Photoelectric Effect

Directions: Log into Gizmos via LCPSgo to go through the simulation. Respond to the questions and prompts in the orange boxes.

Activity A: Wavelength and flux

Get the Gizmo ready: Check that theVoltage is 0.0 volts andPotassiumis selected.

Introduction: Through the centuries, many scientists have debated whether light is a wave or a stream of tiny particles. In the 1800s, most scientists agreed that phenomena such as refraction and diffraction supported the "light as a wave" theory. However, Albert Einstein's explanation of the photoelectric effect showed that light can act like a stream of particles as well.

Question: What factors affect the ability of light to free electrons from a metal surface?

1. Observe: ClickFlash the light with a variety ofwavelength values. What do you notice?

I noticed that the lower I change the wavelength value it bounces back but when I increase it it doesn't bounce back.

1. Observe: Thephoton flux is a measure of how bright the light is. It is equal to the number of photons that are released in a given time. It is given as photons () per millisecond (ms).

ClickFlash the light with a variety ofPhoton flux values. What do you notice?

I noticed that when I increase the number more photons will bounce back and when I decrease it less photons bounces back.

1. Form hypothesis: Answer the following questions based on what you have observed so far.

Which factor determines how many photons will strike the metal?

Wavelength

Explain:

The more I increase the number the lower the amount bounces back.

Which factor determines how much energy each photon has?

Photon flux/ frequency

Explain:

The higher the frequency, the more energy the photon has.

1. Investigate: Set thePhoton fluxto 1 /ms. Use the Gizmo to find the longest wavelength that will dislodge an electron from the metal surface. What is this wavelength?

530 nm

1. Predict: Set theWavelengthto 540 nm. What do you think will happen if you flash the light with a photon flux of 1 /ms? What if you flash the light with a flux of 10 /ms?

I think that with a photon of 10 it will bounce.

1. Test: ClickFlash the light with aPhoton flux of 1 /ms and again with a flux of 10 /ms.

What happened?

The same will happen when the photon is on 1 and 10. Both will go right through it and won't bounce.

1. Explore: Set theWavelength to 400 nm. Experiment with different photon fluxes.

Does the photon flux affect how many electrons are emitted?

Yes it does

Explain:

The higher the photon the higher the electrons are emitted

Does the photon flux affect the energy (speed) of the emitted electrons?

No it doesn't

Explain:

The speed of the electrons travel at the same speed.

1. Infer: For mechanical waves, such as sound waves or ocean waves, increasing the intensity of the wave increases both the amplitude (height) of the wave and the energy it carries. In that situation, a low-frequency but high-intensity wave should have the same effect as a high-frequency but low-intensity wave. How does light behave differently from this model?

1. Think and discuss: How is firing photons at the surface of a metal analogous to rolling different types of balls at a set of bowling pins? If possible, discuss your answer with your classmates and teacher.

Activity B: Voltage gradients

Get the Gizmo ready: Set theWavelengthto 300 nm, thePhoton flux to 10 /ms, and theVoltageto 0.0 volts. Turn onShow voltage gradient.

Introduction: The electrons that are freed from the surface of the metal have a specific amount of kinetic energy. Faster electrons have greater energies than slower ones. The energy of emitted electrons is measured by setting up an electrical field that opposes their motion. Thevoltage of the field is a measure of its strength.

Goal: Use a voltage gradient to measure the energy of emitted electrons.

1. Observe: Check thatPotassiumis selected. ClickFlash the light and observe the emitted electrons. Increase theVoltage to 1.5 volts, and clickFlash the light again.

How does the electrical field affect the motion of the emitted electrons?

1. Measure: The energy of an emitted electron is measured inelectron volts (eV). An electron with an energy of 1 eV can overcome an electrical field of 1 volt. In the Gizmo, increase the voltage until you find the highest voltage that still allows the electrons to reach the light bulb.

What is this value?

This is equal to the energy of the emitted electrons in eV.

1. Gather data: With theWavelengthset to 300 nm, measure the energy of emitted electrons for potassium, calcium, and uranium. Then measure the same values with wavelengths of 250 nm and 200 nm to complete the table.

Element	Energy of emitted electrons (eV)
300 nm	250 nm	200 nm
Potassium
Calcium
Uranium

1. Analyze: What patterns do you notice in your data?

Infer: Based on your data, which element is hardest to extract electrons from?

Explain:

Activity C: Work functions

Get the Gizmo ready: Set theVoltage to 0.0 volts and selectPotassium. You will need a calculator and a copy of the periodic table of the elements for this activity.

Introduction: It is easier to remove electrons from some elements than others. The energy required to free an electron from the surface of a solid is thework functionof the element.

Question: How much energy is required to liberate electrons from a material?

1. Predict: In general, the difficulty of removing electrons increases from left to right across each row of the periodic table. Look up potassium (K), calcium (Ca), and uranium (U). Based on their positions in the periodic table, which of these elements do you expect to have the lowest work function? Which element will have the highest work function?

Lowest work function:

Highest work function:

1. Gather data: Use the Gizmo to determine the highest wavelength for each element that still removes electrons. Fill in the first column below. (Leave the other columns blank for now.)

Element	Wavelength (nm)	Frequency (Hz)	Work function (eV)
Potassium
Calcium
Uranium

1. Calculate: Thefrequency of a wave, measured in hertz (Hz), is the number of waves that passes a point each second. To calculate the frequency (f) of an electromagnetic wave, divide the speed of light (c) by the wavelength ():

The speed of light is 299,792,458 m/s, or approximately 3.0 10¹⁷ nm/s. Using the equation, calculate the frequency of each wavelength given in the table. Fill in the second column.

1. Calculate: The energy of a photon depends on its frequency. The energy of a photon (E) in electron volts is equal to its frequency (f) multiplied by Planck's constant (h):

E (eV) =hf

In this calculation,h is equal to 4.136 10^-15 eVs. Calculate the work function of each element in the table above. (Note: The values in your table are approximations.)

1. Draw conclusions: Based on the calculated work function for each element, which element holds onto its electrons most tightly? Explain.

1. Think and discuss: When the photoelectric effect was discovered, scientists were surprised that low-frequency light was unable to remove electrons, even when the light was very bright In other words, scientists expected the low frequency to be offset by the light's brightness.

How does thinking about light as a stream of particles, rather than a single wave, explain this result? If possible, discuss your answer with your classmates and teacher.

Activity D: The Quantum Side of some modern technology

Watch the short videos and answer the questions.

Semiconductors (Transistors and Diodes)

https://youtu.be/IcrBqCFLHIY

1. What is "doping" in terms of solid state electronics and why is it necessary?

1. What is a hole?

1. What are the parts of a transistor?

LED (Light Emitting Diodes) and Solar cells (Electrovoltaic Cells)

Minute Physics: How Modern Light Bulbs Work

1. What are the types of modern light bulbs discussed in this video?

More specifics on LEDs here (make sure to watch the semiconductor video first)

How does an LED work?

1. What is the difference between an LED and a "regular" diode in terms of what it emits?

1. Based on your understanding of EM waves and practice with the photoelectric effect, why do you think that LEDs are distinct colors?

P-N junction solar cells While you finish, queue up your favorite Electronica Playlist to provide ambience. I recommend deadmau5 Arguru 2k19

1. What similarities do you notice between LEDs and Solar Panels?

1. What isphotogeneration of charge carriers and how is it similar to the photoelectric effect?