The pictures below differ a little bit from those drawn in class but emphasize the same points.  I'm borrowing the scanned images from a previous class.

On Tuesday you were able to see a cloud form when moist air came into contact with liquid nitrogen.  You were able to see a cloud of smog in an earlier demonstration today.  In both cases, the droplets making up the clouds are probably too small to be seen by the naked eye.  You are able to see the cloud depends because the cloud droplets scatter light.  This demonstration will try to show you exactly what that is.

In the first part of the demonstration a narrow beam of intense red laser light was shined from one side of the classroom to the other. 



The students couldn't see the laser beam because the light rays weren't pointing straight at them.  The instructor would have been able to see the beam if he had walked to the wall and looked back along the beam of light (that wouldn't have been a smart thing to do because the beam is strong enough to damage his eyes). 

Students were able to see a bright red spot where the laser beam struck the wall.





This is because when the intense beam of laser light hits the wall it is scattered (splattered is a more descriptive term).  Weaker rays of light are sent out in all directions.  There is a ray of light sent in the direction of every student in the class.  They see the light because they are looking back in the direction the ray came from.  It is safe to  look at this light because the rays are weaker than the initial beam.

Next we clapped some erasers together so that some small particles of chalk dust fell into the laser beam.



Now instead of a single spot on the wall, students saws lots of points of light coming from different positions in a straight line along the laser beam.  Each of these points of light was a particle of chalk, and each piece of chalk dust was intercepting laser light and sending light out in all directions.  Each student saw a ray of light coming from each of the chalk particles.

We use chalk because it is white, it will scatter rather than absorb visible light.  What would you have seen if black particles of soot had been dropped into the laser beam?

In the last part of the demonstration we made a cloud by pouring some liquid nitrogen into a cup of water.  The cloud droplets are much smaller than the chalk particles but are much more numerous.  They made very good scatterers.




The laser light really lit up and turned the small patches of cloud red. The cloud did a very good job of scattering laser light.  So much light was scattered that the spot on the wall fluctuated in intensity (the spot dimmed when lots of light was being scattered, and brightened when not as much light was scattered).

A comment that may not have been mentioned in class (if it was mentioned it certainly wasn't emphasized).  Air molecules are able to scatter light too, just like cloud droplets.  Air molecules are much smaller than cloud droplets and don't scatter much light.  That's why you weren't able to see light being scattered by air before we put chalk particles or cloud droplets into the beam.  Outdoors you are able to see sunlight (much more intense than the laser beam used in the class demonstration) scattered by air molecules.  Sunlight is white and is made up of violet, blue, green, yellow, orange, and red light.  Air molecules have an unusual property: they scatter the shorter wavelengths (violet, blue, green) much more readily than the longer wavelength colors in sunlight (yellow, orange, and red).  When you look away from the sun and look at the sky, the blue color that you see are the shorter wavelengths in sunlight that are being scattered by air molecules.