Overheard: not full moon is not full

Astronomy 210L, spring semester 2012
Cuesta College, San Luis Obispo, CA

(Overheard during presentation introducing moon phases, showing a slide of a nearly full moon.)

Instructor: "So, which phase is this?"

Students: "Waxing gibbous?"

Instructor: (Beat.) "Are you sure? How do you know that this is not a full moon?"

Student: (Beat.) "Because it's not full?"

Astronomy in-class activity: first-day student expectations, questions

Astronomy 210 In-class activity 1, spring semester 2012
Cuesta College, San Luis Obispo, CA

120121-interestingwordle
http://www.wordle.net/show/wrdl/4706913/Untitled
Originally uploaded by Waifer X

Wordle.net tag cloud for potentially interesting astronomy topics, generated by students on the first day of class (http://www.wordle.net/show/wrdl/3935554/Untitled).


120121-confusingwordle
http://www.flickr.com/photos/waiferx/6737065165/
Originally uploaded by Waifer X

Wordle.net tag cloud for potentially confusing astronomy topics, generated by students on the first day of class (http://www.wordle.net/show/wrdl/4706918/Untitled).


On the first day of class, students find their assigned groups of three to four students, and work cooperatively on an in-class activity worksheet to discuss concepts that will potentially be interesting or confusing to them later in the semester. Students are also encouraged to write down a comment or a question for the instructor to go over during the whole-class discussion, after the in-class activity worksheets are turned in.

[Responses have been edited to consolidate common related subjects.]

Discuss in your group astronomy-related concepts you expect to be interesting or confusing later in this course. Use one word or short phrases (e.g., "Pluto," "black holes," "beginning of time") for each concept.

List at least three astronomy-related concepts you expect to be interesting.

Student responses
Sections 30674, 30676

exoplanets, sun, light
blackholes, planetatmospheres, constellations
bigbang, life, light
moons, galaxies, constellations, blackholes, life
constellations, blackholes, supernovae
time, rotations
bigbang, astrology, galaxies
light, blackholes, cosmology
planets, constellations, lightyear
blackholes, bigbang, supernovae
light, galaxies, water
names, blackholes, bigbang
constellations, quarks, tides, blackholes
universe, blackholes, stars
aliens, blackholes, stars, solarflares, northernlights, asteroidimpacts
stars, aliens, blackholes, darkmatter
planets, Mars, atmospheres
Pluto, aliens, galaxies, light
planets, exploration, timetravel, stardeath
universe, sun, Pluto
supernovae, gravity, solarflares, eclipses, meteorshowers
blackholes, aliens, constellations
blackholes, galaxies, darkmatter, lightyear
constellations, blackholes, bigbang, gravity
universe, bigbang, time
blackholes, bigbang, solarflares

List at least three astronomy-related concepts you expect to be confusing.

Student responses
Sections 30674, 30676

blackholes, constellations, bigbang
time, distances, blackholes
bigbang, universe, gravity
blackholes, bigbang, universe
darkmatter, galaxies, blackholes
light, bigbang, starformation
constellations, blackholes, rings
planets, everything, starformation, stardeath
galaxies, names
blackholes, blackholes, bigbang
bigbang, constellations, space
elements, constellations, space, time, light
distances, eclipses, blackholes
galaxies, math, atmosphere
physics, orbits, math
time, cosmology, space, bigbang, knowledge
physics, blackholes, light
names, physics, galaxies
bigbang, spacetime, moonphases
planets, planets, planets
planets, solarflares, blackholes, darkmatter, asteroids
universe, blackholes, moonphases
math, universe, cosmology
space-time, matter, constellations
lightyear, gravity
planets, physics, galaxies

Each week after class you will receive credit for asking a question, or making a comment that the instructor might respond to at the start of the following class (while your identity is kept anonymous). Ask at least one question, or make a comment that you would like the instructor to respond to at the end of this in-class activity.

