is directly proportional to our frequency and we know this because we said that frequency and energy are directly related. It's tied to the frequency rather than the amplitude. Remember, amplitude just measures the intensity of the light. If it has a high amplitude, it's very bright.
We're going to say the photoelectric effect only happens when photons with the certain threshold of frequency strike the surface of the metal. This is the photoelectric effect. It's just saying that if I have a photon with enough energy and I toss it towards the surface of the metal, it can knock off electrons off of the surface of that metal.

The Photoelectric Effect was theorized by Albert Einstein to help explain what would happen to the electrons on the surface of a metal when a photon (light particle) with enough energy struck. 

Concept: Illustrating what happens when a photon strikes the surface of a metal.

Video Transcript

Here we're going to illustrate how this happens. We have a photon here. We'll say it has not enough energy, so when it strikes the surface of the metal, nothing happens. We haven't reached the threshold frequency. But let's say we have another photon and this one is more energetic. It has way more energy. When it strikes the surface of this metal, the metal will start to emit or lose electrons.
Now, how does this work in real life applications? Here's what we have is a called a phototube and this basically puts to practice Einstein's theory of the photoelectric effect.What we're saying here is, we have incoming light. It's from photons. So they shine out and they're going to hit the surface of this metal here. What's going to wind up happening is this metal is going to start to release electrons. These electrons are then going to hit this wire here and this wire we call it an anode, a wire anode.
The electrons escaping, they're hitting that anode. Electrons are just seen as electricity. The electrons or electricity will travel down this wire to this part here called the voltmeter which reads out the amount of energy that's passing through it. Then we're going to say here, this portion here is called the photoelectric cathode.
A cathode is positively charged. Electrons are negatively charge, so it makes sense that the electron would travel through the anode, which is negatively charged, through the voltmeter where its read and then deposit itself on this white bar here, which is our cathode. This generates electricity.
It's this theory that Einstein used to prove the photoelectric effect.

The Photoelectric Effect Additional Practice Problems

It takes 261 kJ/mol to eject electrons from a certain metal surface. What is the longest wavelength of light (nm) that can be used to eject electrons from the surface of this metal via the photoelectric effect?

A) 233

B) 165

C) 552

D) 458

E) 725

Watch Solution

In an experiment of the photoelectric effect, an incident beam of ultraviolet radiation shined on a piece of metal and produced electrons with zero kinetic energy. Which of the following radiation would be most likely to produce electrons with some kinetic energy?

A. radio wave

B. microwave

C. green light

D. infrared

E. gamma ray

Watch Solution

The photoelectric effect provided evidence for the (wave, particle)-like behavior of light. The phenomenon of diffraction provided evidence for the (wave, particle)-like bahavior of light. 

1. wave, wave

2. wave, particle

3. particle, wave

4. particle, particle

Watch Solution

The work function for chromium metal is 4.37 eV. What wavelength of radiation must be used to eject electrons with a velocity of 2500 km/s?

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The carcinogen, CCl4 has been dumped into a holding pond. CClwill begin to decompose by the following reaction: 

     CCl4   → Cl3C•   + Cl•       ΔΕ for C—Cl bond = 327 kJ/mol


Sunlight striking the earth has a frequency range from approximately 6.7×10 14 Hz to 4.4×10 14  Hz.

If you assume the highest energy light must be the same energy of the Cl-Cl bond, can you expect sunlight alone to effect the decomposition?

h = 6.63×10 -34 J•sec; c = 3.00 ×10 8  m/sec



Watch Solution

Magnesium metal is used in a photoelectric effect experiment.  The work function for Mg (i.e. the energy required to remove an electron) is 3.68 eV.  What is the longest wavelength of light, in nm, that can remove an electron from the Mg.  

(1 eV = 1.602 x 10 -19 J; h = 6.63 x 10 -34 J•sec; c = 3.00x10 8 m/sec).


Watch Solution

The ionization energy of sodium is 496 kJ/mol. In an experiment similar Hertz's experiment to which Einstein famously discovered the photoelectric effect, which statement below is true about the wavelength of radiation that could cause sodium to be ionized upon irradiation?

A. If sodium is irradiated with radiation with a wavelength greater than 496 nm, sodium can become ionized

B. If sodium is irradiated with radiation with a wavelength greater than 241 nm, sodium can become ionized

C. If sodium is irradiated with radiation with a wavelength lower than 496 nm, sodium can become ionized

D. If sodium is irradiated with radiation with a wavelength lower than 241 nm, sodium can become ionized

E. Since radiation is not a particle, no form of it may cause an electron to be ejected from sodium

Watch Solution

The figure below shows Ek plotted as a function of v for photoelectric emission from two metals, A and B. Which of the following statements for the two metals is correct?

A. Both metals will produce the approximately equal currents under identical conditions.

B. The threshold frequencies suggest that metal A is an alkali metal.

C. The threshold frequency is the same for both metals.

D. The threshold frequency of A is greater than that of B.

E. The threshold frequency of A is less than that of B.

Watch Solution