Lecture 12 4/8/2013 2:44:00 PM
Light and Telescope Part II
Review: Properties of light:
Light is a wave and carries energy
Travels through space as a wave, but in some of its interactions
with matter, acts more like particles
Visible light makes up a small part of the electromagnetic spectrum
which also includes radio, infrared, ultraviolet, gamma rays, etc.
All of the above can be considered different types of light.
Like all waves, light waves have properties of wavelength,
frequency and wave speed, which are related:
o Wavelength: distance between two wave crests
o Frequency: number of times per second that a wave vibrates
up and down.
Wave speed: wavelength x frequency
Wave speed is fixed by the medium (for like in a vacuum, it is the
speed of light), so that means
The higher the frequency, the shorter the wavelength, and the
other way around.
For visible light, redder light = lower frequencies, longer
wavelengths. Bluer light = higher frequencies, shorter wavelengths.
In the electromagnetic spectrum, radio waves have the longest
waves/lowest frequencies, gamma rays have the shortest
Wave length comparisons:
o Radio: can be as big as a football field, or bigger
o Gamma: comparable to an atomic nucleus
Photons: particles of light:
When it interacts with matter, light energy is emitted to absorbed in
discrete chunks or packets: photons.
The size (energy) of a photon depends on its frequency: E=hf.
o (h = Planck’s constant = 6.62x10^-34 J s)
in other words, photons of higher frequency light have more energy
than for lower frequency light. Photons: higher frequency = higher energy:
Photons of higher frequency light have more energy than for lower
Medical (and other) consequences of this: higher-energy photons
can do more damage to living cells when absorbed. So, visible light
is pretty harmless, but ultraviolet light (with its higher
frequency/more energetic photons) can cause sunburn and skin
X-ray and gamma rays photons have even higher energies, so too
much exposure to them can be more dangerous.
Light: Our Messenger from the Universe:
Not all forms of light can get through Earth’s atmosphere: this is
one of the reasons for building observatories in space.
On earth, we can only see astronomical objects through visible light
and radio waves.
How do telescopes help?
You probably know that they make small things look bigger (they
magnify the apparent angular size of distant objects, allowing you
to see more detail.)
Limitations of the human eye: how telescopes help:
Remember that light intensity (brightness) describes the amount of
energy falling on a surface per unit area.
Some objects (for example, far away stars) are hard to see because
they are too dim. Not enough energy from them is reaching our
But you can increase the amount of energy collected by increasing
the surface area.
Pupils of eyes have a small area: one thing telescopes do is
concentrate light from a larger area.
How do telescopes help: light-gathering power:
A telescope acts as a “light bucket”, or a “light funnel”. Ability of a telescope to collect more light is called “light-gathering
Twice as much area twice as much light.
10 times as much area ten times as much light.
Brightness and Magnitude of Stars:
Stars are classified by apparent visual magnitude, a number
repressing their brightness.
Originally, there wee just six numbered classes:
o First magnitude = brightest stars, sixth magnitude = faintest
visible with the unaided eye.
Ptolemy used this magnitude system in his writing, others may
have used it earlier.
More recently, it became possible to measure flux (energy per
square meter per second) of light more precisely, allowing a more
precise numerical measurement of brightness, mv. (apparent visual
Smaller numbers mean brighter, larger numbers mean
With the new sale, some of the brightest objects have negative
Visual magnitude only counts visible light.
Telescopes: why bigger is better:
A wider telescope means more light gathering power.
Another limitation of human eyes: diffraction.
o The bending of the edges of waves as they pass through an
opening (or around an obstacle)
Diffraction blurs images, so if two stars are too close together in
angular distance, their images blur into one.
Diffraction limit (angular resolution): This puts a limit on the smallest angular sizes we can observe with
a telescope (or eye, camera, etc.)
o Diffraction limit (degrees) = (25/3)^