Download Light and Telescopes - Otterbein University

Stefan’s Law
• A point on the Blackbody curve tells us
how much energy is radiated per frequency
interval
• Question: How much energy is radiated in
total, i.e. how much energy does the body
lose per unit time interval?
• Stefan(-Boltzmann)’s law: total energy
radiated by a body at temperature T per
second: P = A σ T4
• σ = 5.67 x 10-8W/(m2 K4)
Energy & Power Units
• Energy has units Joule (J)
• Rate of energy expended per unit time is
called power, and has units Watt (W)
• Example: a 100 W = 100 J/s light bulb
emits 100 J of energy every second
• Nutritional Value: energy your body gets
out of food, measured in Calories = 1000
cal = 4200 J
Group Activity: Estimate Power
• Estimate how much energy you radiate per
second
• Estimate how many candy bars you would
have to eat per day to be able to do that
• Ponder the paradox
Doppler Shift
• From Wikipedia
Doppler Shift
• Can use the Doppler shift to
determine radial velocity of
distant objects relative to us
• Transverse velocity can be
measured from the motion of
stars with respect to background over a period of years
Homework: Doppler Shift of
Hydrogen spectrum
• The discrepancy between the wavelength of
a line measured in the lab versus measured
on an object is proportional to the velocity
of the object
• Apparent/ true wavelength = 1+ velocity/c
• Example:
– Observed(or apparent): 698 nm
– Actual( or ture or lab) wavelength: 656.3nm
– velocity = (698nm/656.3nm -1) c = 19100 km/s
Atomic Energy Levels
• For Hydrogen, the energies of the atomic levels are
given by a simple formula that just depends on the
(excitation) number n of the orbit: En = – Ry / n2
where Ry = 13.6 eV = 2.179 x 10-18J  E1, E2=¼ E1, E3=1/9 E1,…
• Electrons in higher levels will cascade down,
producing many different spectral lines
• Formula can be converted to frequency, wavelength
Group Work
• What is the wavelength of a photon emitted
as an electron in a Hydrogen atom jumps
from the second to the first level?
• What is the frequency of the emitted
photon?
• How many different photons can be emitted
as hydrogen goes from the fourth to the first
level?
Light hits Matter: Refraction
• Light travels at different speeds in vacuum, air,
and other substances
• When light hits the material at an angle, part of it
slows down while the rest continues at the original
speed – results in a change of direction
– Different colors bend different amounts – prism,
rainbow
Application for Refraction
• Lenses use refraction to focus light to a
single spot
Light hits Matter (II): Reflection
• Light that hits a mirror is
reflected at the same
angle it was incident
from
• Proper design of a mirror
(the shape of a parabola)
can focus all rays
incident on the mirror to
a single place
Application for Reflection
• Curved mirrors use reflection to focus light
to a single spot
Telescopes
• From Galileo to Hubble
• Light collectors
• Two types:
– Reflectors
(Mirrors)
– Refractors
(Lenses)
• Magnification:
– ratio of focal
lengths of
objective and
eyepiece
– M = fobj/feye
– Example:
2000mm
telescope with
40mm eyepiece:
50x
Telescopes
Reflecting Telescopes
Problems with Refractors
• Different colors (wavelengths) bent by
different amounts – chromatic aberration
• Other forms of aberration
• Deform under their own weight
• Absorption of light
• Have two surfaces that must be optically
perfect
Telescope Size
• A larger telescope gathers more light (more
collecting area)
• Angular resolution is limited by diffraction
of light waves; this also improves with
larger telescope size
Resolving Power of Telescopes
Andromeda Galaxy
Telescope 1
Telescope 2 of double size
Resolving Power of Telescopes (II)
Andromeda
Galaxy
Resolution:
