A. Inform students that in this Investigation, they will investigate the nature and properties of light waves. Inform students that all light waves are waves of energy that can be defined according to their shape.

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B. Introduce the description of light waves and the definitions of the terms crest, trough and wavelength using this slide.

1. Indicate to students the structure of a light wave that is determined by the series of repeating crests and troughs.

2. Familiarize students with the definitions of crest, trough, and wavelength by asking the following questions:

• Ask students: What do you think of when you hear the word crest? Student answers will vary. Crest describes the crest of a wave in the ocean or the peak of a mountain.

• Ask students: What do you think of when you hear the word trough? Student answers will vary. Trough describes a low point between two high sides such as the low container out of which animals eat or drink.
• Ask students: Into what two words can you divide the word wavelength? Student answers will vary. Wave and length.
• Ask students: From breaking up the word, can you think of a definition of wavelength? Student answers will vary. The term can be defined as the length of a wave or the distance between two crests or two troughs of a wave.

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C. Introduce the term frequency.

1. Familiarize students with the definition of frequency by asking the following questions:

a. Ask students: What is the definition of the word frequent? Student answers will vary. Frequent usually means often.

b. Inform students that the word frequent suggests that something occurs during a certain time period.

c. Ask students: What is the definition of the word frequency? Student answers will vary. Frequency means how often something occurs. Something that occurs less often occurs with a low frequency. Something that occurs more often occurs with a high frequency.

d. Indicate to students that the light wave on the top has a frequency that is higher or greater than the frequency of the light wave on the bottom. The crests of the wave on top occur more often, in a given period of time, than the crests of the wave on the bottom.

Notice that during the time period bracketed by the two dotted red lines, the top wave has 6 crests while the bottom wave has only half that (two full-crests and two half-crests). 

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2. Ask students the following questions to illustrate the concept of low- and high-frequency events:

a. Ask students: How many Saturdays are there in one year? There are 48 Saturdays in one year.

b. Ask students: How many times during one year do you celebrate your birthday? Everyone’s birthday occurs only once during one year.

c. Inform students that everyone celebrates their birthday with a low frequency compared to how often a Saturday occurs during one year which occurs with a higher frequency.

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D. Establish the relationship between wavelength and frequency using the following.

1. Introduce students to the relationship between wavelength and frequency by using this slide.

a. Indicate to students that the light wave on the top has a short wavelength since the distance between crests is small.

b. Indicate to students that the light wave on the bottom has a long wavelength since the distance between crests is large.

c. Are the crests of the wave on top closer or farther apart than the crests of the wave on the bottom? Student answers will vary. The crests of the wave on the top are spaced closer than the crests of the wave on the bottom.

d. Inform students that the wave on top has a frequency that is greater than the frequency of the wave on the bottom.

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2. Use the analogy of ocean waves to illustrate the relationship between frequency and wavelength.

a. Tell students to imagine that waves are washing on a beach. On a day at the beach when the ocean is calm, the waves gently hit the beach very infrequently.

• • Ask students: Do the waves have a frequency that is high or low? Student answers will vary. On a calm day, the frequency of the waves hitting the beach is low since they do not come in as frequently.
  • Ask students: Do the waves have a wavelength that is large or small? Student answers will vary. On a calm day, the waves have a large distance between crests, so the wavelength is large.

b. On a day at the beach when there is a storm, the ocean is violent with the waves crashing on the beach very frequently.

• • Ask students: Do the waves have a frequency that is high or low? Student answers will vary. On a stormy day, the frequency of the waves hitting the beach is high since they come in frequently.
  • Ask students: Do the waves have a wavelength that is large or small? Student answers will vary. On a stormy day, the waves have a small distance between crests, so the wavelength is small.

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E. Introduce the general concepts of light waves and how we perceive them.

1. Ask students the following questions:

a. Ask students: Can you see in the dark? No.

b. Ask students: What must we have in order to see the world around us? Student answers will vary. Light is necessary to see the world around us.

c. Ask students: What color is light from the Sun? Student answers will vary. Light from the Sun is white.

d. Ask students: Do you always see things in black and white? No, we can see our surroundings in color.

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e. Ask students: How many of you have seen a rainbow? Student answers will vary. 

• Ask students: How does white light from the Sun form a rainbow? Student answers will vary.

f. Inform students that raindrops in the air act to refract or bend the light from the Sun and separate it into the colors of the rainbow. Inform students that the colors of the rainbow are termed the Visible Spectrum.

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F. Introduce students to the Visible Spectrum. Explain the order of the colors of the Visible Spectrum to students.

a. Inform students that the Visual Spectrum is the only portion of light energy that we are able to see.

b. The six most obvious colors of the Visible Spectrum are red, orange, yellow, green, blue, and violet. Using the acronym, ROY G. BV, the first letters of their names, the colors, and their order are easily remembered.

c. Inform students that the Visible Spectrum contains many colors between the ones that are most often named. The colors of the Visible Spectrum that are most often named are simply those that are most easily recognized as specific colors.

2. Relate the order of the colors of the Visible Spectrum to their wavelengths.

a. Direct students’ attention to the scale of wavelengths in the diagram. Inform students that the colors of the Visible Spectrum can be differentiated by their wavelengths.

b. Ask students: What is the wavelength of light waves corresponding to red light? Student answers will vary. 700 nm* (0.00007 cm).

c. Ask students: What is the wavelength of light waves corresponding to violet light? Student answers will vary. 400 nm (0.00004 cm).

d. Inform students that the wavelengths of the other colors of the Visible Spectrum are between (700 nm) 0.00007 and 400 nm (0.00004 cm).

* nm stands for nanometers. A nanometer is one billionth of a meter. A human red blood cell is about 10,000 nanometers across! Therefore, light waves in the visible spectrum are about 20-times smaller that a single human blood cell.

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3. Relate the wavelengths of the colors of the Visible Spectrum to their frequencies.

a. Ask students: Does red or violet light have a greater frequency? Student answers will vary. Red light has a longer wavelength than violet light, so red light has a lower frequency.

b. Ask students: Does orange or green light have a greater frequency? Student answers will vary. Orange light has a longer wavelength than green light, so green light has a lower frequency.

c. Remind students of the relationship between wavelength and frequency. As the wavelength of a wave increases, the frequency decreases.