lecture 12 (10/10), chapter 6 & 7

1

Next quiz: WednesdayChapters 2-5; emphasis on Chp. 5Newton’s laws of motionNewton’s law of gravitation

No homeworkSample questions at end of lecture files archived online.

Monday, October 10, 2011

Light and TelescopesChapter 6


I. Radiation: Information from Space A. Light as a Wave and a Particle B. The Electromagnetic Spectrum

II. Optical Telescopes A. Two Kinds of Telescopes B. The Powers of a Telescope C. Buying a Telescope D. New-Generation Telescopes E. Interferometry

III. Special Instruments A. Imaging Systems B. The Spectrograph

Outline


IV. Radio Telescopes A. Operation of a Radio Telescope B. Limitations of the Radio Telescope C. Advantages of Radio Telescopes

V. Astronomy from Space A. The Ends of the Visual Spectrum B. Telescopes in Space C. Cosmic Rays

Outline (continued)


5

Wave-like properties:

• Reflection, refraction, diffraction, interference

• Electromagnetic “waves”

Particle-like properties:

• Energy carried in bundles we call “photons”

• Can propagate through a vacuum

Properties of Light


6

“Light” = Electromagnetic Radiation = Photons

Different names for the same thing:

Vocabulary


7

Frequency

The number of peaks which pass you per second. Units: Hz


8

Speed of wave

Speed = distancetime

= wavelength × frequency

c = λ × f = 3 × 105 km/s


Light as Particles• Light can also appear as particles, called

photons (explains, e.g., photoelectric effect).



photons (explains, e.g., photoelectric effect).• A photon has a specific energy E,

proportional to the frequency f:




E = h*f

• A photon has a specific energy E, proportional to the frequency f:




E = h*f

h = 6.626x10-34 J*s is the Planck constant.





E = h*f

h = 6.626x10-34 J*s is the Planck constant.

The energy of a photon does not depend on the intensity of the light!!!



Atoms and StarlightChapter 7


Wavelengths and Colors


Wavelengths and Colors

Different colors of visible light correspond to different wavelengths.


12

Electromagnetic Spectrum


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15

Rank the following types of radiation is order of increasing 1) wavelength, 2) frequency, and 3) energy:

microwave, visible, x-ray, radio, gamma ray, infrared

A) gamma ray, x-ray, visible, infrared, microwave microwave, infrared, visible, x-ray, gamma ray microwave, infrared, visible, x-ray, gamma rayB) microwave, infrared, visible, x-ray, gamma ray gamma ray, x-ray, visible, infrared, microwave gamma ray, x-ray, visible, infrared, microwaveC) gamma ray, x-ray, visible, infrared, microwave microwave, infrared, visible, x-ray, gamma ray gamma ray, x-ray, visible, infrared, microwave


16

In which way does a photon of blue light NOT differ from a photon of red light?a. Energyb. Speedc. Wavelengthd. Colore. Frequency


17

Some light sources are comprised of all colors (white light).

Other light sources contain just a few colors.

Some are missing just a few colors.

Types of Spectra


18Wavelength

Brightness

Graphical Representation


19

Graphical Representation


Color and Temperature

Orion

Betelgeuse

Rigel

Stars appear in different colors,

from blue (like Rigel)

via green / yellow (like our sun)

to red (like Betelgeuse).

These colors tell us about the star’s

temperature.


Black Body Radiation (1)The light from a star is usually concentrated in a rather narrow range of wavelengths.

The spectrum of a star’s light is approximately a thermal spectrum called a black body spectrum.

A perfect black body emitter would not reflect any radiation. Thus the name “black body”.


Two Laws of Black Body Radiation1. The hotter an object is, the more energy it emits:

F = σ*T4

where

σ = Stefan-Boltzmann constant

F = Energy Flux =

= Energy given off in the form of radiation, per unit time and per unit surface area [J/s/m2];

Energy Flux


Two Laws of Black Body Radiation

2. The peak of the black body spectrum shifts towards shorter wavelengths when the temperature increases.

→ Wien’s displacement law:

λmax ≈ 3,000,000 nm / TK

(where TK is the temperature in Kelvin)


lecture 12 (10/10), chapter 6 & 7

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