(a) 10’
(b) 1’
(c) 5”
(d) 1”
Magnification
• The magnification of a telescope can easily
be changed by plugging in a different
eyepiece with a different focal length
• M= focal length of main lense or mirror
focal length of eyepiece
Example: F= 2000mm, f = 40 mm  M= 50
Atmospheric Limitations
Atmospheric Limitations
Optical Window
Radio Window IR Window
Largest Earth-Based Telescopes
• Hobby-Eberly Telescope,
Davis Mountains, TX
– 11 m diameter
– Cannot see all parts of the
sky
• Keck I and II, Mauna Kea,
HI
– 36  1.8 m hexagonal
mirrors;
equivalent to 10 m
– Above most of atmosphere
(almost 14,000 ft ASL)
– Operating since 1993
Other Techniques
• Put telescopes on
satellites
– Hubble Space
Telescope: 2.4 m,
since 1990
• Use computers to
correct optics during
light gathering:
adaptive and active
optics
• Interferometry
• Radio astronomy
Other Wavelengths
• Must be carried out on satellites (or rockets,
balloons, etc.) due to strong absorption in the
atmosphere
• Infrared astronomy
• High-energy (UV, X-ray, gamma-ray) astronomy
Full-Spectrum
Coverage
• Radio
• Infrared
Each region of the
• Visible
electromagnetic
spectrum gives us
valuable information
about the universe
• X-Ray
(only these frequency bands can
be observed with groundbased telescopes)
• Gamma
-ray
Activity - Telescopes
• Pick up a telescope
• Tasks:
–
–
–
–
–
–
–
What type of telescope do you have?
How big is it, i.e. what is its biggest optical device?
What is its focal length?
What is the focal length of the eyepiece?
What is its magnification?
Try to focus on an object. Describe the image. Is it upside down?
If the telescope is on a mount:
• How many axes does the mount have? Does it have a motor? What for?
• Do you know how the mount is called?
You’ll probably have a lot of questions. Ask me!
Famous Telescopes - Galileo
• Galileo’s first telescope was 3x magnifying
• his last one 32 x
Famous Telescopes -Newton
• First Reflector ever
• Built around 1670
• After this: gargantuan
Telescopes!
Famous Telescopes - Hevelius
Rooftop observatory of Johannes Hevelius (1670)
Famous Telescopes - Hevelius
60 inch ^
140 inch 
Famous Telescopes - Herschel
Herschel detected Uranus (1781)
Famous Telescopes – Lord Ross
• 72 inch Reflector
• built during potato famine in Ireland
• Largest Telescope until Mt Wilson
(1917)
Famous Telescopes – Yerkes
• Largest Refractor
Telescope ever
• 40 inch lens
• Built 1897
Famous Telescopes – Mt Palomar
• 5 Meter Telescope – Huge and heavy mirror
• On Mt. Palomar in California
Visiting
Mount
Palomar
• November 2005
Model of the 200” Telescope
Model of
the Mirror
• The telescope
is located
between Palm
Springs and
San Diego on
top of 5000 ft
high Mount
Palomar
Actual 5m
Mirror
• Took 3 years to
cool down
• 10 years to
polish
Transport of the
Mirror ~ 1945
Famous
Telescopes –
Hubble Space
Telescope
• In orbit around earth
• No limitations due
to earth’s
atmosphere
• Brilliant pictures
Famous Telescopes – Arecibo Radio
Telescope
• Located in
Puerto Rico
• 300m
diameter
• Receives
Radio waves
• Built 1963
• SETI
Visiting
Arecibo
• Spring Break 2006
Famous People
Hubble in prime focus of
Mt Palomar.
Einstein visits Mt Wilson
Hubble detected the Expansion of the Universe
 Proof of Einstein’s General Relativity Theory
Largest Earth-Based Telescopes
• Keck I and II,
Mauna Kea,
Hawai’i
– 36  1.8 m
hexagonal mirrors;
equivalent to 10 m
– Above most of
atmosphere
(almost 14,000 ft
ASL)
– Operating since
1993
Visiting Mauna Kea – December 2004
Mauna Kea
• Elevation: 14,000 ft.
• Oxygen: 60%
• Freezing on top,
snorkeling at sea level
• Road: strictly 4 wheels!
Mauna Kea
Maui
• 325 observing
days per year!
• Darkest skies on
the planet!
The biggest Telescopes in the World
Sunset on
Mauna Kea