Student responses
Sections 30674, 30676

"What do you think is the most exciting thing to learn in astronomy?"

"What tools are used to measure the distance of the known universe?"

"We've landed on the moon, we've sent telescopes deep into outer space--what do you think technology will let us or allow us to do next?"

"How many dimensions are there?"

"What makes stars go supernova?"

"How did the beginning of time happen?"

"What's your favorite planet, P-dog?"

"Why does a star combust after the gas is gone?"

"Recent study found a planet that could hold life?"

"What is your favorite constellation, and why?"

"What's most interesting to you in astronomy?"

"Currently, what is the general consensus in the science community on there being life on another planet in the universe?"

"Can we, should we, be concerned about space debris?"

"How often are we threatened by a comet or asteroid or other large astronomical disaster?"

"Am I going to need a tutor?"

"How did you become an astronomy teacher, and hired to do it, without actually taking an astronomy course?"

"What is the ring around Saturn made of?"

"What's responsible for creating the northern lights?"

"How would we know what a star in the sky is? It could be a solar system, or even a galaxy, right?"

"If a star is brighter, does that mean it could be closer, bigger, either of those?"

"What is Nibiru/Planet X?"

"What is the 2012 scare all about and the reason for it?"

"Do you believe in extraterrestrials?"

"How have you been known as 'P-dog?'"

"How do asteroid belts maintain structure and orbits?"

"What's the deal with Pluto?"

"Do you believe in aliens? Or life on other planets?"

"Do you believe in extraterrestrials?"

"What is your favorite planet, and why?"

"How do we know the universe is 5 billion light years?"

Previous posts:

• Astronomy in-class activity: first-day student expectations, questions (fall semester 2011)
• Astronomy in-class activity: first-day student expectations, questions (fall semester 2010)

Astronomy in-class activity: astronomy in the marketplace tags

Astronomy 210 In-class activity 1, spring semester 2012
Cuesta College, San Luis Obispo, CA

120120-carwordle
http://www.flickr.com/photos/waiferx/6733751333/
Originally uploaded by Waifer X

Wordle.net tag cloud for astronomy-related car brand names, generated by responses from Astronomy 210 students at Cuesta College, San Luis Obispo, CA (http://www.wordle.net/show/wrdl/4705552/Untitled).


120120-foodwordle
http://www.flickr.com/photos/waiferx/6733751421/
Originally uploaded by Waifer X

Wordle.net tag cloud for astronomy-related food brand names available in a supermarket, generated by responses from Astronomy 210 students at Cuesta College, San Luis Obispo, CA (http://www.wordle.net/show/wrdl/4705559/Untitled).


120120-nonfoodwordle
http://www.flickr.com/photos/waiferx/6733751513/
Originally uploaded by Waifer X

Wordle.net tag cloud for astronomy-related non-food brand names available in a supermarket, generated by responses from Astronomy 210 students at Cuesta College, San Luis Obispo, CA (http://www.wordle.net/show/wrdl/4705569/Untitled).


Students find their assigned groups of three to four students, and work cooperatively on an in-class activity worksheet to discuss car brand names, and food and non-food brand names found in supermarkets (adapted from D. Schatz, "Why Should We Care About Exploding Stars?" Universe in the Classroom, no. 8, Spring 1987 (http://www.astrosociety.org/education/publications/tnl/08/stars2.html).

There are many astronomy-related brand names. Consider car brand names (old and new); and brand names for food and non-food items that are typically found in the supermarket. Do not consider titles of TV shows, movies, or books.

List at least three astronomy-related car brand names.

