Nature of Waves Chapter 20

Bellringer What do you think of when you hear the word “wave”? Write a brief description of what you know about them.

What do you think? (True or False) Light waves are mechanical waves because they must travel in a medium. In space, no one can hear an explosion. Water waves are a combination of longitudinal and transverse waves.

Think about this… Imagine that your family has just returned home from a day at the beach. You had fun, but you are hungry from playing in the ocean under a hot sun. You put some leftover pizza in the microwave for dinner and you turn on the radio. Just then the phone rings. In the events described above, how many different waves were present? List them.

Answers…. There are at least 5 Water waves in the ocean Microwaves in the microwave oven Light waves from the sun Radio waves transmitted TO the radio Sound waves from the phone and radio Did you come up with any others?

Nature of Waves If you are seen, heard or otherwise detected, it is the result of waves!!! A wave is any disturbance that transmits energy through matter or space.

ONLY the Energy Moves!! When you shake a rope, the rope only goes up and down…..the ENERGY is all that moves forward and backwards. Sound moves through the air, but you don’t feel a breeze every time you hear a noise….the air doesn’t travel with the sound wave…

Medium A medium is a substance through which a wave can travel. A medium can be a solid, liquid or gas It travels through vibrating particles…..they pass the energy from one particle to the next…..

Mechanical Waves Any type of wave that requires a medium to travel through is called a mechanical wave Sound waves, water waves and seismic waves require a medium… No air……means no sound!!!

Electromagnetic Waves Waves that DO NOT require a medium to travel through are called electromagnetic waves. Visible light, microwaves, radio waves (radio waves are NOT sound waves), ultraviolet light, x-rays and gamma rays are all electromagnetic waves.

Types of Waves Waves are classified based on the direction in which the particles of the wave move.

Transverse Waves Transverse waves are waves in which the particles vibrate in an up and down motion. The crest is the highest point on the wave. The trough is the lowest point between each crest.

Transverse Waves

Longitudinal Waves Longitudinal Waves are waves in which the particles vibrate in a back and forth motion. The area where particles are crowded together is called a compression. The area where particles are spread out is called a rarefaction.

Longitudinal Waves

Concept Review A wave is a disturbance that travels through ______________ or _____________.

Concept Review A wave is a disturbance that travels through matter or space.

Concept Review A wave carries _____________.

Concept Review A wave carries energy.

Concept Review Waves that require a medium are called _____________ waves. Waves that DO NOT require a medium are called _____________ waves.

Concept Review Waves that require a medium are called mechanical waves. Waves that DO NOT require a medium are called electromagnetic waves.

Review Questions Describe how transverse waves differ from longitudinal waves. Why can’t you cause a floating leaf to move to the edge of a pond by throwing stones behind it? Explain why supernova explosions in space can be seen but not heard on Earth. Sometimes people at sports events do “the wave”. Do you think this a real example of a wave? Why or why not?

Bellringer What would happen if a transverse and longitudinal wave happened at the same time? What type of movement would the energy have?

Properties of Waves How do we measure the properties and characteristics of waves? There are 4 basic properties. Amplitude Wavelength Frequency Wave Speed

Amplitude Amplitude is the maximum distance a wave travels from its resting position.

Amplitude If you had a wave whose distance from trough to crest was 6 cm, how would you calculate the amplitude? Large Amplitude = High Energy Small Amplitude = Low Energy

Wavelength In a transverse wave, wavelength is the distance between 2 adjacent crests (or troughs) In a longitudinal wave, wavelength is the distance between 2 adjacent compressions (or rarefactions)

Wavelength Transverse Wave Rarefaction Longitudinal Wave Compression

Frequency Think about making rope waves again. The number of waves that you can make in 1 second depends on how quickly you move the rope. If you move the rope slowly, you make only a small number of waves each second. If you move it quickly, you make a large number of waves.

Frequency The number of waves produced in a given amount of time is the frequency of the wave. You can measure frequency by counting either the number of crests or troughs that pass a point in a certain amount of time.

