# Practice Page

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### 901-O: Speed of Sound and Light in Media

• Topic Cluster: Waves
• Topic: Wave Structure
• Objective: Identify how media change the speed of sound and the speed of light; identify why media has opposite effects on the speed of sound and of light.
• Content: Sound moves faster in dense media because closer together particles can push each other quickly and more efficiently; light moves faster in less dense media because electromagnetic waves do not depend on media and any particles are in the way of the wave's progress.
• Level: 3

#### Speed of Sound

The speed of sound depends upon the material sound moves through. It moves fastest in solids, second fastest in liquids, and slowest in gasses.

Material
Speed of Sound (m/s)
Air (20°C)
343
Water
1484
Glass
4540
Steel
6100
Diamond
12000
Vacuum
---- Sound does not move through a vacuum
1. Where does sound move the fastest?
2. Where does sound move the slowest?
3. Where does sound not move at all?
##### Why?
• Sound is a longitudinal, mechanical wave.
• A sound wave travels when each molecule pushes the molecule in front of it forward.
• If the substance is more dense and compact, sound will be able to move faster. Solids are the densest material, and gasses are the least.
1. Where does sound move faster: water or steel?
2. Where does sound move faster: air or steel?
3. Where does sound move faster: air or water?

#### Speed of Light

You should know that "The speed of light is 300,000,000 m/s. However, this is really the speed of light in a vacuum. In a medium (matter), light moves slower than it does in a vacuum (outer space).

Material
Speed of Light (m/s)
Very Accurate Speed of Light
Vacuum
$$3.00 \cdot 10^8$$
299,792,458 m/s
Air
$$3.00 \cdot 10^8$$**
299,686,445 m/s
Water
$$2.56 \cdot 10^8$$
Glass
$$1.97 \cdot 10^8$$
Diamond
$$1.24 \cdot 10^8$$
Steel
---- Light does not move in steel

** The speed of light is a very little bit less in air than a vaccum. The difference comes in the fourth significant figure!

1. Where does light move the fastest?
2. Where does light move the slowest?
3. Think of three materials that light does not move through at all:
##### Why is this?
• Light is an electromagnetic wave. Unlike sound, it doesn't depend on matter at all!
• Any atoms of matter that are there simply get in the way of the light wave and slow it down.
• The more dense the matter, as in a diamond, the more it slows down the light.
##### One very important point:

Even though light slows down in matter, it is still really really really really fast. Light moves slowest in a diamond, but light in a diamond is still much much much much faster than anything that isn't light.

The main relevance of the changing speed of light is that it causes light to refract, or bend, creating weird effects and distorted images.

1. Where does light move faster: water or diamond?
2. Where does light move faster: water or glass?
3. Where does light move faster: water or air?
4. Where does light move faster: air or vacuum?
##### True or False?
1. Sound is faster than light.
2. Sound moves slower in air than in water.
3. Light moves slower in a vacuum than it does in matter.
4. Light moves slower in water than it does in air.
5. Sound moves faster in a steel train track than it does in air.
6. Light moves faster in a diamond than it does in water.
7. Sound moves faster in solids than in liquids.
8. When light slows down in a diamond, it moves slow enough that you could run after it.
9. Sound moves faster in solids because the particles are packed closer together.
##### More Practice:
1. For each material, rank the speed of light and sound. For example, the material where sound moves the fastest will have a "1" for sound. The material where sound moves the second fastest will have a "2" for sound, etc. Any materials in which sound does not travel will have an "X" for sound. 2. Do you notice any general patterns in the ranking table above? Specifically, are light and sound affected differently?
3. You can tell if a train is coming by putting your ear on a train track. Using facts from this pod, explain why this is. Also, NEVER EVER ACTUALLY DO THIS!