Student responses
Sections 30674, 30676

Saturn, Mercury, Titan
Saturn, Mercury, Astro
Infiniti, Mercury, Saturn
Saturn, Mercury, Subaru, Volt, Sky, Celeste
Mercury, Subaru, Eclipse
Taurus, Saturn, Mercury
Saturn, Mercury, Subaru, Astro
Saturn, Infiniti, Nova
Saturn, Subaru, Volt
Nova, Saturn, Galaxy
Astro, Galaxy, Taurus
Saturn, Infiniti, Mercury
Mercury, Saturn, Taurus
Mercury, Saturn, Infiniti
Aerostar, Explorer, Saturn, Aura, Mercury
Mercury, Eclipse, Saturn
Mercury, Saturn, Comet
Saturn, Eclipse, Scion, Astro
Saturn, Mercury, Taurus, Eclipse, Nova, Voyager
Saturn, Mercury, Galaxy
Nova, Saturn, Mercury, Astro
Saturn, Mercury, Fusion
Mercury, Cobalt, Nova, Astro, Saturn
Mercury, Saturn, Taurus, Subaru, Galaxy
Saturn, Nova, Mercury, Galaxy, Astro
Mercury, Saturn, Comet

List at least three astronomy-related food brand names typically found in the supermarket.

Student responses
Sections 30674, 30676

MoonPie, MilkyWay
starfruit, Starburst, MilkyWay, Marsbar
MilkyWay, Starburst, Orbit
Starburst, MilkyWay, LuckyCharms
Marsbar, MilkyWay, Starburst
MilkyWay, Starburst, StaroftheSea
Sunkist, SunChips, Starburst
MoonPie, MilkyWay, Sunkist
Starburst, MilkyWay, SunChips
MilkyWay, MoonPie, Marsbar, SunChips
Sunkist, Starburst, MilkyWay
MilkyWay, Marsbar, MoonPie
MilkyWay, MoonPie, Orbit
Starburst, Marsbar, MilkyWay
CapriSun, Sunkist, SunnyD
MilkyWay, Starburst, Eclipse, Marsbar
Sunburst, MilkyWay, Starburst, starfruit
MoonPie, MilkyWay, Lunabar
Marbar, MilkyWay, Moonshine, Starbucks, BlueMoon
MilkyWay, Starburst, Marsbar
AstroPop, SunnyD, MilkyWay
MilkyWay, MoonPie, Marsbar
BlueMoon, MilkyWay, Sunkist, SunnyD, Starburst
Trident, Orbit, AstroPop, MilkyWay, Starburst
MilkyWay, starfruit, Marsbar
MilkyWay, Marsbar, Corona, Sunkist, SunnyD, BlueMoon

List at least three astronomy-related non-food brand names typically found in the supermarket.

Student responses
Sections 30674, 30676

BlueMoon
Venusrazor, Comet, BlueMoon
Starbucks, light, gravity
BlueMoon, Skyline, Comet
Venusrazor, Orbit
Comet, Venusrazor
Sextantwine, Venusrazor, Comet
SkyyVodka, BlueMoon, VerizonGalaxy
Skyviewwine, sunscreen
BlueMoon, Sunkist, Starburst
Comet, Moonshine, Ka-boom
Comet, Orbit, Venusrazor
Comet, BlueMoon, Tide
sunscreen, Tide, PopularScience
Sun, Comet, Dawn
Venusrazor, BlueMoon, Comet
Comet, sunscreen, Dawn
Nova, SunnyD, Starbucks
Comet, tequilasunrise, LAGalaxy, Globetrotters, Venusrazor
sunglasses, Venusrazor, SpaceBags
Comet, BlueMoon, Nova
BlueMoon, Telescopicmascara, Comet
Comet, SpaceBags, Venusrazor
Comet, Ajax, Venusrazor, Tide, Trojans
Comet, sunscreen, Moonshine
Comet, Tide, sunscreen

Previous posts:

• Astronomy in-class activity: astronomy in the marketplace tags (Cuesta College, fall semester 2011).
• Astronomy in-class activity: astronomy in the marketplace tags (Cuesta College, fall semester 2010).
• Astronomy in-class activity: astronomy in the marketplace tags (Cuesta College, spring semester 2010).

