Monday Oct. 8, 2012
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A busy 10 minutes before the start of class with Green Cards to distribute, Experiment #2 reports and materials to collect, a demonstration to set up, and music playing in the background.  You heard "Swing 49", "Gypsymania", and "Turkish Delights" from the Live at Birdland Django Reinhart NY Festival.  I wasn't able to find any of those songs on YouTube, but here is "Caravan" which is a good substitute.

The Upper Level Charts Assignment has been graded.  If you lost 3 pts or less on the assignment you earned a Green Card.  Everyone that turned in an assigment earned 0.5 pts of extra credit.

The last of the 1S1P Assignment #1 reports (the radon topic) has been graded and was returned in class.

The final version of the Quiz #2 Study Guide is now online.

The world would not look the same if we were able to see IR light instead of visible light.

The picture at left was taken using normal film, film that is sensitive to visible light.  The picture at right used infrared film.  In both pictures we are looking at sunlight that strikes the tree or the ground and is reflected toward the camera where it can be photographed (i.e. these aren't photographs of light emitted by the tree or the ground).  The tree at left is green and relatively dark (it reflects green light but absorbs the other colors of visible light).  The tree at right and the ground are white, almost like they were covered with snow.  The tree and grass on the ground are reflecting infrared light.  Here are many more images taken with infrared film.



Here's another example, photographs of the ground taken from an air plane using ordinary film at left (responds to visible light) and  infrared  film at right.  Notice how the IR photograph is able to "see through" the  haze.  The haze at left is scattered light.  IR light is not scattered as readily as visible light.

Another example was shown in class, a thermal image of a house.  These are photograps of  infrared light that is being emitted (not reflected light) by a house.  Remember that the amount of energy emitted by an object depends strongly on temperature (temperature to the 4th power in the Stefan-Boltzmann law).  Thus it is possible to see hot spots that emit a lot of energy and appear "bright" and colds spots.  Photographs like these are often used to perform an "energy audit" on a home, i.e. to find spots where energy is being lost.  Once you locate one of these hot spots you can add insulation and reduce the energy loss.

Here are the rules for the amount and kind (wavelength of peak emission) of radiation emitted by an object.


Let's look at the light emitted by the sun and the earth.



The curve on the left is for the sun.  We have used Wien's law and a temperature of 6000 K to calculate λmax and got 0.5 micrometers.  This is green light; the sun emits more green light than any other kind of light.  The sun doesn't appear green because it is also emitting lesser amounts of violet, blue, yellow, orange, and red - together this mix of colors appears white.  44% of the radiation emitted by the sun is visible light,  Very nearly half of sunlight (49%) is IR light (37% near IR + 12% far IR).  7% of sunlight is ultraviolet light.  More than half of the light emitted by the sun (the IR and UV light) is invisible.

100% of the light emitted by the earth (temperature = 300 K) is invisible IR light.  The wavelength of peak emission for the earth is 10 micrometers. 

Because the sun (surface of the sun) is 20 times hotter than the earth the sun's surface emits energy at a much higher rate than the earth.  Note the vertical scale on the earth curve is different than on the sun graph.  If both the earth and sun were plotted with the same vertical scale, the earth curve would be too small to be seen.

In the demonstration in class last Friday we also learned that ordinary tungsten bulbs (incandescent bulbs) produce a lot of wasted energy.  This is because they emit a lot of invisible infrared light that doesn't light up a room (it will warm up a room but there are better ways of doing that).  The light that they do produce is a warm white color (tungsten bulbs emit lots of orange, red, and yellow light and not much blue, green or violet). 

Energy efficient compact fluorescent lamps (CFLs) are being touted as an ecological alternative to tungsten bulbs because they use substantially less electricity, don't emit a  lot of wasted infrared light, and also last longer.  CFLs come with different color temperature ratings.



The bulb with the hottest temperature rating (5500 K ) in the figure above is meant to mimic or simulate sunlight (daylight).  The temperature of the sun is 6000 K and lambda max is 0.5 micrometers.  The spectrum of the 5500 K bulb is similar.

The tungsten bulb (3000 K) and the CFLs with temperature ratings of 3500 K and 2700 K produce a warmer white. 

Three CFLs with the temperature ratings above were set up in class so that you could see the difference between warm and cool white light.  Personally I find the 2700 K bulb "too warm," it makes a room seem gloomy and depressing (a student in class once said the light resembles Tucson at night).   The 5500 K bulb is "too cool" and creates a stark sterile atmosphere like you might see in a hospital corridor.  I prefer the 3500 K bulb in the middle.

The figure below is from an article on compact fluorescent lamps in Wikipedia for those of you that weren't in class and didn't see the bulb display.  You can see a clear difference between the cool white bulb on the left in the figure below and the warm white light produced by a tungsten bulb (2nd from the left) and 2 CFCs with low temperature ratings (the 2 bulbs at right).



