Pick up the handout in back. Waves Pick up the handout in back. Mac and Tosh are resting on top of the water near the end of the pool when Mac creates a surface wave. The wave travels the length of the pool and back in 25 seconds. The pool is 25 meters long. Determine the speed of the wave.

Homework Test next Monday. Make arrangements if you won’t be here.

Emission Spectrum We learned how light can tell scientists about which direction a star is moving. Today we learn about how scientists can tell what faraway stars are composed of. Some pieces of information needed to understand it: In your notebook, draw what you recall from middle school about the parts of an atom.

Emission Spectrum

Emission Spectrum Second, recall from last week: What determines the color of light? What does that quality mean?

Emission Spectrum Light is weird because it isn’t just a wave, it also comes in things like “packets,” called photons. Each photon, or bit of light, does behave like a wave. Each light photon has a wavelength, frequency, etc. And each photon is worth a certain amount of energy. The more energy, the higher the frequency and the lower the wavelength. With your partner: What color will a light with high energy photons be? What color will low-energy photons be?

Emission Spectrum Here’s what that has to do with atoms: The visible light spectrum is a range of frequencies and wavelengths, which determines color, and is related to energy. When a photon of light strikes an object and is absorbed, that photon never makes it to our eyes, we don’t know about it. But photons that strike an object and are reflected or released and make it to our eyes, make that object appear to be that color.

Emission Spectrum When an ion of an element absorbs energy (from heat, for instance), its electrons become energized and “bounce” further away from the nucleus. Those electrons quickly “fall back” to their normal positions. But when they do, they release the energy that they absorbed, in the form of a photon of light. The energy of the photon (and therefore the wavelength and frequency) depends on how far the electron had to “fall back.”

Emission Spectrum Because every element has a unique pattern and arrangement of electrons, every element gives off photons of different wavelengths when it absorbs energy. Emission spectrum: The particular colors of light emitted by an element when it absorbs energy.

Emission Spectrum So an emission spectrum is like a fingerprint for an element. There are different ways of quantifying emission spectra. We’re going to use the broadest method in class, examining the light with our unaided eyes.

Emission Spectrum A flame test can’t show you the wavelengths of each kind of photon given off by an element, but it shows you the combined effect of all of them put together.

Emission Spectra A tool called a spectroscope works kind of like a prism: it can take light, and isolate each individual wavelength. So it’s more precise than a flame test with the naked eye. If a flame appears orange, for instance, a spectroscope may show that it looks that it looks that way because the element is emitting some red and some yellow, or that the element is just emitting orange, or some combination.

Emission Spectra Bright Line Spectrum = The individual colors of light photons emitted by an energized element. http://profmokeur.ca/chemistry/spectemien.htm

Bright Line Spectra

Dark Line Spectra Dark Line Spectrum = The individual color of light that an unenergized element absorbs when lit. Notice, bright line vs dark line for hydrogen:

Interference

Interference Wave interference = When two (or more) waves become superimposed, creating a new wave pattern.

Slinky Interference Lab Your goal is to demonstrate different kinds of interference. Take out a sheet of paper and make a table. Decide with your group what some different hand motions will be. Draw these motions along the top and side of the table. Then, in each table cell, draw what the wave looks like when Person 1 does one movement, and Person 2 does the other.

Interference Constructive interference = When the troughs and peaks line up, the amplitudes of the two waves will combine. Destructive interference = When the troughs are opposite of the peaks, the amplitudes will subtract/cancel.

Interference Down = wavelength increases Right = distance between origins increases

Interference With your partner, consider these questions: “Surround Sound”: Two speakers on either side of the room blast the same song. You stand in the middle of the room, and the song seems even louder than it does when you stand in front of one of the speakers. Draw a diagram, and explain in words why.

Interference Two people jump into a pool at opposite ends, one after the other, each one creating a nice big splash. In the middle of the pool, you continue to float undisturbed by waves. Draw a diagram and explain this.

Interference CHALLENGE: A light is aimed at a barrier with two small slits in it. It shines through the slits, but rather than getting two points of light on the wall behind them, a pattern shows up. Explain this.

Double Slit Experiment An extremely famous experiment in physics (and philosophy!)