Presentation: redirecting light

Now that we've already been introduced to "light," that is, the electromagnetic spectrum, let's take a look at redirecting (visible) light. Note that like presentations in the previous first semester of this college physics sequence, this presentation will hopefully give you a sense of what Bill Nye ("The Science Guy") likes to describe as "PBJ"--the "passion, beauty, and joy" of reflecting and refracting light. We simply don't have time for an exhaustive, comprehensive discussion of this material in class--that's what your textbook is for!

Reflection is redirecting light by bouncing it off of a surface.

Conventionally we consider reflections off of flat surfaces. (Don't worry about what's about to happen here--it's art!)

Even with reflections off of curved surfaces, each point on it can be considered a locally flat surface.

And more "PBJ" for reflections--they have the curious property of reversing front-to-back symmetry (or here in this perspective, left-to-right symmetry).

The law of reflection is simple geometry--for a (visible) ray of light incident on a flat surface, if we measure the angle of incident with respect to the normal (a line drawn perpendicular to the surface), the reflected ray will make the same angle as it leaves the surface. (This law also applies for curved surfaces, provided we look close enough such that it look locally flat.)

Instead of bouncing light off of surfaces, if light can pass into a transparent material, we will have refraction, where it is redirected by being "bent."

Refraction occurs when light starts in one material, and passes into another. (This art installation only gives the illusion of seeing light from people underwater; instead there is only a thin layer of water supported by sheet of glass between these two levels.)

When light does start in one medium, and pass into another medium, it will refract, or bend, which can produce curious results.

Look at the exhaust plume from this jet: heat warms the air and changes its density, making light travel at a different speed through it, and it will be bent in interesting directions. Also note the shockwaves from leading edges on the jet--here air is compressed, and again light traveling through it will be bend it in interesting directions.

Now take a look at this transparent block. Light will travel with a different speed through it than through air, and so the light will be bend in interesting directions. Why can't we see the block when we pour water around it? (Video link: "100108-1140566.")

Quantitatively, "Snell's law" describes how light will bend as it passes from one medium into another medium. Note that angles for both the incident ray and refracted ray are measured with respect to the normal (that imaginary line drawn perpendicular to the interface between the two media). The medium with the lower index of refraction will have the larger angle (actually, the larger sine of that angle). (This law of refraction is commonly known as "Snell's law," but in France it is referred to as "Descartes' law," as René Descartes was French, while Willbrord Snellius was Dutch.)

Since the index of refraction is a measure of the "optical slowness" of a material, a faster speed of light corresponds to a lower index of refraction, and a larger angle, as it travels into a material with a slower speed, a higher index of refraction, and a smaller angle. Mnemonic: "Fast-to-slow, bend towards the normal."

Consider starting in a medium with a greater index of refraction. Note that angles for both the incident ray and refracted ray are still measured with respect to the normal (that imaginary line drawn perpendicular to the interface between the two media). The medium with the higher index of refraction will have the smaller angle (actually, the smaller sine of that angle).

And since the index of refraction is a measure of the "optical slowness" of a material, a slower speed of light corresponds to a higher index of refraction, and a smaller angle, as it travels into a material with a faster speed, a lower index of refraction, and a larger angle. Mnemonic: "Slow-to-fast, bend away from the normal."

Presentation: electromagnetic waves

We'll start off the second semester of college physics with light, or more correctly, electromagnetic waves. Since we are covering this topic after rope and string waves, but before electricity and magnetism, this will concentrate on describing their behavior in terms of one-dimensional waves rather than explaining it in terms of three-dimensional electromagnetic fields.

Consider all types of "light."

The electromagnetic spectrum encompasses all types of "light," here listed from low frequency to high frequency. Notice that visible light is only a very small portion of the entire electromagnetic spectrum, which we perceive as colors. The vast majority of the electromagnetic spectrum is invisible to our eyes, but we can detect their presence indirectly with certain instruments, or even different parts of our bodies. (When discussing all types of "light," we'll use "electromagnetic waves," as often "light" refers only to visible light.) Let's introduce these types of light...oops, electromagnetic waves, from lowest to highest frequency.

