Rainbow Dust, 6A46.16

Topic and Concept:

• Geometrical Optics, 6A46. Rainbow

Location:

• Cabinet: Optics (OP)

• Bay: (A2)

• Shelf: #1

Abstract:

A sample of glass (or possibly polymeric) spheres collected from University Ave. outside Chamberlin Hall following the painting of the crosswalks. Typical sphere diameter 400 microns.

Setup and Procedure:

Darken room except for single spotlight.

Place large sheet of white paper below spotlight, empty spheres on paper, and spread them out so that paper is more or less evenly covered.

Stand so that shadow of head falls in center of paper.

Observe colored halo surrounding head.

Discussion:

Small spheres are excellent retroreflectors--that's why transportation engineers embed them in paint used to mark roads.

The exact size of "small" depends on the index of refraction.

This is an example of Mie scattering.

One way to motivate an interest in Mie scattering (which is an advanced topic): when sunlight illuminates falling spherical raindrops, the result is a rainbow.

However, clouds are made of more or less spherical droplets of water. Why do we not see a rainbow every time there is a cloud?

The answer turns out to be that there is a significant difference between the scattering of light by drops of about 300

Videos:

• Lecture Demonstration's Youtube Channel

References:

• From the transportation engineer's point of view: http://www.dot.state.fl.us/construction/Engineers/MOT/Presents/PavementMarkingTraining.pdf

• From the physicist's point of view: http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html

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