What Is a Wave? Lesson 5

Show slide 11. Shine a 650nm red laser pointer through the glow water. Ask students to share what they observe. No fluorescence should appear. Note that the red laser passes directly through the glow water and appears on the other side of the bottle.  This can be done by holding up a white card behind the bottle.

Show slide 12. Shine a 405nm blue/violet laser through the glow water. Ask students to share what they observe. The water should fluoresce a bright yellow/green color.

Suggest to students that perhaps two red laser pointers will make the water fluoresce. Shine two red laser pointers at one time into the bottle. Ask students to share why the blue/violet laser produces a change, but the red lasers do not. Explain that red lasers have a lower frequency than blue/violet lasers, so regardless of how many red lasers are shined into the water, it will not result in a fluorescence. 

Also note that the blue laser does not pass through the glow water.  It is completely absorbed by the process of fluorescence.  A white card can also be used to show that no blue light appears on the other side of the bottle.

Move to slide 13 and pose the question, “How do electromagnetic waves interact with matter?” to your students.  At this time your students should have a good understanding of EM waves and how they react.  They may not know the academic vocabulary at this point, but they should be able to share that at least one of four things can happen when EM radiation interacts with matter:

1. Waves do not penetrate and are reflected

2. Waves are transmitted through the material with little energy loss

3. Energy from the waves is absorbed by the material and the

   waves disappear

4. Energy from the waves is absorbed, then converted to heat energy or emitted as new EM

   waves of a different frequency

Tell students to use their copy of the attached Wave Vocabulary Packet for taking notes while reviewing the definitions on the following terms. By this time in the unit, students should have notes and examples for most of the terms on the handout. Students can add notes to their vocabulary packets at this time. Ask students to review their vocabulary words to make sure they have filled in all the boxes on the handout. Discuss any questions they may have about their vocabulary words.

Show slide 14 and share the definition of Reflection.  You’ll want to make sure and share the following information with your students as well.

• Many materials will cause light to bounce off their surface. This “bouncing off” is called reflection.

• When reflection occurs, little to none of the light enters the material.

• Even “dull” looking materials reflect light. If you can see it, it is reflecting.

• Color is determined by reflection.  For example, a red object reflects red light but absorbs other colors.

Show slide 15 and share the definition of Transmission.  You’ll want to make sure and share the following information with your students as well.

• Many materials allow light to pass through while absorbing virtually no energy from them.

• These materials look clear and are transparent.

• Translucent materials allow light to pass through, but scatter the wave direction, making the material look cloudy

Show slide 16 and share the definition of Refraction.  You’ll want to make sure and share the following information with your students as well.

• When light travels from a less dense medium into a more dense medium (air into water, for example) its speed and wavelength both slightly decrease.  But its frequency (color) remains unchanged.  

• A change in the direction of light always accompanies this speed/wavelength change. When this happens, the path of the waves appears to bend. We call this bending refraction.

Show slide 17 and play the video, titled “Refraction Explained.”

After the video, discuss the remaining vocabulary words on each of the upcoming slides. 

Show slide 18 and share the definition of Absorption.  You’ll want to make sure and share the following information with your students as well.

• Absorption occurs when matter absorbs the energy contained in EM waves.  Total absorption results in all of the energy being transferred to the matter, with the EM wave vanishing.

Show slide 19 and share the definition of Emission.  You’ll want to make sure and share the following information with your students as well.

• Emission is a special property of some materials that is observed when they absorb EM waves.

• Fluorescent materials absorb energy from EM waves.  This excess energy causes the material to emit new EM waves with a lower energy and different color than what was absorbed.

• In our demonstration the light from the red laser did not have enough energy to trigger emission.  It passed right through the dye. The light from the blue laser has much higher energy and triggered the emission of bright yellow/green light.  All of the blue light was absorbed in this process. 

• Materials react differently to EM radiation depending on how much energy it contains.