Photon frenzy

What is a photon? How does the quantization of light affect the way light interacts with matter?

Electromagnetic waves are waves that result from oscillations between electric and magnetic fields. These waves can propagate through a vacuum and travel at the speed of light. Radio waves, microwaves, x-rays, and light are all forms of electromagnetic waves. The type of wave depends on its wavelength, with visible light having wavelengths 400 - 700 nm. This is less than 1/100 of the width of an average human hair.

Quantum mechanics is used to describe systems that are very small. In this theory, physical systems are quantized, forcing them to exist in discrete states. This contrasts with classical physics, where states are continuous. For example, quantities such as momentum and energy, which we think of as continuous variables, can only take on a set of discrete values in quantum mechanics. The quantized states in large, macro-scale systems are very close to each other, which makes them look like continuous systems. For smaller, quantum-scale systems, this discrete spacing becomes much more important, resulting in significant, observable effects. The quantized nature of light emitted from Cdots was explored in Activity 2 of this set.

Another consequence of quantum mechanics is that waves, which carry energy, are also quantized as discrete particles. For electromagnetic waves, these quantized particles are called photons. Like the classical wave description of light, photons also have a wavelength, which determines their color. Red light has a longer wavelength, while blue light has a shorter wavelength. Each photon carries a discrete set of energy, which is determined by its wavelength: shorter wavelengths have a higher energy. A fixed color light source’s brightness is adjusted by varying the number of photons emitted.

When photons hit matter, they can knock electrons loose and cause a current to flow. This effect is known as the photoelectric effect. (Fun Fact: Einstein received the Nobel Prize for theorizing the photoelectric effect.) Because electron energies are quantized in matter, a photon must have enough energy to raise the electron to the next energy level; otherwise, it does not interact with the electron. It was this observation that lower energy photons were unable to create a current that led to the discovery that light is quantized. The experiment that we are performing today is meant to mimic this discovery.

The sensor board will turn on an indicator LED when there is incident light detected on the sensor LED. When light is shone at the sensor LED, this causes a small current to flow, which is amplified by a pair of transistors, which then lights up the indicator LED. The current generated by the LED is due to the photoelectric effect, where an electron is knocked loose by the energy of a sufficiently high-energy photon. The sensor LED used in this experiment was chosen to be sensitive only to photons with blue or higher energy.

This forms the basis of the experiment, where students will find that yellow photons cannot light up the LED, but blue ones can. The solution to this finding can only be reconciled with quantum mechanics.

Advanced background:

Light-emitting diodes (LEDs) are formed using specially designed crystal structures that force the electron energy levels to be quantized into two main bands: the conduction band and the valence band. There is an energy gap between these bands, known as a band gap. When electrons are forced to cross this gap (for example, using a battery), the electron drops in energy as it crosses the band, emitting the energy difference as a photon. Conversely, when a photon with sufficient energy (higher energy than the color light the LED emits) hits an LED, an electron will be forced up across this band gap in the opposite direction, and a current will flow.

The first observation of the photoelectric effect was done by shining ultraviolet light on a metal surface, which physically ejected electrons from the surface. Thus, the energy of the ultraviolet photons was large enough to completely overcome the binding energy of the electrons to the metal and impart kinetic energy to eject them. In the experiment today, we have much lower energy photons, which do not completely eject the electrons from the surface. Instead, there is just enough energy to move them across a bandgap into the conduction band, allowing a current to flow.

Responses to common questions:

• You cannot knock an electron loose by hitting it with multiple photons because they would need to hit the electron simultaneously, which is extremely unlikely to occur.
• Students may be able to get the sensor to turn on with a yellow light if they hold the light source very close to the sensor LED. This is because the yellow filters are not perfect, and some blue light will still get through.
• Light can behave as a wave or as a particle depending on the situation. However, it can only act like one at a time. In the experiment today, we are observing the particle-like behavior of light.

Teacher tips:

• Suggested STEP UP Everyday Actions to incorporate into the activity.
• Consider using whiteboards during discussions, so students have time to brainstorm and work through their ideas before saying them out loud.
• As students experiment, roam around the room to listen in on discussion and notice experiment techniques. If needed, stop the class and call over to a certain group that has hit on an important concept.
• Consider these responsive tools and strategies and/or open ended reflection questions to help push student thinking, and to help students track their thinking during the activity.
• Connect to students’ lives and create opportunities to develop STEM identity using these suggested extensions.
• Allow the work of physicists to come alive by signing up for a virtual visit from a working physicist using APS’ Physicist To-Go program. You can request a quantum physicist to talk about the concepts students learned in this activity!

• Real world connections
  • Make connections between photon quantization and polarization
    • Quantum Atlas: https://quantumatlas.umd.edu/entry/polarization/
    • Visualizing Qubits: https://quantum.bard.edu/
    • Quantum for All: https://quantumforall.org/quantum-opportunities/
  • Sign up for Physicists To-Go to have a scientist talk to your students.

**Real-world situations/connections can be used as is, or changed to better fit a student’s own community and cultural context.