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Energy Carried by Photons
• A photon carries energy with it
that is related to its wavelength
or frequency
E
hc

 h 
• From this we see that long
wavelength (low frequency)
photons carry less energy than
short wavelength (high
frequency) ones. This is why
UV waves give us a sunburn,
and X-rays let us look through
skin and muscles!
Seeing Spectra
• Seeing the Sun’s
spectrum requires a few
special tools, but it is
not difficult
– A narrow slit only lets a
little light into the
experiment
– Either a grating or a
prism splits the light
into its component
colors
– If we look closely at the
spectrum, we can see
lines, corresponding to
wavelengths of light
that were absorbed.
Emission Spectra
•
Imagine that we have a hot
hydrogen gas.
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–
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Collisions among the hydrogen
atoms cause electrons to jump
up to higher orbitals, or energy
levels
Collisions can also cause the
electrons to jump back to lower
levels, and emit a photon of
energy hc/
If the electron falls from orbital
3 to orbital 2, the emitted
photon will have a wavelength
of 656 nm
If the electron falls from orbital
3 to orbital 2, the emitted
photon will have a wavelength
of 486 nm
• We can monitor the gas, and count how many
photons of each wavelength we see. If we
graph this data, we’ll see an emission
spectrum!
Emission spectrum of hydrogen
• This spectrum is
unique to hydrogen
– Like a barcode!
• If we were looking
at a hot cloud of
interstellar gas in
space, and saw
these lines, we
would know the
cloud was made of
hydrogen!
Different atom, different spectrum!
•
Every element has its
own spectrum. Note the
differences between
hydrogen and helium
spectra below.
Absorption Spectra
• What if, instead of hot
hydrogen gas, we had a cloud
of cool hydrogen gas between
us and a star?
– Photons of an energy that
corresponds to the
electronics transitions in
hydrogen will be absorbed
by electrons in the gas
– The light from those photons
is effectively removed from
the spectrum
– The spectrum will have dark
lines where the missing light
would be
– This is an absorption
spectrum!
– Also like a barcode!
Types of Spectra
•
Kirchoff’s Laws:
– If the source emits light that is
continuous, and all colors are present,
we say that this is a continuous
spectrum.
– If the molecules in the gas are wellseparated and moving rapidly (have a
high temperature), the atoms will emit
characteristic frequencies of light. This
is an emission-line spectrum.
– If the molecules of gas are wellseparated, but cool, they will absorb
light of a characteristic frequency as it
passes through. This is an absorption
line spectrum.
Spectra of Astronomical Objects
If absorption lines are seen in the spectrum of an
object, what are we seeing?
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•
•
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a. A gas in front of a source of continuum radiation.
b. We are seeing the glow from a gas by itself.
c. A source of continuum radiation is in front of a gas.
d. We are seeing the glow from a continuum source
by itself.
Doppler Shift in Light
• If an object is emitting light and
is moving directly toward you,
the light you see will be shifted
to slightly shorter wavelengths –
toward the blue end of the
spectrum, or blue-shifted
• Likewise, if the object is moving
away from you, the light will be
red-shifted.
• If we detect a wavelength shift of
 away from the expected
wavelength , the radial (line-ofsight) velocity of the object is:
VR =
Dl
l
´c
Telescopes
• Telescopes have been
used for hundreds of years
to collect light from the
sky and focus it into an
eyepiece. An astronomer
would then look through
this eyepiece at planets,
nebulae, etc.
• The human eye is not very
sensitive to dim light, and
was replaced in astronomy
by the film camera.
• Film is sensitive to only
around 10% of the
impinging light, and is
usually replaced by a…
The Charge-Coupled Device (CCD)
•
•
The CCD, similar to those
found in commercial digital
cameras and phones, utilizes
the photoelectric effect to
collect around 75% of the
visible light that is focused
on it!
It has revolutionized
astronomy – images can be
recorded and downloaded to
a computer anywhere in the
world for analysis
•
The science of developing new methods for
sensing, focusing and imaging light in
astronomy is called instrumentation
Outside the visible spectrum
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Many objects of astronomical interest are
visible only in wavelengths other than the
visible!
Much can be learned from studying a star,
planet or nebula in multiple wavelengths.
Radio telescopes can be used from the ground
to image pulsars and other bodies
•
Observations in other wavelengths
require instrumentation to be lifted
above the Earth’s atmosphere.
X-ray, Gamma ray and infrared
wavelength telescopes are currently
in orbit!
Radio Telescopes
• Radio telescopes,
like the one in
Arecibo, Puerto
Rico, collect radio
waves from
astronomical objects
and events
Size Matters!
•
•
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Aperture size is very important
when collecting light!
A large collecting area allows
astronomers to image dim and
distant objects.
For a telescope with an aperture
a distance D in diameter,
Collecting Area =
p
4
´ D2
Refraction
•
•
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Light moves at a fixed speed, c.
The value of c changes depending
on what substance, or medium, it
moves through.
The speed of light in vacuum is
around 300,000 km/s. Its speed
through glass or water, however, is
slightly slower
•
•
If a beam of light (or light ray) enters a
new medium at an angle, light on one
side of the ray enters first, and slows.
This slowing of one part of the ray
causes the ray to change direction,
similar to driving a car from asphalt onto
sand can make a car swerve.
This bending of a light ray’s path is
called refraction
Refraction in Water
Dispersion
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•
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The amount a light is diffracted depends on its wavelength
A prism spreads the light out, using this effect
This dispersion of light is a problem in refracting telescopes, as the focal planet will be
at a slightly different location for each wavelength of light
This leads to chromatic aberration, a blurring effect.
Lenses
• A lens is a specially
shaped piece of glass that
bends light rays passing
through it so that they
focus a particular
distance away (the focal
length) at a particular
location (the focal plane).
• A sensor such as a human
eye, a camera or CCD, if
placed in the focal plane
can image the light
Refracting Telescopes
• Telescopes that use lenses to focus
light are called refracting telescopes,
or refractors.
• Large refractors are difficult to build!
– Glass is heavy, and glass lenses must be
supported only by their rims, a difficult
engineering problem
– Glass sags under its own weight,
defocusing the light!
– Refractors suffer from chromatic
aberration, a blurring effect due to
changes in the focal plane of the lens
for different wavelengths of light
Reflecting Telescopes
• Reflecting telescopes, or
reflectors, use a curved
mirror to focus light
• Mirrors can be supported
from behind, and so can be
much larger than refractors
• Larger sizes mean that more
light can be collected and
focused, allowing
astronomers to image
dimmer or more distant
objects
• Most modern telescopes are
reflectors.
Different styles of reflectors
X-Ray reflectors
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X-rays only reflect at glancing angles,
otherwise they are absorbed or pass
through the mirror!
X-Ray mirrors are designed to gently
reflect the incoming photons, focusing
them at the end of a long tube-shaped
array of mirrors