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Scattering of light


Scattering of light makes the sky appear blue, clouds white, and turns the sun red at sunset.
A simple demonstration will give you a pretty good idea what scattering is.

A thin beam of bright red laser light was shined across the front of the classroom.  No one in the class could see this beam of light.  To see the beam you would need to stand over where the beam struck the wall and look back toward the laser (the laser light is intense enough that you shouldn’t do this – it could damage your eyes).

Students in the class could see a red spot on the wall because the light hitting the wall was scattered or splattered and sent off in a multitude of directions.  A ray of that light was sent to everyone in the class (and because the intense light is split up into many rays, these rays are weaker and safe to look at).

Next we clapped a couple of chalkboard erasers together.  When particles of chalk dust fell into the laser beam they intercepted some of the laser light and scattered it.  Again everyone in the room got their own personal ray of light coming from each of the particles of chalk.  We use chalk because it is white, it scatters rather than absorbs light.  What would you have seen if black particles of soot had been dropped into the laser beam?

In the 3rd part of the demonstration we made a cloud by pouring some liquid nitrogen into a cup of water.  The numerous little water droplets made very good scatterers.  So much light was scattered that the spot on the wall fluctuated in intensity (weak when lots of light is being scattered, brighter when not as much light was scattered).

Just about everything in the atmosphere can scatter light (some particles and gases might absorb light).  These scatterers fall into two categories: (1) those that have sizes equal to or greater than the wavelength of the light being scattered and (2) those that are much smaller.  Air molecules fall into the 2nd group.  Members of this group scatter short wavelengths of light much more readily than long wavelengths.  We will see what effects this has.

In this first figure we imagine going outside at midday, turning toward the south and looking up at the sun when it is high in the sky (you shouldn’t do this of course, sunlight is too intense and will blind you).  We assume that the sunlight arriving at the top of the atmosphere is made up of equal amounts of all the colors.  This isn’t true, but that’s what we’ll assume.  As the sunlight passes through the atmosphere some of the shorter wavelengths will be scattered by air molecules.  The unscattered light that makes it to the ground will be most of the original red, orange, and yellow with some of the green, blue, and violet light removed.  The resulting mixture will be a warm white color.

If you were to turn toward the north, away from the sun and look up at the sky you see blue light.

First of all you see light coming from the sky because some of the sunlight has been intercepted by air molecules and redirected (just like the chalk dust and cloud droplets made the laser beam visible in the demonstration).  The sky would appear black if it weren’t for the fact that air molecules scattered light.

Two things to notice about this scattered light: first it is much weaker than the unscattered sunlight and is safe to look at (imagine if that weren’t the case and it was dangerous to look at the sky), second the light is blue because it is mainly the shorter wavelengths that are being scattered.

Why is the sky blue and not green or violet?  The sky isn’t violet because there isn’t as much violet light in sunlight as there is green and blue.  Also our eyes might not be as sensitive to violet as they are to blue and green.  The sky probably isn’t green because that color isn’t scattered as readily as the blue and violet light.  Blue is a sort of compromise.

You might have noticed looking west late in the day that the setting sun is not as bright and is redder than it is at midday (it is still not safe to look at the setting sun).  The sunlight follows a much longer path through the atmosphere at this time of day and much more of the sunlight is scattered.  Essentially all of the shorter wavelengths are removed from the unscattered beam of light.  You are left with a mixture of yellow, orange, and red.  Sometimes just the orange and red light are left.

As we saw with the laser demonstration, the water droplets in clouds are very good scatterers of light.  The cloud droplets (typically around 10 or 20 micrometers in diameter) are larger than the wavelength of visible light (0.4 to 0.7 micrometers).  Cloud droplets scatter all of the colors equally.  When white light strikes a cloud, the scattered light is also white.

Here are a couple of more common phenomena produced by the scattering of light (these weren’t mentioned in class)

The person in this figure would see a crepuscular ray, a shaft of sunlight that passes through a hole in a cloud layer.  The sunlight is scattered by particles in the air.  Rays of sunlight that would ordinarily pass through the adjacent parts of the sky are reflected by the clouds.  These parts of the sky appear darker.  You’ll find a nice photograph of crepuscular rays on in Fig. 15.6 on p. 407 in the textbook (p. 403 in the 3rd ed.).

Scattering of sunlight by air molecules turns distant mountains blue and eventually makes them fade from view (there is a limit to how far you can see even when the air is very clean).

A nearby mountain might appear dark green or brown.  You are mainly seeing light reflected off the mountain.  As the mountain gets further away you start seeing appreciable amounts of blue light (sunlight scattered by air molecules).  As the mountain gets even further the amount of this blue light from the sky increases.  Eventually the mountain gets so far away that you only see blue sky light and none of the light reflected by the mountain itself.  You’ll find a nice photograph of the changing colors of distant mountains in Fig. 15.5 on p. 406 in the text (p. 402 in the 3rd ed.)



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This entry was posted on February 27, 2013 by in Science & Technology.
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