The lowest frequency form of electromagnetic waves are collectively known as radio waves, which are subdivided into microwave, TV, FM and AM bands depending on the type of device used to send and receive these forms of electromagnetic waves.

Slightly higher in frequency along the electromagnetic spectrum is infrared, also known as "heat waves," which our eyes cannot directly see (but we can indirectly feel), but certain devices allow us to "see" in the infrared. (Video link: "infrared heat cam.")

Then slightly higher in frequency on the electromagnetic spectrum is visible light, the only type of "light" we can directly see with our eyes. (What is this person looking at?)

Higher in frequency on the electromagnetic spectrum, we're back again to types of "light" we cannot directly see with our eyes, but we can indirectly "see" with special devices, or in this case, materials that react in certain ways to being exposed to ultraviolet, such as our skin, or this Milky Way^TM candy bar.

"Who was Count Dooku's Jedi Mentor?"

So either children are supposed be Star Wars trivia experts, or have to go to a nightclub with "blacklights" in order to answer this correctly... (Video link: "090529-1090772.")

Higher in frequency is x-rays, which again cannot be seen by our eyes (well, maybe for Superman), but can be made visible with special devices or certain materials. Note that tissue is relatively transparent to x-rays, while bone, and especially metals are opaque. (What is that thing in this person's nose?)

And highest in frequency are gamma rays, which have higher penetration than x-rays, allowing inspection inside metal shipping containers.

Now let's look at the parameters used to describe these forms of "light," and the connections between them. (From here on, since we'll be discussing the transmission of visible light through different types of transparent media, we'll drop the quotation marks around "light.")

(This is a review of a previous discussion of one-dimensional rope and string waves from last semester.) Note the hierarchy of these wave parameters. Since the wave speed is determined by properties of the material it travels through (independent of the source), and the frequency is determined by the source (independent of the medium), these are said to be independent wave parameters. In contrast, the wavelength of the wave is dependent on both the independent speed and frequency parameters. Algebraically there is nothing wrong with expressing this relation as v = λf and f = v/λ, as long as you recognize that the dependency of λ doesn't change.

This is a very simple but adequate model of how the independent and dependent parameters remain constant, or change. Consider light passing from a medium where it has a fast speed, into a different medium where it has a slow speed. The frequency of this light does not change (which depends on its source), so ideally it is still visible light, and hasn't changed frequency to become some other type of electromagnetic waves such as radio or gamma rays! Since the frequency (and type of light) is unchanged, but the speed does change (due to the change in medium), then the wavelength changes, here decreasing ("scrunching") due to the decrease in speed in the new medium.

(N.b.: we are ignoring dispersion for the purposes of this discussion.)

Light changes speed (and wavelength) as it travels through different materials. Commonly used instead of the speed of light through different materials is the "index of refraction."

The index of refraction is defined as the ratio of speed of light in vacuum (c = 3.00x10⁸ m/s) to the actual speed of light in that material. Thus for vacuum, n = 1, while for all other materials, light will travel slower, such that n > 1, and the index of refraction can be considered a measure of "optical slowness" of a material.

Light will travel quickest through vacuum, and the index of refraction is 1.

Through air, light will travel very slightly slower than it does through vacuum, so the index of refraction for air is slightly greater than 1, but to three or fewer significant figures, n can be approximated as 1.

Light will travel noticeably slower through ice, so the index of refraction for ice is noticeably greater than 1.

Light travels a bit slower through water, and so the index of refraction for water is slightly greater than for ice.
Light typically travels slower through glass than through water, so again, a greater index of refraction for glass than water. (Note that different types of glass will have slightly different indices of refraction.)

At the extreme is light traveling through diamond, which for our purposes has the greatest "optical slowness," and the greatest possible index of refraction.