Optional Unit VI: Optics

C. Physical Optics

3. Colour

Key Concepts

White light contains at least three additive primary colours: red, green, and blue.

A surface appears black if it absorbs the light that strikes it.

By combining red, green, and blue light (or any two of them) at different intensities, a wide range of other colours can be produced. (Additive theory of light.)

Cyan, magenta, and yellow are called secondary colours of light. Each contains two primary colours and lacks a third, complementary colour.

Nonluminous objects have colour because they reflect light at certain wavelengths, absorb light at different wavelengths, and transmit light at other wavelengths. The reflected and transmitted wavelengths combine to produce the resulting colour, e.g. paint mixing. (Subtractive theory of light)

A filter selectively absorbs and transmits light. (This definition allows other filters besides coloured ones to be considered i.e., red filter, U.V. filter, polarizing filter, etc.)

Two overlapping primary colour filters can remove (virtually) all visible light. A primary colour filter overlapping with its complementary secondary colour filter will also remove (virtually) all visible light. (Virtually, since it is extremely difficult to produce filters with a sharp cut-off of light transmission at a certain wavelength. Most coloured filters "leak" some light at other wavelengths.)

Two overlapping secondary colour filters will transmit one of the primary colours.

Colour theory is extremely important in art, photography, colour printing, and other applications. (Several should be discussed or researched. Opportunities also exist for webbing with other subject areas.)

The following table summarizes the relationships between the primary and secondary colours of light. This table applies for additive colour mixing only. It does not apply to pigments.

Colour	Contains	Missing
red	red	green,blue
green	green	red, blue
blue	blue	red, green
cyan	green,blue	red
magenta	red, blue	green
yellow	red, green	blue

This table should be developed experimentally. Colour wheels could be used to illustrate the relationships between primary and secondary colours.

Learning Outcomes

Students will increase their abilities to:

1. Define the following terms: additive primary colours, secondary colours, filter, complementary colours, colour filter.

2. State the three additive primary colours.

3. Explain that "white light" contains at least the three additive primary colours.

4. Explain that usually nonluminous objects appear to be a certain colour because they reflect light at those wavelengths that combine to produce the resulting colour observed.

5. Suggest what causes some nonluminous surfaces to appear to be black or white.

6. Explain that by combining red, green, and blue light at varying intensities, a wide range of other colours can be produced.

7. Suggest some applications of the additive theory of light.

8. State the three secondary colours of light.

9. Give examples to illustrate what is absorbed and what is transmitted by various different types of filters.

10. Suggest some important applications of colour theory.

Teaching Suggestions, Activities and Demonstrations

1. Record the standard T.V. colour bar test pattern on videotape. Play it back in class. Note how the colours in the test pattern can be used to develop a table summarizing the relationships between the primary and secondary colours of light.

2. Develop a table or colour wheel experimentally, illustrating the relationships between the primary and secondary colours.

3. Using three slide projectors, or other light sources, place different types of primary filters in the light paths of each projector and observe what happens when the filtered lights overlap on a white screen. Repeat using secondary filters.

   Using different coloured paper, examine the resulting colours produced when the filtered light and its combinations strikes the coloured surfaces.

4. Using a light source, a triangular prism, and a converging lens, illustrate the separation of white light into a spectrum through the prism and its recombination into white light (and subsequent inversion of the spectrum) through the lens.

5. Observe what happens when a secondary colour combines with its primary complementary colour.

6. Perform activities to observe what happens when two primary colour filters overlap, and what happens when two secondary colour filters overlap.

7. View the complete spectrum of various light sources through a diffraction grating or a spectrometer. Use sodium (yellow) and mercury (blue-white) street lamps. Account for any differences observed.

8. Extract chlorophyll from a green plant (or use red cabbage leaves or a coleus for some interesting results) and investigate its absorption spectrum.

   To prepare the chlorophyll solution, chop leaves up into very small pieces. Place the pieces into a pestle and pour hot alcohol over them. Grind the leaf-alcohol mixture with a mortar. Decant the solution. Allowing the alcohol to evaporate for a short period of time will help to concentrate the solution.

9. A separation using paper or column chromatography could also be done on the prepared sample.) Using the results, students should be able to explain why plants look a certain colour in sunlight.

10. Based on the above considerations, design an experiment to determine how plants respond to different colours and types of artificial light.

11. Research the colours of plants and their flowers found in various different environments. How do their colours enable them to adapt and survive?

    Activities such as these help to show students how physics is closely intertwined with the other sciences, to the point where it can cease to be important whether one is actually "doing" physics, chemistry, or biology. Students get an opportunity to experience the interrelatedness of different branches of science.

12. If there is a colour filter head on a darkroom enlarger in your school, take students to see what happens to the resulting image of a negative or a slide viewed on the enlarger baseboard when the filter pack in the enlarger head is changed.

    This forms a good springboard activity into a discussion of applications of colour theory in colour photography. It may also form an interesting research project that students could pursue individually or in small groups.

13. Using a simple spectroscope or a spectrometer, perform flame tests to analyze the spectral characteristics of different chemical solutions. Develop this activity in conjunction with Chemistry 20.

14. Illustrate that some nonluminous objects appear to be a certain colour because they transmit light at certain wavelengths (e.g. some aniline textile dyes exhibit this phenomenon).