Scientists don’t always agree, but their arguments are a crucial part of progress. A century ago, physicist Max Planck represented one side in a debate among scientists: Was light a wave or a particle? The combined work of many physicists produced a model of light that was both! One of the key lines of evidence in the argument came from Max Planck. He began by researching the radiation that came from hot materials. He asserted that energy did not flow in a steady way, but in tiny packets called quanta. He used the example of a hot piece of iron (like a blacksmith might use). One side can be red and the other white because different numbers of quanta are being emitted.

Planck found that although quanta activity (energy) varied it was still equivalent to radiation levels multiplied by a universal constant: “Planck’s constant.” These findings came to be known as quantum theory, which provides a theoretical foundation for the relationship and tendencies of matter and energy on atomic and subatomic levels. Planck won the 1918 Nobel Prize in recognition of his groundbreaking work.

Planck's constant, "h", can be used to find the energy of a photon of electromagnetic radiation. We use the equation E = hν, where E is energy in Joules (J), h is Planck's constant, 6.626×10−34J⋅s, and ν is the frequency. The unit for frequency is Hertz (Hz).[b][c]

We began with a film (The Last Artifact) that showed the process through which metrologists around the world debated how to redefine the kilogram in 2018. One proposal involved creating a 1-kilogram sphere of silicon that contains a specific number of silicon atoms and was therefore based on the constant Avogadro’s number. [Link to Mole Reading in 2 Extend][d]. Ultimately, metrologists agreed that Planck’s Constant provided a more direct connection and path to accuracy using a tool called a Kibble Balance. It also helped in the redefinition of units for voltage, resistance, and amperage. Of course, they are all connected!

Let’s begin with an analogy for energy in packets. It will help you visualize the core of last century’s great debate. Then look more carefully at the model that Max Planck described.

Materials:

Procedure I:

• Materials for brainstorming
• Web Link - NIST in 90: Measuring Planck’s Constant

Procedure II:

• Web Link - NIST in 90: Measuring Planck’s Constant
• Web Link - PhET - Blackbody Spectrum [e]
• Internet research access

Procedure I:

Begin by thinking about the difference between something that is continuous and something that is discrete. At a sleep-over everyone is getting silly. One friend pellets you with small pillows. Another hits you with a water hose. The first “ammo” is discrete. The second is “continuous.”

You finally fall asleep, and morning comes far too early. Is that light coming through the window discrete or continuous? Max Planck proposed to the world’s physicists that it was a little of both. When energy was emitted from an object, it was released in discrete packets called quanta that had wavelike qualities! The amount of energy in the packet, incredibly small, is known as h, Planck’s constant 6.62607004 × 10-34 m2 kg / s

Watch Darine Haddad use a cup of coffee as an analogy for quanta: Web Link - NIST in 90: Measuring Planck’s Constant

Analogies are powerful ways to model and explain big ideas in science. Think of an analogy of your own, of something that is emitted or used in a continuous way, and then a similar emission that occurs in a discrete way (packets or discrete units.) Develop diagrams to help you explain your example to others in the class.

Procedure II:

Packets of Light and Heat

The value of a model is whether it can accurately predict real world phenomena. Max Planck’s model of energy correctly predicted spectra emitted by a “blackbody”--a theoretical object that absorbs all of the radiant energy that falls on it[f].

A blackbody also emits all wavelengths of light in a continuous spectrum. As the temperature of a blackbody increases, the wavelength at which the peak radiation is emitted becomes shorter.

Go to the PhET simulation on Blackbody Spectrum and explore the functions: Web Link - PhET - Blackbody Spectrum

1. Click on the graph values and labels. Make sure you understand them.
2. Next change the temperature on the thermometer. Observe how the emission spectrum changes on the graph. Describe the emission spectrum of the sun and a light bulb in words using the units on the Y axis in your explanation.                                                                                                                                 
3. Compare the spectrum of a light bulb and Earth. To see the changes more clearly, use the magnification buttons in the lower right.                                                                                                                                                         
4. Find the temperature of 5 other objects using Internet resources. Move the slider on the thermometer to the appropriate level and describe those spectra.

1. Create a graph of temperature vs wavelength of the maximum intensity λmax for the temperatures you have selected. Because the x axis of the simulation does not have very precise units marked you may have to estimate.
2. Is the relationship direct or inverse? (Does the Y value increase when the X value increases?) What does that mean?

Analysis:

1. Explain in your own words what Planck’s constant represents.

1. Return to Darine Haddad’s short explanation of her work defining Planck’s constant. How precise was her team’s calculation?                                                                                                                                
2. Planck’s constant is used in many areas of physics. Why is it related to a kilogram? (Hint: Einstein used a pun to explain why he was able to come to his famous equation E = mc2[g][h][i][j]: “I had a Planck to stand on.”)

Conclusion:

What have you learned?

What more do you want to know?