Astronomy

Homework

# Scholars Online Astronomy - Chapter 5: 1-4: The Nature of Light and Matter

## Homework

Reading: Astronomy, Chapter 5: Sections 1-4 The Nature of Light and Matter

• Section 5.1 Note the difficulties involved in calculating certain phenomena when the insturmentation to make proper measurements is not available. Galileo and Huygens worked without adequate clocks (Huygens invented the pendulum escapement to make better measurements of time, but it still didn't help). Nevertheless, the first reliable determination of the speed of light in a vacuum was accomplished by using observations of the moons of Jupiter. The speed of light varies depending on its medium; it is higher if the medium is rarer (less dense), so the speed in a vacuum is the maximum speed light can achieve.
• Section 5.2 Light is the result of interacting electric and magnetic fields. As each changes, it causes changes in the other, creating a self-propagating wave that carries a specific amount energy determined by the frequency of the wave. The product of wavelength (distance) times frequency (cycles per second) determines the speed (distance/time): c = λν. Be sure that you understand this, since much of physics and astronomy depends on this simple idea. Spend some time looking at the range and types of electromagnetic radiation: what colors are associated with short wavelengths? With long wavelengths? With low frequencies? With high frequencies? With low energies? With high energies?
• Section 5.3 Look at the temperature conversion scales, because most astronomers use the Kelvin scale when talking about the temperatures of most celestial objects (planetary temperatures is sometimes an exception, to make it easier to compare with earth-type temperatures). Both the Kelvin and Celsius (centigrade) scale divide the temperature difference between freezing point of water and the boiling point of water into 100 equal degrees, but where the Celsius scale sets its zero point at the freezing point of water (which leads to readings of negative temperatures), the Kelvin scale uses a principle of physics to set its zero point at the lowest possible temperature -- the point where all motion stops. There are no "negative" Kelvin temperatures!

The amount and frequency of energy radiating from a body depends on its temperature. As temperature rises, more energy is radiated and most of the energy radiates at higher frequencies. Stars are similar to idealized objects called blackbodies, which re-emit as radiation all energy falling on them.

• Section 5.4 Wien's law describes the relationship between temperature and the wavelength at which the most light is radiated by a blackbody, a spherical object which absorbs all light falling on it, then re-radiates that light at wavelengths depending on its own temperature. The Stefan-Boltzmann law describes the relationship between temperature and the total energy radiated by a blackbody. Be sure that you can use Wien's law and the flux and luminosity relationship to calculate stellar temperatures according to the reasoning in box 5-2. Don't be thrown by the math: this stuff isn't that hard. But do reason from the math to the physical object being modeled: 4*π*R2 isn't just a set of numbers, it is surface area for a sphere.

### Key Formulae to Know

• Light speed:
• Temperature conversion
• Wien's Law
• Stefan-Boltzman Law (for blackbodies)

### Web Lecture

Read the following weblecture before chat: Light and how it tells us what's out there (Part 1)

### Study Activity

Planetarium program: Use your planetarium program or Wikipedia to lookup the temperatures and wavelength of maximum intensity of the following stars: Altar, Procyon, Epsilon Indi, Tau Ceti, Epsilon Eridani, Laland 21185, the sun. Plot the wavelength vs temperature vs temperature roughly on a graph. What relationship do you find?

Optional websites:

### Chat Preparation Activities

• Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
• Go over the list of Key Words and Key Ideas at the end of the chapter. If you don't remember the definition of the key word, review its use (the page number on which it is explained is given).
• Read through the Review Questions and be prepared to discuss them in class. If any of them confuses you, ask about it!
• Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.

### Chapter Quiz

• There is no chapter quiz yet -- we haven't finished the chapter.

### Lab Work

Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.