Chapter 6: Light and Telescopes
Wave: a disturbance which carries energy through a material or
through empty space.
Often a regular series of disturbances
[slinky demos,wave1.gif fig 6.1,2, Trans_n_Long_harmonic.avi]
Wavelength (λ ): distance between successive crests
Frequency (f ): rate at which crests pass a fixed point
Energy Flux: rate at which the wave carries energy through a given
area, Intensity
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Electromagnetic Waves
Electric and Magnetic disturbance
[fig 6.2, em_plane_polarization1.avi]
wave speed: 3x108 m/s
=186,000 miles per second
= speed of light (c)
Ey
Bz
x
Electromagnetic Flux
energy per area = ‘brightness’
Flux-distance-energy relationship: Inverse Square Law
[figure 6-3 ,inverse_square_law.mov]
Intensity decreases with distance in a particular way (mathematically)
F=
E
4πd 2
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Wavelength and frequency
λf=c
[wave1.gif ]
higher frequency shorter wavelength
spectrum : different frequencies [Figure 6-5]
Doppler Effect: moving wave source’s motion affects received
frequency
[tuning fork demonstration, Figure 6-6, doppler_shift_interacti.swf]
wavelength shift radial speed related
∆λ =
λ 0vr
c
“red” shift
= lower frequency/longer wavelength
= moving away
“blue” shift
= higher frequency/shorter
wavelength
= moving towards
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Photons: particles of light
light comes in “quanta” (or packets) of energy = photons
photoelectric effect: an experiment whose results are inconsistent with the wave nature
of light [figure 6.7]
photon energy depends upon frequency [fig 6.8]
E=hf
higher f higher E lower λ
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Light “rays” [ http://www.falstad.com/ripple/, optical disk demos]
i
r
Reflection (light rays bounce) [Fig 6-9]
i
Refraction (light rays bend) [Fig 6-11,12]
index of refraction n = c/v
= the optical “thickness” of a material
r
also
Diffraction: light (as a wave) bends around corners
[http://www.falstad.com/ripple/]
Dispersion: light rays separated according to wavelength (color)
prism [Fig 6-13]
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Optical Telescopes
Refractor: objective is a lens
Lens [demonstration optical disk, fig 6.15,16]
focal point
focal length
focal point
focal length
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Optical Telescopes
Reflector: objective is a mirror
Mirror [demonstration optical disk, fig 6.18]
focal point
focal length
focal point
focal length
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Forming an Image [figure 6-19]
Focal Plane: where lens or mirror forms image
Brightness of an image depends upon:
•Light-gathering power of objective (area A)
•Size of image (focal length f )
~(Diameter/f)2
f-number is the “speed” of a lens or mirror = f/D
smaller f-number → more light → shorter exposure
Resolution: ability to resolve fine detail [figure 6.20, resolver.avi]
Diffraction: light (as a wave) bends around corners
→ smallest resolvable angle
θ = 250,000 λ/D
resolution depends upon wavelength! [resolver_wavelength.avi]
Magnification: angular size of image/angular size of object
m = fp/fe
[figure 6-21]
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Detectors: beyond the human eye (requirements for: sensitivity,
non-visible wavelengths, numerical measurements, permanent
record)
Photography: Telescope acts as a camera
Photometer: measures brightness of an object
CCDs (Charge Coupled Devices): digital electronic
imaging, very sensitive [fig6-22, 23 A=photo B=CCD of same field]
Spectroscopy: light is dispersed according to wavelength
[grating demonstration, suna.jpg (sun's spectrum in detail)]
Spectrograph: also known as a Spectrometer, a device
that measures spectra [figure 6-24]
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Telescopes
Large Refractors: largely pre 1900
40 inch (1 m) Yerkes Observatory [fig 6.25]
Large Reflectors
Mount Wilson (1.5 m in 1908, 2.5 m in 1917)
Mount Palomar (5m in 1948)
Mauna Kea (10-m Keck) telescope
[fig 6.26]
36 adjustable mirror segments
uses Adaptive Optics to improve seeing
seeing: blurring of an image because of atmospheric turbulence
[laser demonstration]
[06ex1.jpg, 06ex2.jpg, fig 6.31 (same telescope w/wout adaptive optics)]
Very Large Telescope (VLT) in Chile [figure 6-27]
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Observatories
Transmission through earth’s atmosphere depends upon wavelength [Fig 6-29]
Light Pollution [figure 6.30]
Optical Observatories [fig 6.32(Kitt Peak), fig 6.33 (Chile) fig 6.34 Hawaii]
Radio Telescopes: larger wavelengths = more resolution problems
Large Dishes (arecibo [fig 6.40])
increased resolution using interferometry (arrays)
VLA ~ 27 km [fig 6.41]
VLBA ~ continental USA [Figure 6-42])
[fig 6.44 length scale of resolvable objects at distance of moon by A:Arecibo B:VLA C:VLBA]
Space Observatories
Hubble Space Telescope [Figure 6-35,36]
James Webb Telescope (2013) Hubble's successor (visible & infrared)
Spitzer Space Telescope (Space Infrared Telescope Facility) [fig 6.37]
International Ultraviolet Explorer (1978 -1996)
Einstein X-Ray Observatory(1970's), Chandra (1999-) [fig 6.38,39]
Compton Gamma Ray observatory (1991-2000)
Planes, Rockets, Balloons: means to get above some of the atmosphere
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