AY 101 Lecture Notes - Lecture 6: Chromatic Aberration, Refraction, Wavelength
Electromagnetic Radiation (Light)
The second great area of physics necessary to address the universe is the subject
of light, or electromagnetic radiation. Visible light is the relatively narrow frequency
band of electromagnetic waves to which our eyes are sensitive. Wavelengths are
usually measured in units of nanometers (1 nm = 10 −9m) or in units of angstroms (1
Å = 10 −10m). The colors of the visible spectrum stretch from violet with the shortest
wavelength to red with the longest wavelength.
However, electromagnetic radiation consists of more than just visible light; it also
includes (from short wavelength to long wavelength) gamma‐radiation, X‐radiation,
ultraviolet, visible, infrared (heat), microwaves, and radio waves (see Figure 1). All of
these forms of light have both electrical and magnetic characteristics. The properties of
light (see the section, Particle properties of light) allow us to build devices to observe
the universe and to deduce the physical nature of the sources that emit the radiation
received during these observations. However, these same properties mean that light
interacts with other matter before it reaches the observer and this often complicates our
ability to observe other objects in the universe. Note that the word radiation can refer
to any phenomena that radiates (moves) outwards from a source, here electromagnetic
or light radiation. The term should not be confused with radiation associated with a
radioactive source, i.e. nuclear radiation.
Particle properties of light
Light is such a complicated phenomena that no one model can be devised to explain its
nature. Although light is generally thought of as acting like an electric wave oscillating in
space accompanied by an oscillating magnetic wave, it can also act like a particle. A
particle of light is called a photon, or a discrete packet of electromagnetic energy.
Most visible objects are seen by reflected light. There are a few natural sources of light,
such as the Sun, stars, and a flame; other sources are man‐made, such as electrical
lights. For an otherwise non‐luminous object to be visible, light from a source is
reflected off the object into our eye. The property of reflection, that light can be
reflected from appropriate surfaces, can most easily be understood in terms of a particle
property, in the same sense that a ball bounces off a surface. A common example of
reflection is mirrors, and in particular, telescope mirrors that use curved surfaces to
redirect light received over a large area into a smaller area for detection and recording.
When reflection occurs in particle‐particle interactions (for example, colliding billiard
balls), it's called scattering — light is scattered (reflected) off molecules and dust
particles that have sizes comparable to the wavelengths of the radiation. As a
consequence, light coming from an object seen behind dust is dimmer than it would be
without the dust. This phenomena is termed extinction. Extinction can be seen in our
own Sun when it becomes dimmer as its light passes through more of the dusty
atmosphere as it sets. Similarly, stars seen from Earth seem fainter to the viewer than
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they would if there were no atmosphere. In addition, short wavelength blue light is
preferentially scattered; thus objects look redder (astronomers refer to this
as reddening); this occurs because the wavelength of blue light is very close to the size
of the particles that cause the scattering. By analogy, consider ocean waves — a row
boat whose length is close to the wavelength of the waves will bob up and down,
whereas a long ocean liner will scarcely notice the waves. The Sun appears much
redder at sunset. The light of stars also redden in passing through the atmosphere. You
can see the scattered light by looking in directions away from the source of the light;
hence the sky appears blue during the day.
Extinction and reddening of starlight are not caused by just the atmosphere. An
exceedingly thin distribution of dust floats between the stars and affects the light that we
receive as well. Astronomers must take into account the effect of dust on their
observations to correctly describe the conditions of the objects that emit the light. Where
interstellar dust is especially thick, no light passes through. Where dust clouds reflect
starlight back in our direction, the observer may see blue interstellar wispiness like thin
clouds surrounding some stars, or a nebula (to use the Latin word for cloud). A nebula
formed by scattering of blue light is called a reflection nebulae.
Wave properties of light
Most properties of light related to astronomical use and effects have the same
properties as waves. Using an analogy to water waves, any wave can be characterized
by two related factors. The first is a wavelength (λ) the distance (in meters) between
similar positions on successive cycles of the wave, for example the crest‐to‐crest
distance. The second is a frequency (f) representing the number of cycles that move by
a fixed point each second. The fundamental characteristic of a wave is that
multiplication of its wavelength by its frequency results in the speed with which the wave
moves forward. For electromagnetic radiation this is the speed of light, c = 3 ×
10 8 m/sec = 300,000 km/sec. The mid‐range of visible light has a wavelength of λ =
5500 Å = 5.5 × 10 −7 m, corresponding to a frequency f of 5.5 × 10 14 cycles/sec.
