Light and Matter I Exam #1 Notes

The first midterm exam is Wednesday, March 8th during class time, administered through CANVAS with live Zoom proctoring. Please be on time.
The exam will cover the material in Chapters 1, 2, 3, 4, and 5 of the optional textbook.
Most questions will resemble in-class “Conceptual Questions” or the Lecture Tutorial and Ranking Task questions, so the LT book is the key study resource.
Cell phones must be off and put away during the exam. Seeing or hearing a cell phone during the exam is grounds for failure.
Special Relativity will be studied that day as well, so students should do the reading as usual.

Light and matter interact on a macroscopic level in four ways:
- Emission
- Absorption
- Transmission
- Reflection/scattering
Interaction of light with matter involves the creation (emission) or annihilation (absorption) of photons by atoms and molecules.

Matter comes in different types, but most detectable matter is made of atoms.
There are 92 naturally occurring elements, each with atoms in its pure form.
Combinations of atoms form molecules.
All atoms consist of three kinds of particles: electrons, protons, and neutrons.

The number of protons (Z) determines the element type.
For a given element, the number of neutrons can vary, creating different versions called isotopes.
Examples of Hydrogen Isotopes:
- Normal hydrogen ( $1H$ ): 1 proton.
- Deuterium ( $2H$ ): 1 proton and 1 neutron.
- Tritium ( $3H$ ): 1 proton and 2 neutrons.
Carbon also has isotopes (e.g., $12C$ , $13C$ , $14C$ ).
$14C$ is radioactive and used in “carbon dating”.

Electrons in an atom exist in well-defined energy levels, characterized by precise, quantized values of energy.
When matter is at rest (0 K), all electrons are in their lowest energy level (ground state).

Electron orbits in the electron cloud are restricted to specific radii and energies ( $r1, E1$ , $r2, E2$ , $r3, E3$ ).
Characteristic electron energies are different for each element.

A photon can only be absorbed if it has the correct energy to move an electron to one of the possible energy levels.
If a high-energy photon (UV or X-ray) is absorbed, the electron would hold a great deal of energy within an allowed state.
The photon is absorbed (ceases to exist), and the electron moves to a higher energy level.
Electrons tend to get rid of extra energy and drop back down to a lower energy level, emitting light.

When an electron drops to a lower energy level, it emits a photon with energy exactly equal to the energy difference between the levels.
For example, if an electron absorbs a sky-blue photon and then drops back down, a new sky-blue photon would be emitted.

If a continuous spectrum of light hits a cloud of atoms, certain wavelengths get absorbed.
The resulting spectrum on the other side is an absorption spectrum.

If one could only see the light emitted by a cloud of these atoms, an emission spectrum would be observed.

The spectrum of a cloud of hot glowing gas contains emission lines.
When viewed through a cloud of cool gas, a continuous spectrum will have absorption lines.

The number of protons (atomic number) in a nucleus determines the element.
Each element has a number of electrons equal to the number of protons.
The electron orbitals and energy differences between the orbitals are unique for each element.

Work with partners on Lecture Tutorial: Light and Atoms I, pp. 65-67 (#1-#6) and Lecture Tutorial: Light and Atoms II, pp. 65-67 (#7-#11).
Read instructions and questions carefully.
Discuss concepts and answers with one another to reach a consensus.
Write clear explanations for your answers.
Seek help from other groups or the instructor if you get stuck.