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Introduction to Interactions Between Light and Matter

Objective: To familiarize students with the reflection, transmission, and absorption of light by matter.

New Mexico Standards and Benchmarks: 

Time: 1 hour

Vocabulary 

Reflection, Absorption, diffuse reflection, random scattering, coherent reflection, Law of reflection, angle of incidence, transparent, photons, heat, temperature, refraction, index of refraction, prism, spectrum, visible spectrum, white light, black-body spectrum.

Materials 

  1. A sunny day, or a bright tabletop directed light source (preferably a spotlight, 200 watts or more). 

  2. Some items to absorb sunlight in varying amounts:

  3. A mirror

  4. flashlight

  5. A piece of plexiglas

  6. Large jar full of water

  7. A straight stick (a ruler will do)

  8. A prism (many stores sell decorative prisms that will do. A laboratory prism will probably work better).

Preparation

  1. If using rocks;

    1. Cover one rock with foil, shiny side out.

    2. Paint one rock white.

    3. Paint one rock black.

Procedure

Solar heating and dependence of absorption on color

Place the various rocks, or the colored pieces of paper in the sun. If using paper, place the thermometers on them. Or, if your using cars, go on out to the parking lot if the Sun has been shining on the cars.

After about 15 minutes, have them feel the rocks or the cars, or read (and record) the temperatures indicated on the thermometers.

Discuss why the darker rocks, cars, or paper get hotter than the lighter colored ones. Explain that:

Ask the students what kind of energy is responsible for the high temperatures. Explain that:

Reflection

If you're using some foil covered objects, ask the students why the white rock and the rock with foil feel about the same (Actually, some white paints absorb more light than you would expect. Its possible that your white rock will be somewhat warm). Explain that:

Ask the students why the foil and white colored things look so different. Explain that:

Law of Reflection: The angle of reflection equals the angle of Incidence

Now turn off the lights, and use the flashlight and mirror to demonstrate the Law of Reflection by reflecting the light to make a spot on the ceiling.

Explain that light bouncing off the white rock behaves differently - it is scattered randomly in all directions. This is called diffuse reflection:

You may want to scatter the flashlight beam off the rock with the lights out.

Transparency

Now place some objects in the Sun again, but this time with some plexiglas covering them.

Discuss why it is that the dark objects still get warm. Stress that this implies that energy, as light, can travel right through some things, and that these things are called transparent.

Now turn the lights out, and demonstrate that not all the light is transmitted through the plexiglas - some is reflected. Explain that this is almost always the case, even if the surface is very smooth.

Light Spectra 

The basic colors that everyone should know are summed up by the phrase "Roy G Biv": R (red), O (orange), Y (yellow), G (green), B (blue), I (indigo), V (violet). Go over these carefully on the board, and make a large chart to refer to.

Explain that:

To demonstrate these ideas in an experiment, hold the prism up to the sunlight, and turn it around slowly until the rainbow spectrum can be seen on the walls:

 

Refraction

Now place the straight stick partially into the jar of water. Point out how, when one looks from above, the stick appears to be bent by the water. Explain that this is because the rays of light traveling from the stick to your eye are bent at the surface of the water, and that this is called refraction:

Explain that:

S.O.L. in material = S.O.L. in vacuum / Index of Refraction

where S.O.L. = speed of light.

 

Black Body Spectrum

Discuss the observation that when an electric stove is really hot, the coils give off reddish visible light. This is an example of the black-body spectrum: All things at finite temperature emit light, and do so with a same spectrum that depends, roughly, only on the temperature. That is, all bodies at the same temperature emit the same spectrum.  

Explain that:

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