EM Waves 04

OPENING QUESTION:

  • Give an example of how we experience each type of EM light here on Earth in our day to day lives....

  • Back in the day, there was a CB ("Citizens Band") Radio craze... lots of folks bought 'em and pretended to talk trucker linkgo like THIS. Anywhoooo.... as you might expect CB Radio is part of the Radio Band of the EM spectrum. So, let's say you are tuned into a CB wavelength of 11.0 meters:

    Calculate the frequency of that light wave

    Calculate the energy contained in ONE (count 'em ONE) photon of light having that wavlength.

  • Gamma Ray light has incredibly high frequencies.
    • Calculate the wavelength AND energy of a gamma ray with a frequency of 1.0 x 1019 hz.

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LEARNING TARGET: I will be able to diagram how each type of light in the electromagnetic spectrum is created HERE ON EARTH.

WORDS O' THE DAY:

  • hertz (Hz): a measure of wave frequency in waves/second
  • electromagnetic waves (light waves from radio - gamma)
  • quantum jump
  • energy levels
  • excited (electron) = ("electrons 'jolted' into a higher energy level")
  • rest state (electron) = ("electrons existing on the lowest energy level")

FORMULAE OBJECTUS:

h = Plank's Constant = (6.63 x 10-34 m2kg/s)

c=λυ: The speed of light (c) = wavelength (λ "lamda") x frequency "nu")

E = hυ (The energy in a photon of light is given by Plank's Constant times the frequency of that light)

WORK O' THE DAY:

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Let's volunteer some folks to get their opening ?'s up on the board...

Now let's have a wee bit of fun with light. We have talked about the very odd concept that a photon of light has Zero mass.

However, a photon of light very definitely has momentum (remember how gamma rays are used to zap cancer cells?):

Work with your group to see if you can derive a formula for the "equivalent" mass of a photon (sometimes referred to as a "rest mass" but that is rather misleading, since photons don't ever 'rest')

 

 

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Hint:

E = mc2

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Once you have the "equivalence" formula derived, please use it to find the "equivalent" mass of each of the two photons discussed in today's opener.

Now let's talk about what those formulae mean:

E = mc2 is one of the most (if not THE MOST) easily recognizable formula in all of science. Generally stated, it says that energy and mass are interchangeable.

That is by way of saying that we can determine the amount of energy released when a certain amount of mass is converted into energy...

That may seem AWFULLY theoretical until we look at THIS:

The "Tsar Bomb" was the largest nuclear detonation every achieved. It was the heart of the "Cold War" and the Soviets designed a bomb that would be equivalent to 100 million tons of TNT. Either the bomb didn't *quite* go off as calculated or the Soviets got nervous and backed it down by about half (I've read both happend, I dunno).

Anywho...

Let's do some math:

1.000 kiloton yield =

4,184,000,000,000 joules of energy = 4.184 x 1012 J

58.00 megatons = 58,000 x 1.00 kt =>

(5.8 x 103 kilotons) x (4.184 x 1012J)

= 2.427 x 1017 J

Now let's work backwards using E = mc2:

E = mc2

Isolate for mass:

E/c2 = m

Substitute for c (speed of light) and E (energy of the Tsar Bomb calculated in Joules above):

(2.427 x 1017 J)/(3.00 x 108 m/s)2

Do the math:

2.70 kg of mass turned into energy