2.5 Waves and Mediums Study Guide

Types of Waves
There are two main types of waves. Light waves (also known as electromagnetic waves) transmit energy in the form of light and do not require a medium.
Mechanical waves transfer energy through matter by moving particles and so must have a medium present in order to transfer energy. Sound waves and water
waves are examples of mechanical waves. Waves, both light and mechanical, can be transmitted through various mediums. They can also be reflected and
 absorbed. Light waves move as transverse waves and mechanical waves can move as either transverse or longitudinal waves. 

We see an image when waves of light reflect off of the object and enter our eye. In this way, light waves transfer light energy to our eyes. Have you ever turned out all the lights in a room and then tried to walk around? You probably noticed it was a little difficult to see things. That is because there are no light waves present to reflect off the objects around you. A light’s brightness is controlled by the amplitude of the wave whereas the wavelength determines the color of the light. Light waves with the longest wavelength appear red and light with the shortest wavelength appears violet. In between these two is the spectrum of all the other colors of light that goes in the order of the rainbow red, orange, yellow, green blue indigo and violet. 

Waves and Sound
Sound is defined as the transfer of energy from a vibrating object in waves that travel through matter. Most people commonly use the term sound to mean what
they hear when sound waves enter their ears and vibrate their eardrum.

Sound waves are mechanical waves. Sound waves begin with vibrating matter. Consider a guitar string. Plucking the string makes it vibrate. The diagram shows
the wave generated by the vibrating string. The moving string repeatedly pushes against the air particles next to it, which causes the air particles to vibrate. The
vibrations spread through the air in all directions away from the guitar string as waves. Once the wave hits your ear, the energy vibrates your eardrum. Your
brain interprets the vibration patterns as sound.

Reflection of Waves
When a light ray strikes a reflecting surface like a mirror the light ray is bounced back in a different direction (reflected). Sound waves can also be reflected.
Echoes are an example of reflecting sound waves.

Watch this video for demonstration of natural echo:    https://www.youtube.com/watch?v=TAliq8lXCZE

Transmitting Waves
Most of the sounds we hear reach our ears through the air but sounds can also travel through liquids and solids. If you swim underwater—or even submerge
your ears in bathwater—any sounds you hear have traveled to your ears through the water. Some solids, including glass and metals, are very good at transmitting
sounds. Foam rubber and heavy fabrics, on the other hand, tend to muffle sounds. They absorb rather than pass on the sound energy. We will talk more
about absorption of waves later. Transmission of sound and light occurs when the wave travels through a medium.

Refracting Waves
Refraction is the bending of light due to the change in its speed when it moves from one medium to another. A prism, like the one in the figure below, can be
used to separate white (visible) light into its different wavelengths using refraction. The prism transmits light through, but also slows it down. When light
passes from the air to the glass of the prism, the change in speed causes the light to bend. Different wavelengths of light bend at different angles because they
have different amounts of energy. This causes the beam of light to separate into light of different wavelengths. What we see is a rainbow of colors. 

When light waves refract they don’t always bend into the different colors to make a rainbow. Sometimes we use glass to bend the waves to make an image larger for us to see more detail like in a microscope or a telescope. Refraction is also used in contact lenses or glasses so that we can bend the light so that it focuses on the correct location in our eye and makes a clearer image. 

ABSORBED WAVES

Sometimes a wave will not be able to reflect off a medium or be transferred through a medium. Instead the wave is stopped and absorbed by the medium.
When waves are absorbed by an object the energy is changed into heat energy. Radiation happens when heat moves as energy waves, called infrared waves, directly from its source to something else. For example sound proof walls absorb sound waves and turn the sound energy into heat which explains why they make it quieter. Black pavement absorbs the sun's light waves and turns it into heat which explains why the road’s asphalt is so much hotter than a sidewalk on a summer day

Heat rising off the road meets cool air just above it causing a shimmering mirage on the road as light is refracted through the air temperature differences. According to the dictionary, “an obscuration of the atmosphere in hot weather, especially a shimmering in the air near the ground that distorts distant views.”

The image above shows how a wave can be either absorbed, transmitted, reflected or refracted depending on which medium it is traveling through. In some
cases, waves can be both transmitted and reflected such as when you can see the display in a store window (transmitted) through the glass and also see a
reflection of yourself. 

Waves in the World Around Us
Each year there are firework displays all over the United States to celebrate the Fourth of July. Thanks to waves, millions of people are able to enjoy the fireworks displays. Can you explain why waves are necessary to experience fireworks?

You hear the fireworks because sound waves transmit through the air to your
ears. You see the fireworks because the light waves transmit through the air and are detected by your eyes. The different wavelength structures of light allow you to see a variety of different colors. The fireworks can be seen before we hear their sound. This is because light
waves travel faster through the medium of air than sound waves. Because sound waves are mechanical waves they travel at different speeds, depending on the
medium through which they are moving.

SPEED OF SOUND AND WAVES THROUGH MEDIUMS

In old westerns many times the outlaws trying to rob the oncoming train would put their ear to a railroad track to see if a train was coming. They would be able
to hear the train through the train tracks long before they could hear it through the air. The reason this works is because sound waves travel faster through
solids than they do through gases. Sound waves are passed on because of molecules bumping into the next molecule. Below is a chart that shows the speed of sound traveling through different substances. 

Question: Which phase does sound travel through the slowest? If you remember back to learning about solids liquids and gases the solids are
more dense and so the molecules are closer together compared to liquids or gases. Since the molecules are farther apart in a gas, it takes longer for the
 wave’s energy to pass the vibrations from one molecule to the next so it takes longer to move the sound along. When sound travels through a solid the
molecules are closer together when they start to vibrate it easily passes on the vibration to the next. This causes the sound waves to be passed along more
quickly. 

The solid steel railroad tracks transmit the train’s rumblings faster than the air does. In general, the more rigid the matter, the faster sound travels through it.
Therefore sound travels faster through solids than liquids and faster through liquids than gases.