Download Class Exercise10 Basic Em1 Spectrum Light

Name: ___________________________
Introductory Exercise: Very Basics of Lights and Electromagnetic Waves
Basics: Electromagnetic (EM) waves are
traveling waves of electric and magnetic
fields moving through space. “Light”
often is used unofficially to refer to all EM
waves but what human eyes see as light is
only the “optical” or “visible” range of
the entire EM spectrum. The colors of
rainbows essentially are the visible light spectrum perceived by human eyes and the
wavelength is from about 400 nm (blue) to about 700 nm (red).
(Explanation of waves - NASA:
http://missionscience.nasa.gov/ems/02_anatomy.html)
(Credit: http://sob.nao-rozhen.org/content/green-flash-phenomenon)
Other examples of (non-electromagnetic) traveling waves are ripples, sound waves
and seismic waves of an earthquake.
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Ripples, or Surface Waves
Sound Wave
http://missionscience.nasa.gov/ems/02_anatomy.html
http://imagine.gsfc.nasa.gov/YBA/M31-velocity/Doppler-shift-2.html
Radi
o, infrared, visible, ultraviolet, x-ray and
gamma-ray are other EM waves of different spectrum range, that is, with different
wavelength and frequency. Human eyes cannot see these waves that are not in the
optical (visible) range but sophisticated detectors are built to observe in various
frequencies.
Basic Exercise – Ask some questions about UV, IR etc.
1) Radio Astronomy is a very important part of Astronomy but receives lots less
coverage. What is the wavelength of a typical (middle of the road) long radio wave?
Speed of Light, Frequency and Wavelength
Light or all EM waves travel at a constant speed. The speed of light is denoted by
the letter c and the value is c = 300,000,000 m/s = 3 x 108 m/s (meters per second).
The general relation of the velocity (or speed), frequency and wavelength for any
traveling wave is
c = f * velocity or speed = frequency * wavelength )
where f (frequency) is number of cycles repeated per unit time (second), and 
(wavelength) is the physical length or distance traveled per oscillating cycle (peak-topeak distance of the wave). The unit for frequency is “hertz”, which is 1/s, or cycle
per second, and the standard unit for wavelength is “meter”, of course.
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Name: ___________________________
Online information:
1. Frequency (Wikpedia) : http://en.wikipedia.org/wiki/Frequency
2. Read the section “Light and the Electromagnetic Spectrum” in the following:
http://elmhurst.edu/~ksagarin/color/discussion1.html
Practice 1)
What is the speed of red light? (hint: read above description carefully….)
Does red light travel through space slower or faster than other lights?
How far does it travel in one second?
Practice 2) Red light has wavelength of about 700 nm (nanometer), what is it
frequency – that is, how many times does it oscillate or wiggle every second? Use c
= f * and simply solve algebraically and numerically for f, since you now know the
value of c and . NOTE that 700 nm needs to be converted properly so that you
can match unit properly with the unit of c, the speed of light. To do so, recall
previous learning exercises and convert 700 nm to meter!
Practice 3) Compare red light to blue light. Which one is “longer” (wavelength)?
Which one “wiggles faster” (frequency)? Which one travels faster through space?
Practice 4) Refer to the EM spectrum illustration. Can human eyes see Cosmic
“Microwave” Background Radiation that permeates the entire universe? What about
the radiations inside your tiny home “microwave” oven?
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Name: ___________________________
Light as Photons:
“Photon” is the modern language for light in the context of quantum mechanics,
which describes light as particles. Thus photons can be thought of as tiny “roundish
objects of pure energy” traveling through space. Each photon has energy that is
directly proportional to its frequency f:
E= h f
where h=6.626 x 10-34m2 kg/s is “Planck's constant”. The standard unit for energy is
Joule.
For example, an x-ray photon has frequency of about f =1018 hertz (1/s), and
therefore the energy per x-ray photon is:
E=hf
= (6.626 x 10-34m2 kg/s ) (1018 hz)
= 6.626 x 10-34+18 Joules
= 6.626 x 10-16J.
Neither the photon concept nor the above frequency-energy relation is known prior
to the advent of quantum mechanics in early 1900s and cannot be established using
the older “wave” formulation of electromagnetism. It is also worth noting that
modern optics experiments are capable of detecting lights of extreme low intensity to
observe individual photons.
Practice 5) Gamma rays are highly energetic with a typical frequency of 1020 hertz
(1/s). How much energy does each gamma ray photon carry? Refer to above
example.
Practice 6) Does green light have longer or shorter wavelength than red light? Have
higher or lower frequency than red light? From that information answer the question
whether a green light photon have more or less energy than a red light photon.
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Name: ___________________________
Practice 7) How much more energetic is a gamma ray photon compare to a red light
photon? Hint: Since energy is directly proportional to frequency (see above formula),
as h is a constant, then comparison of 2 energies simply requires a comparison of
their frequency without needing to compute energy. It is just a simple ratio (but of
what?). Still if you are not sure you can compute energy explicitly for practice.
(Bonus: +1 ) Practice 8) Given a very common 40 watts lights bulb, let's estimate how
many photons it emits per second.
 A Watt, the standard unit of power measuring flow of energy per unit time,
means one Joule per second, J/s, so the question translates to “how many
photons is emitted per second so that the total is 40 joules of energy”?
 A common light bulb emits white light so it include all colors of light. But we
can ignore that and simply use red light (or any color) since the wavelengths
are relatively close. We are, after all, only making an estimate. Use previous
practice for the energy of a single red light photon, expressed in joule.
 Calculate “how many red light photons would total up to 40 joules?”
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Survey: On a scale of 0 to 5, how familiar are you with the exercise materials? Add
your thoughtful and constructive comment (for 1 extra pt):
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