There is one downside to these energy efficient CFLs.  The bulbs shouldn't just be discarded in your ordinary household trash because they contain mercury.  They should be disposed of properly (at a hazardous materials collection site or perhaps at the store where they were purchased).

It probably won't be long before LED bulbs begin to replace tungsten and CFL bulbs.  At the present time the LED bulbs are pretty expensive.

We now have most of the tools we will need to begin to study energy balance on the earth.  It will be a balance between incoming sunlight energy and outgoing energy emitted by the earth.  This will ultimately lead us to an explanation of the atmospheric greenhouse effect. 

We will first look at the simplest kind of situation, the earth without an atmosphere (or at least an atmosphere without greenhouse gases).  The next figure is on p. 68 in the photocopied Classnotes.  Radiative equilibrium is really just balance between incoming and outgoing radiant energy.  Pages 68 and 69 were somehow left out of the photocopied ClassNotes.  Copies of these two pages were handed out in class.



You might first wonder how it is possible for the earth (with a temperature of around 300 K) to be in energy balance with the sun (6000 K).  At the top right of the figure you can see that because the earth is located about 90 million miles from the sun and it only absorbs a very small fraction of the total energy emitted by the sun.

To understand how energy balance occurs we start, in Step #1, by imagining that the earth starts out very cold (0 K) and is not emitting any EM radiation at all.  It is absorbing sunlight however (4 of the 5 arrows of incoming sunlight in the first picture are absorbed, 1 of the arrows is being reflected) so it will begin to warm  This is like opening a bank account, the balance will be zero at first.  But then you start making deposits and the balance starts to grow.

Once the earth starts to warm it will also begin to emit EM radiation, though not as much as it is getting from the sun (the slightly warmer earth in the middle picture is now colored blue).  Only the four arrows of incoming sunlight that are absorbed are shown in the middle figure.  The two arrows of reflected sunlight have been left off because they don't really play a role in energy balance (reflected sunlight is like a check that bounces - it really doesn't affect your bank account balance).  The earth is emitting 3 arrows of IR light (in red).  Because the earth is still gaining more energy than it is losing the earth will warm some more.  Once you find money in your bank account you start to spend it.  But as long as deposits are greater than the withdrawals the balance will grow.

Eventually it will warm enough that the earth (now shaded brown & blue) will emit the same amount of energy as it absorbs from the sun.  This is radiative equilibrium, energy balance (4 arrows of absorbed energy are balanced by 4 arrows of emitted energy).  The temperature at which this occurs is about 0 F.  That is called the temperature of radiative equilibrium (it's about 0 F for the earth).

Note that it is the amounts of energy not the kinds of energy that are important.  Emitted radiation may have a different wavelength than the absorbed energy.  That doesn't matter.  As long as the amounts are energy the earth will be in energy balance.

Before we start to look at radiant energy balance on the earth with an atmosphere we need to learn about filters.  The atmosphere will filter sunlight as it passes through the atmosphere toward the ground.  The atmosphere will also filter IR radiation emitted by the earth as it trys to travel into space.

We will first look at the effects simple blue, green, and red glass filters have on visible light.  This is just to be able to interpret a filter absorption curve or graph.




If you try to shine white light (a mixture of all the colors) through a blue filter, only the blue light passes through.  The filter absorption curve shows 100% absorption at all but a narrow range of wavelengths that correspond to blue light.  The location of the slot or gap in the absorption curve shifts a little bit with the green and red filters.

The following figure is a simplified, easier to remember, representation of the filtering effect of the atmosphere on UV, VIS, and IR light (found on p. 69 in the photocopied notes).  The figure was redrawn after class.



You can use your own eyes to tell you what effect the atmosphere has on visible light.  Air is clear, it is transparent.  The atmosphere transmits visible light.

In our simplified representation oxygen and ozone make the atmosphere pretty nearly completely opaque to UV light (opaque is the opposite of transparent and means that light is blocked or absorbed; light can't pass through an opaque material).  We assume that the atmosphere absorbs all incoming UV light, none of it makes it to the ground.  This is of course not entirely realistic.

Greenhouse gases make the atmosphere a selective absorber of IR light - the air absorbs certain IR wavelengths and transmits others.  It is the atmosphere's ability to absorb jcertain wavelengths of infrared light that produces the greenhouse effect and warms the surface of the earth.  I didn't mention it in class but greenhouse gases also emit infrared light.  This is an important part of the greenhouse effect, something we'll return to on Friday after the quiz.

Note "The atmospheric window" centered at 10 micrometers.  Light emitted by the earth at this wavelength (and remember 10 um is the wavelength of peak emission for the earth) will pass through the atmosphere.  Another transparent region, another window, is found in the visible part of the spectrum.