1. What is light? Is it a wave, or a particle? Light is a wave… –It reflects off surfaces, and refracts through mediums. –Light has a frequency, and wavelength. –Light diffracts (only waves do that). –Light interferes with itself (only waves do that). –Light can be polarized (only transverse waves do that).

2. What is light? Is it a wave, or a particle? Light is a particle… –Particles reflect off surfaces as well as waves. –Particles refracts also when passing from on medium into another. –Light can travel through the vacuum of space (waves can’t, they need a medium). –Light can knock electrons out of atoms (waves can only vibrate matter).

3. So what does this mean? Light CAN act as a wave when it travels, and interacts with other rays of light. Light CAN ALSO act like a particle traveling though vacuums, and by interacting with matter. This phenomenon is called the duality of nature. Since light can act as both a wave or a particle, it maybe neither. Light maybe something else that is far more complex. We call them Photons.

4. So what is a Photon? In short, a photon is a very small “packet of energy” that acts as both a wave and a particle. –Because it is a “packet”, it can move matter when they hit. –Because it is very small (relating to mass), it moves not is a straight line, but as a wave weaving back and forth with a wavelength.

5. How little mass is a “little mass”? The photon’s mass is unlike any mass we have ever known. The photon’s mass comes solely from its kinetic energy (motion), –If E = mc 2, then m = E/c 2 But if the photon ever stops moving, it’s kinetic energy and mass go to zero. Photons have no rest mass (the mass we know). Rest mass is the mass you have when you’re not moving.

6. The energy of large and small. In the “large world” that we see, changes in energy are considered to be continuous –Any change in the energy is a smooth transition. –A continuous line from point A to point Z that goes through points B-Y, and everything in between. However, on the atomic level energy is what we call discrete –Any change in energy jumps from one value to another, and there is no transition. –There is no discrete line, you start at point A and then you are at point Z, without ever passing through B-Y.

7. Continuous vs. Discrete Continuous – smooth Transition. Everything Is connected. Discrete- values are not Connected, they jump from One value to another.

8. What is quanta? The smallest discrete amount of energy that a photon can have is called a quanta. It is from this that the term Quantum Mechanics comes from.

9. Calculating the energy of a ray of light. Energy of light = (# of photons)(6.63*10 -34 J*s)(frequency) Or simply : E = nhf –n = quantum number (number of photons) –h = Planck’s constant = 6.63*10 -34 J*S –f =frequency The energy of a single photon depends only on it’s frequency, not its speed!! [which is the speed of light of course (c =3*10 8 m/s)] Because (V photon =c= f) we can also solve for the Photon’s energy in terms of wavelength instead of frequency. E = nh[c/ 

10. Units of energy. Photons tend to have a small amount of energy. So it is sometimes inconvenient to work in Joules. Another unit that we can use is called an electron volt (eV). 1 eV = 1.6 * 10 -19 J Many times we will be given the energy of a photon in electron volts, as we will have to convert the energy into Joules before we can solve any problem.

11. Sample problem: What is the frequency, and wavelength of a 50 eV photon? 50 eV 1.6 * 10 -19 J 1 eV = 8 * 10 – 18 J X 8*10 -18 J=(6.63*10 -34 J*s)(f) (6.63*10 -34 J*s) f = 1.21 * 10 16 s -1 = 1.21 * 10 16 hz

12. Sample problem continued f = 1.21 * 10 16 s -1 = 1.21 * 10 16 hz 3 * 10 8 m/s = (wavelength)(frequency) 3 * 10 8 m/s = (wavelength)(1.21 * 10 16 hz) 1.21 * 10 16 hz Wavelength = 2.48 * 10 -8 meters

13. How do Neon lights work? Electrical energy (photons) is emitted into a tube filled with Neon gas. When the photons hit the atom they excite the electrons to a higher energy level The electrons don’t stay excited for long and fall back to their original state. As the electrons go back they release the extra energy they absorbed to become excited This released energy is photons that we can sometimes see.

14. How do Neon lights work? Photon Nucleus e Velocity A photon hits a Neon atom Electron’s orbital path

15. How do Neon lights work? The electron absorbs the photon's energy and becomes excited Change in energy level Original orbital path Excited orbital path = Energy of Photon

16. How do Neon lights work? The electron returns to its original orbit releasing the absorbed energy Change in energy level Original orbital path Excited orbital path = Energy of Photon Velocity

17. Change in energy level Original orbital path Excited orbital path = Energy of Photon 2 Photon Velocity Change in energy level = Energy of Photon 1 Photon Velocity Energy of original photon = Sum of the energies of the emitted photons

18. Is that it? In a nutshell, yes. The real process is a little more complicated. The incoming photons tend to give the electrons enough energy to pass several possible orbital paths. So, when the electrons fall back to their original path they release several small photons each time it passes one of the possible orbits. The total energy of all the “smaller” photons equals that of the original photon.

19. Does it work for all photons No, for an electron to be excited by a photon the photon must have just enough energy to excite an electron to an energy level. Too much, or too little energy means the photon will just bypass the atom

20. So how do we know what an atom looks like? From the light produced from exited atoms (such as Neon, or hydrogen). The light we see can be broken down into a specific set of colors (wavelength or frequencies) called a line spectra. –Each element has a different set of colors Each color represents an energy level –Since the photon that makes that light is created when an electron falls past an energy level.

21. Viewing Line Spectra White Light Produced by a heated gas Every line represents a given wavelength, & a change in energy level Pictures from http://csep10.phys.utk.edu/astr162/lect/light/absorption.html

22. Potential wells A visual tool that is used to illustrate the different energy levels of a atom. n = 1 Ground state The bottom of the well represents the atoms ground state. Each line above the ground state represents another energy level n = 2 n = 3 n = 4 Notice as you move up in the well the energy level gets closer. This means that it requires less energy to move from level 4 to level 5, than from level 1 to level 2.

23. How many energy levels are there? Every well goes up to ten energy levels (n=10) At the tenth energy level we consider the atom ionized. (The atom has lost the electron, it popped out of the well)

24. Other number on the well You will find two sets of energies for the well. One set is positive numbers that starts from 0 eV at the top of the well and gets larger as you go down the well. This is the energy the electron gives when it falls from that level to the ground state. E 1 = 0 eV E 3 = 12.08 eV E 4 = 12.75 eV E 5 = 13.06 eV E 6 = 13.22 eV E 7 = 13.32 eV E 8 = 13.39 eV E 9 = 13.43 eV E 10 = 13.6 eV E 2 = 10.2 eV Ground state of Hydrogen

25. Other numbers on the well The second set is made up of negative numbers and 0 eV is at the top. The numbers become more negative as you go down This is the energy the electron needs to escape the atom. The energies are negative because it is what the electron needs, not has. Ground state of Hydrogen E 1 = -13.6 eV E 2 = -3.4 eV E 3 = -1.5 eV E 4 = -.85 eV E 5 = -.54eV E 6 = -.38eV E 7 = -.28eV E 8 = -.21eV E 9 = -.17eV E 10 = 0 eV

26. Reading the Well Pick a side (positive or negative, it does not matter which). Find the difference between the energy level values. That difference is how much energy an electron needs to gain (or release) to move from one energy level to another. Electrons gain energy moving up. They lose energy falling down. Ground state Electrons emit energy Electrons gain energy

27. Finding the energy levels for other single electron atoms (He +, Li ++ ) Z 2 (-13.6 eV) E n = ----------------- n 2 Electron’s energy at state n = (Atomic #) 2 (ground state energy of H) ----------------------------------------------- (state n) 2