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Chapter 06 - Light and Telescopes
CHAPTER 6
LIGHT AND TELESCOPES
CHAPTER OUTLINE AND LECTURE NOTES
Introduction
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Spatium Lux, the chapter opening painting by Dennis Davidson is inaccurate in that it shows too much of
the Moon illuminated by sunlight. The Moon should be shown as a much thinner crescent. The painting is
so beautiful, however, that I just couldn’t resist using it.
Waves
Because the students can’t actually see the wavelength and frequency of electromagnetic waves, it is a good
idea to make sure that the students have a firm idea about wavelength, frequency, the speed of a wave, and the
relationship among them before introducing electromagnetic waves. Figure 6.4 shows this relationship. An
even better way to show it is a standard piece of physics lecture demonstration apparatus that is very helpful.
The apparatus is a “wave machine” consisting of a number of thin rods supported and connected at their
midpoints by a wire. When one of the rods is moved from its rest position it twists the wire and causes the
next rod to move. If the system of rods is damped at one end, by shaking one end up and down you can get a
series of waves to travel down the system of rods. By changing the frequency, you can show the relationship
between frequency and wavelength.
Electromagnetic Waves
Although there are many ways to illustrate the Doppler effect, including Figure 6.6, my favorite is a batteryoperated whistle on the end of a piece of string. You can use it anywhere, even outside, and can put it in your
pocket. You can also whirl it slowly enough that the students can clearly tell when the whistle is moving away
(and the frequency is lowered) and toward them.
Reflection and Refraction
Optical Telescopes
The standard line drawings showing the properties of lenses, mirrors, and telescopes, can be improved on
using the standard “chalk board optics” demonstration apparatus, which consists of a projector that emits
parallel beams of light that graze the surface of a blackboard or whiteboard to show the direction of the beams
and mirrors and lucite lenses of different curvature. Laser versions of this apparatus are also available.
Optical Observatories
The original print of Figure 6.29 shows even towns as small as Riverside, Iowa, with a population of a few
hundred. You can also see some interstate highways outlined by lights. The Astronomical Society of the
Pacific and Kitt Peak National Observatory both sell a spectacular poster of the world at night. It shows not
only artificial lighting around the world but also the aurorae and gas fires in oil producing regions. A similar
map was included with the November 2004 issue of National Geographic.
Space Observatories
I have devoted relatively little space to infrared and ultraviolet space observatories because the techniques
used to gather and measure radiation are essentially the same at those wavelengths as for visible light.
Radio Telescopes
The advantages of interferometry are something I like to emphasize in my classes. I got the idea of using
Figure 6.44 to compare the angular resolutions of the Arecibo telescope (Figure 6.40), the VLA (Figure
6.41), and the VLBA (Figure 6.42) from a similar comparison presented in a lecture by Vincent Icke.
KEY TERMS
adaptive optics — A system for modifying the shape of the mirror of a telescope to compensate
for atmospheric seeing and produce sharp images.
charge coupled device (CCD) — An array of photosensitive electronic elements that can be
used to record an image falling on it.
detector — A device used to measure light once it has been brought to focus by a telescope.
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Chapter 06 - Light and Telescopes
diffraction – The change in the direction of a wave after passing an obstacle or through an
aperture that has about the same size as the wavelength of the wave.
dispersion — The separation of white light according to wavelength. Dispersion produces a
rainbowlike spectrum.
Doppler effect — The change in the frequency of a wave (such as electromagnetic radiation)
caused by the motion of the source and observer toward or away from each other.
electromagnetic spectrum — The range of frequency or wavelength of all possible
electromagnetic radiation. The electromagnetic spectrum includes (in order of increasing
wavelength) gamma ray, X ray, ultraviolet, visible, infrared, and radio.
electromagnetic wave — A periodic electrical and magnetic disturbance that propagates
through space and transparent materials at the speed of light. Light is an example of an
electromagnetic wave.
energy flux — The rate at which a wave carries energy through a given area.
focal length — The distance between a mirror and lens and the point at which the lens or mirror
brings light to a focus.
focal plane — The surface where the objective lens or mirror of a telescope forms the image of
an extended object.
focal point — The spot where parallel beams of light striking a lens or mirror are brought to a
focus.
frequency — The number of oscillations per second of a wave.
gamma ray — The part of the electromagnetic spectrum having the shortest wavelengths.
index of refraction — The ratio of the speed of light in a vacuum to the speed of light in a
particular substance. The index of refraction, which always has a value greater than 1.0,
describes how much a beam of light is bent on entering or emerging from the substance.
infrared — The part of the electromagnetic spectrum having wavelengths longer than visible
light but shorter than radio waves.
interferometry — The use of two or more telescopes connected together to operate as a single
instrument. Interferometers can achieve high angular resolution if the individual telescopes
of which they are made are widely separated.
light — The visible part of the electromagnetic spectrum.
light-gathering power — A number, proportional to the area of the principal lens or mirror of a
telescope, that describes the amount of light that is collected and focused by the telescope.
objective — The main lens or mirror of a telescope.
photometer — An instrument used to measure the brightness of a source of electromagnetic
radiation such as a planet, star, or galaxy.
photon — A massless particle of electromagnetic energy.
pixel — A “picture element,” consisting of an individual detector in an array of detectors used
to capture an image.
reflection — The bouncing of a wave from a surface.
reflectivity — The ability of a surface to reflect electromagnetic waves. The reflectivity of a
surface ranges from 0% for a surface that reflects no light to 100% for a surface that reflects
all the light falling on it.
reflector — A telescope in which the objective is a mirror.
refraction — The bending of light when it passes from a material having one index of
refraction to another material having a different index of refraction.
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refractor — A telescope in which the objective is a lens.
resolution — The ability of a telescope to distinguish fine details of an image.
seeing — A measure of the blurring of the image of an astronomical object due to turbulence in
the Earth’s atmosphere.
speckle interferometry — A technique to overcome seeing by analyzing very brief images.
spectrograph — A device used to produce and record a spectrum.
spectroscopy — The recording and analysis of spectra.
ultraviolet — The part of the electromagnetic spectrum having wavelengths longer than X rays,
but shorter than visible light.
wave — A regular series of disturbances that moves through a material medium or through
empty space.
wavelength — The distance between crests of a wave. For visible light, wavelength determines
color.
ANSWERS TO QUESTIONS AND PROBLEMS
Conceptual Questions
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The photoelectric effect shows that light behaves like a stream of particles. The operation of a radio
interferometer, in which electromagnetic radiation interferes like waves, shows that electromagnetic radiation
behaves like waves.
Snow has a reflectivity near 100%, coal has a reflectivity near 0%.
To make a drawing, consult Figure 6.19. Replace the lens with a mirror and draw rays as if reflected from
mirror rather than refracted.
CCDs are more sensitive and automatically produce quantitative rather than qualitative data.
Astronomers seek transparency, clear skies, good seeing, and dark skies.
In using space observatories astronomers seek to overcome atmospheric seeing and access to parts of the
spectrum blocked by Earth’s atmosphere.
Problems
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The wavelength of A is 1/4 the wavelength of B.
24 m/s
60 m
15 m
7.5 × 108 Hz, this is a radio wave
10-8 m, this is ultraviolet radiation
1/100
The spacecraft would have to be at 0.22 AU.
300 km/s away from each other
466.7 nm
A has twice the energy of B.
1.0
36 times as great
32 times as large
The focal length of the f/15 mirror is twice as long.
800 m
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100 m
Figure-Based Questions
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3 × 1022 Hz
300 m
50%
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