Optional Unit VI: Optics
C. Physical Optics

2. Electromagnetic Radiation

Key Concepts

Maxwell predicted that electromagnetic waves should be produced by the vibration of electric charges. The transverse waves were also predicted to travel at the speed of light.

Newton's experiments with prisms and lenses demonstrated the dispersion of white light into the spectrum, and the subsequent recombination into white light. Monochromatic light was incapable of further dispersion or recombination.

Monochromatic light consists of a single wavelength and hence has only one colour. A laser produces this type of light. (Laser light is also coherent. The emitted photons are all in phase.)

The spectrum produced by sunlight is continuous. (It also contains Fraunhoffer lines due to absorption at certain specific wavelengths.)

Some other light sources are capable of producing discontinuous line or band spectra, called an absorption spectrum, consisting of discrete lines or bands emitted at certain specific wavelengths. (e.g., atomic hydrogen, helium, neon, etc. These examples will only produce line spectra. A molecular gas is required to give band spectra.)

An excited gas will emit a line (or band) spectrum. The wavelengths emitted are characteristic of the gas and can be used to determine what gas it is. (an emission spectrum)

If a non-excited gas is between a light source and the observer, a continuous spectrum will be seen, with dark bands or lines at certain wavelengths. The wavelengths which are absent are the same wavelengths which would be emitted by the same gas if it were excited. These wavelengths have been absorbed by the gas (hence, absorption spectrum). An absorption spectrum also can give the chemical composition of the gas.

Visible light has wavelengths ranging from about 400 to 750 nm. Violet light has the shortest wavelengths, from about 400 to 450 nm. Red light

has the longest wavelengths, from about 610 to 750 nm.

Infrared light, with longer wavelengths just beyond the red region, and ultraviolet light, with shorter wavelengths just beyond the violet region, are two important types of electromagnetic radiation which the human eye is not capable of detecting.

Microwaves and radiowaves are two types of electromagnetic radiation with long wavelengths.

Short wavelength electromagnetic radiation includes x-rays, gamma rays, and cosmic rays.

Cosmic rays is a collective term for high energy particles of extraterrestrial origin reaching the Earth. They are mainly charged particles travelling at high speed, and only partly composed of high energy photons (electromagnetic radiation).

The energy of electromagnetic radiation depends on its frequency. High frequency electromagnetic radiation possesses larger amounts of energy. (Note, however, that a high intensity visible light source can deliver more energy than a low intensity x-ray, even though the individual x-ray photons "contain" more energy than the visible light photons.)

The electromagnetic spectrum is a continuous range of electromagnetic radiation from radio waves to cosmic rays. The physical nature of the radiation is the same throughout the entire spectrum. Frequency, wavelength, and the energy possessed are the only things that vary throughout the spectrum.

Learning Outcomes

Students will increase their abilities to:

1. Define the following terms: electromagnetic spectrum, electromagnetic radiation, monochromatic light, continuous spectrum, line spectrum, visible light, infrared light, ultraviolet light.

2. State what predictions Maxwell made about the nature of electromagnetic waves.

3. State the range of wavelengths for visible light.

4. Describe the infrared and ultraviolet regions of the electromagnetic spectrum.

5. Investigate several practical applications of infrared and ultraviolet light.

6. Describe regions of the electromagnetic spectrum on both sides of the visible spectrum and state several applications that arise from them.

7. Explain that different regions of the electromagnetic spectrum vary in their frequency, wavelength, and the amount of energy possessed.

Teaching Suggestions, Activities and Demonstrations

1. As a research project, students could investigate how a microwave oven cooks food. The discovery of the effect of microwaves on food, established as a result of the "serendipity effect," could also be investigated to reveal some important aspects about the nature of science.

2. The SHARP project (Stationary High Altitude Relay Platform), developed by the Department of Communications, involves a high altitude microwave-powered plane which acts as a telecommuni- cations relay platform. Students could research this project further. For more information on this project, refer to "Technology and Innovation in Canadian Industry: An Information Kit for Science Teachers" available in your school resource centre.

3. Allow students to design an activity to investigate the dispersion of light through a prism.

4. Describe, demonstrate, or attempt to have students replicate some of the experiments performed by Newton to investigate the nature of light.

5. Observe and compare continuous and discontinuous spectra.

6. Give examples to illustrate that predictions in science are subject to experimental verification. Without the experimental verification science is not likely being performed.

7. Examine the characteristics of different types of objects under ultraviolet light. Certain kinds of minerals, fabrics, paints, and even some insects have an interesting appearance when viewed with an ultraviolet light source.