When light passes from one medium to another (for example, from water to air; from air
to glass to air; from warmer, less dense regions of air to cooler, denser regions and
vice‐versa) its direction of travel changes, a property termed refraction. The result is a
visual distortion, as when a stick or an arm appears to bend when put into water.
Refraction allowed nature to produce the lens of the eye to concentrate light passing
through all parts of the pupil to be projected upon the retina. Refraction allows people to
construct lenses to change the path of light in a desired fashion, for instance, to produce
glasses to correct deficiencies in eyesight. And astronomers can build refracting
telescopes to collect light over large surface areas, bringing it to a common focus.
Refraction in the non‐uniform atmosphere is responsible for mirages, atmospheric
shimmering, and the twinkling of stars. Images of objects seen through the atmosphere
are blurred, with the atmospheric blurring or astronomical seeing generally about one
second of arc at good observatory sites. Refraction also means that positions of stars in
the sky may change if the stars are observed close to the horizon.
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Related to refraction is dispersion, the effect of producing colors when white light is
refracted. Because the amount of refraction is wavelength dependent, the amount of
bending of red light is different than the amount of bending of blue light; refracted white
light is thus dispersed into its component colors, such as by the prisms used in the first
spectrographs (instruments specifically designed to disperse light into its component
colors). Dispersion of the light forms a spectrum, the pattern of intensity of light as a
function of its wave length, from which one can gain information about the physical
nature of the source of light. On the other hand, dispersion of light in the atmosphere
makes stars undesirably appear as little spectra near the horizon. Dispersion is also
responsible for chromatic aberration in telescopes — light of different colors is not
brought to the same focal point. If red light is properly focused, the blue will not be
focused but will form a blue halo around a red image. To minimize chromatic aberration
it is necessary to construct more costly multiple‐element telescope lenses.
When two waves intersect and thus interact with each other, interference occurs. Using
water waves as an analogy, two crests (high points on the waves) or two troughs (low
points) at the same place constructively interfere, adding together to produce a higher
crest and a lower trough. Where a crest of one wave, however, meets a trough of
another wave, there is a mutual cancellation or destructive interference. Natural
interference occurs in oil slicks, producing colored patterns as the constructive
interference of one wavelength occurs where other wavelengths destructively interfere.
Astronomers make use of interference as another means of dispersing white light into
its component colors. A transmission gratingconsisting of many slits (like a picket
fence, but numbering in the thousands per centimeter of distance across the grating)
produces constructive interference of the various colors as a function of angle.
A reflection grating using multiple reflecting surfaces can do the same thing with the
advantage that all light can be used and most of light energy can be thrown into a
specific constructive interference region. Because of this higher efficiency, all modern
astronomical spectrographs use reflection gratings.
A number of specialized observing techniques result from application of these
phenomena, of which the most important is radio interferometry. The digital radio
signals from arrays of telescopes can be combined (using a computer) to produce high‐
resolution (down to 10 −3 second of arc resolution) pictures of astronomical objects.
This resolution is far better than that achievable by any optical telescope, and thus,
radio astronomy has become a major component in modern astronomical observation.
Diffraction is the property of waves that makes them seem to bend around corners,
which is most apparent with water waves. Light waves are also affected by diffraction,
which causes shadow edges to not be perfectly sharp, but fuzzy. The edges of all
objects viewed with waves (light or otherwise) are blurred by diffraction. For a point
source of light, a telescope behaves as a circular opening through which light passes
and therefore produces an intrinsic diffraction pattern that consists of a central disk
and a series of fainter diffraction rings. The amount of blurring as measured by the width
of this central diffraction disk depends inversely on the size of the instrument viewing
the source of light. The pupil of the human eye, about an eighth of an inch in diameter,
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Document Summary
The second great area of physics necessary to address the universe is the subject of light, or electromagnetic radiation. Visible light is the relatively narrow frequency band of electromagnetic waves to which our eyes are sensitive. Wavelengths are usually measured in units of nanometers (1 nm = 10 9m) or in units of angstroms (1. The colors of the visible spectrum stretch from violet with the shortest wavelength to red with the longest wavelength. However, electromagnetic radiation consists of more than just visible light; it also includes (from short wavelength to long wavelength) gamma radiation, x radiation, ultraviolet, visible, infrared (heat), microwaves, and radio waves (see figure 1). All of these forms of light have both electrical and magnetic characteristics. The properties of light (see the section, (cid:1688)particle properties of light(cid:1689)) allow us to build devices to observe the universe and to deduce the physical nature of the sources that emit the radiation received during these observations.
