Objective: To familiarize students with the reflection, transmission, and absorption of light by matter.
New Mexico Standards and Benchmarks:
Science 4-B: Describe energy and matter, and explain the processes that transform energy and matter
Science 5-A-2: Explore and determine which materials will reflect, which absorb, and which transmit light.
Science 1-B-1: Develop predictions based upon prior knowledge
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
A sunny day, or a bright tabletop directed light source (preferably a spotlight, 200 watts or more).
Some items to absorb sunlight in varying amounts:
Several large smooth rocks, white and black paint, some aluminum foil, OR,
Several pieces of paper of different colors (white, black, red, etc), and one or several thermometers, OR,
A safe parking lot full of different colored cars, OR,
A mirror
flashlight
A piece of plexiglas
Large jar full of water
A straight stick (a ruler will do)
A prism (many stores sell decorative prisms that will do. A laboratory prism will probably work better).
Preparation
If using rocks;
Cover one rock with foil, shiny side out.
Paint one rock white.
Paint one rock black.
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:
Light is energy; Light is made up of little packets of energy called photons.
Dark colored things look dark because they absorb this energy.
Therefore, things that are darker absorb more energy and therefore become hotter.
Ask the students what kind of energy is responsible for the high temperatures. Explain that:
The warm feeling or high temperature indicates the presence of heat energy, and this energy came from the photons of light.
Heat energy corresponds to the microscopic vibrations of molecules in the rocks.
Temperature measures how large these vibrations are. It takes energy to make these vibrations. Hence, the higher the temperature, the more heat energy an object has.
Heat cannot be seen - for this reason, people didn't understand what is was for many centuries. But it is a very important form of energy, and so our bodies have evolved special touch sensors to detect it.
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:
Both white and shiny surfaces are about the same temperature because in both cases the light is bouncing off of them.
The bouncing of light is called reflection.
Ask the students why the foil and white colored things look so different. Explain that:
There are two different types of reflection: coherent and diffuse.
The foil reflects (more or less) coherently, that is, the light rays bouncing off obey the Law of Reflection, because the foil is so smooth:
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.
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.
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:
The Sun's light is made of up all these colors.
The amounts of each color is called the spectrum of the light.
"White light" is an equal mixture of all the visible colors.
The strongest color in the Sun's light corresponds roughly to yellow. This is right in the middle of the visible spectrum - the colors we can see. Thus we have evolved to best see the strongest color of sunlight!
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:
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:
Refraction happens because the speed of light is actually different in different materials, for example, it is slower in water than in air, and slower in air than in a vacuum (outer space).
The speed of light in a given material is quantified with the index of refraction; the higher the index of refraction, the lower the speed. The actual formula for the speed (you may want to skip this) is:
S.O.L. in material = S.O.L. in vacuum / Index of Refraction
where S.O.L. = speed of light.
Not only does the index of refraction (and hence the speed of light depend on the type of material, it also depends on the color (wavelength) of the light. Light at shorter wavelengths (blue as opposed to yellow), generally have a higher index of refraction (lower speed), and therefore tend to bend more. This is demonstrated in the diagram above.
A great example of refraction is a rainbow: As rays of sunlight pass into and get turned around by raindrops, they get separated into different colored rays just like light through a prism, giving rise to the rainbow!
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:
The strongest color in the black body spectrum depends on temperature: the higher the temperature the shorter the wavelength.
For really hot objects, like the stove, the hotter the object, the more towards the violet end of the visible spectrum it will appear, as opposed to redder end.
The Sun is a very good example of an object that emits a black-body spectrum. In this case, the peak color is yellow (as discussed previously).