Frequency Frequency is usually expressed in hertz (Hz). One hertz = one wave per second 1Hz = 1/s

Higher Frequency = More Energy It takes more energy to vibrate a rope quickly than to vibrate a rope slowly….thus high frequency waves carry more energy than low frequency waves.

Wavelength & Frequency Draw a wave with an amplitude of 5 and a wavelength of 10 across a piece of graph paper. Draw a wave with an amplitude of 10 and a wavelength of 4 across a piece of graph paper.

Wavelength & Frequency If 10 centimeters is equal to one second, calculate the frequency of each wave. What can you conclude about the energy contained in each wave?

Wavelength & Frequency In general, a wave with a short wavelength carries more energy than a wave with a long wavelength.

Wave Speed Wave speed is the speed at which a wave travels. Remember speed = distance/time Wave speed can be calculated by measuring the distance a single crest or compression travels in a certain amount of time.

Wave Speed The speed of a wave can change depending on the medium through which it travels…. Sound travels at 340 m/s in air, but travels at 5,200 m/s through steel…why do you think this happens?

Summary High Energy Short wavelength High frequency Larger amplitude Draw one Low Energy Longer wavelength Lower frequency Smaller amplitude Draw one

Bellringer Draw a wave that has an amplitude of 3 and a wavelength of 2. Label the crest, trough, resting line, amplitude and wavelength.

Bellringer Why are boats instructed to stay at slow speed when they get close to other boats or as they approach the dock?

Bellringer Why are we able to see the moon and planets?

Wave Interactions We have been studying how waves behave by themselves…this is not realistic. Waves are constantly interacting with other waves, running in to objects and passing through different mediums. What happens when they do this?

Reflection Reflection occurs when a wave bounces back after striking a barrier. A reflected sound wave is called an echo. An object that reflects light waves allows us to see that object.

Refraction Refraction is the bending of a wave as it passes from one medium to another. Changing mediums forces the wave to change the speed at which it travels. This causes it to bend.

Diffraction Suppose you are walking down the street and hear music playing. It seems to be coming from around the corner. There is a building on the corner blocking your view, so you can’t see who is playing the music. Why does the sound wave travel around the corner, but not the light wave?

Diffraction Diffraction is the bending of waves around a barrier or through an opening.

The amount of diffractions depends on the wavelength of the wave and the size of the opening or barrier. If the wavelength is larger than the object or opening you get lots of diffraction. If the wavelength is smaller than the object or opening, you get very little diffraction.

Interference Interference occurs when 2 or more waves occupy the same space and overlap. There 2 categories of interference: Constructive and Destructive Check out this link on interference

Interference Constructive Destructive Occurs when crests and troughs of the waves match up. Increases the amplitude of the resulting wave. Destructive Occurs when the crest of one wave overlaps with the trough of another wave. Decreases the amplitude of the resulting wave.

Standing Waves Check out this link on standing waves Also known as a stationary wave, in this form of interference the interaction between waves gives the appearance of a wave staying in one place. However, in reality, the waves are continuing to move in both directions. Check out this link on standing waves

Resonance Do you remember what frequency is? Every object has a natural frequency at which it will vibrate when it is disturbed. If something nearby is vibrating at that frequency then the object will absorb that energy….to better understand, look at these examples. Check out this video on the Tacoma Narrows Bridge.

Concept Review 1. When a wave bounces back from a barrier, _____________ has occurred.

Concept Review 1. When a wave bounces back from a barrier, reflection has occurred.

Concept Review 2. __________________ occurs when a wave bends as it passes at an angle from one medium to a difference medium.

Concept Review 2.Refraction occurs when a wave bends as it passes at an angle from one medium to a difference medium.

Concept Review 3. _____________ happens when two or more waves overlap.

Concept Review 3. Interference happens when two or more waves overlap.

Ticket out the Door Name 2 wave interactions that can occur when a wave encounters a barrier. Describe what happens when a wave is refracted.

Bellringer What are the characteristics of a high energy wave?