6F40.10 Sunset Demo

PURPOSE: To demonstrate the effects of scattering on white light

DESCRIPTION: This demonstration closely mimics the effects of light being scattered by our atmosphere - hence the title. The experiment itself is quite simple. A strong white light source is directed down the long axis of a fishtank and focused on the opposite wall. The fishtank is filled with water and "scattering centers" are added to the water. The more the scatterers, the "redder" the focused spot will become until, eventually, the light is completely scattered and none reaches the far wall.

SETUP TIME: Medium to Long

COMPONENT LIST:

    


  1. White light source (usually a slide projector with small circular slide inserted)


  2. Fish Tank


  3. Water


  4. Scattering Centers 


  5.  Something to stir with

SUGGESTIONS: Many people have their own favorite way to do this demonstration. We usually use a pine-scented liquid floor cleaner as our scatterer, just because it's less toxic (tho admittedly less impressive) than other methods. Make sure to point out to students how the water looks "blue" as the scatterers are added. Another nice thing to do is to put color filters in front of the white light source to show where the blue/red light is scattered. The scattered light can be seen to be polarized by rotating a polaroid sheet in the light path.
Students remember this demonstration.

REFERENCES: PIRA 6F40.10

OTHER NOTES: As the pine-scented liquid floor cleaner is poured into the water, the particles act as scattering centers for the light (especially blue light at first). Pouring more scattering solution into the tank will allow the particle size to increase and more light (and light of a longer wavelength) will be scattered. (Note: I don't think it takes much scattering solution, and I believe the scattering centers "grow" (clump) as time progresses.) (Again, this is not as impressive as the 2.5 ml hypo in 100 ml water, and 2.5 ml concentrated HCl 1:4 dilution in 650 ml water demonstration in which sulfur crystals actually grow during the course of the demonstration. However it is less nasty, chemically. Plus we get a nice fresh pine scent!)