Physical Science Ch. 10: Waves

Physical Science Ch. 10: Waves

A wave is a rhythmic disturbance which carries energy NOT matter. A medium is a material through which a wave transfers energy. Some Waves, but not all, require a medium to move between points. Any solid, liquid, or gas can serve as a medium. A wave will travel as long as it has energy All waves are produced by vibrations All waves have the following Frequency (Hz) Wavelength (λ)

Wavelength- distance from crest to crest or trough to trough on a wave

Wave Frequency Frequency is a property of a wave, not a part of one. Frequency is the number of waves which pass a given point in 1 second. Hertz (Hz) is the SI unit for frequency. 1 Hz = 1 wave passing a given point in 1 second

Types of Waves Based on the way they move, there are 2 different types of mechanical waves: 1. Transverse Waves 2. Compressional Waves

Transverse Waves In a transverse wave, the wave's medium move perpendicular to the direction of the wave. Ex. ocean waves, light waves For example, as the wave moves left to right, the medium moves up and down.

Say you're waterskiing down at the lake and you wipe out. As you float in the water and a wave goes by, what does your body do as the wave passes?

Waves of the electromagnetic Spectrum are Transverse waves. You will need to be able to put these in order of energy, frequency (Hz), and Wavelength (λ)

Parts of a Transverse Wave There are 4 main parts to a transverse wave: Crest- the highest point on a transverse wave Trough- the lowest point on a transverse wave

Amplitude- the distance from either the crest or trough to the resting point of the wave

Compressional Waves A compressional wave vibrates in the direction that the wave is travelling. These waves are also known as longitudinal waves. Examples: slinky, sound waves

The 2 main parts of a compressional wave are: A. Compression- area of higher density within the wave B. Rarefaction- area of lower density within the wave pg. 296

When you tune in to a radio station, the numbers on the dial represent the frequency that a particular station is sending their signal out at. FM MHz (megahertz), 98.9 = 98,900,000 Hz AM KHz (kilohertz), 610 = 610,000 Hz

Sound Waves Sound waves are a type of compressional wave which require a medium to travel. Sound waves travel at about 750 mi/hr. This speed is effected slightly by the type of medium, temperature of medium, and elevation.

When an object such as a jet travels at a speed greater than 750 mi/hr it is said to be supersonic. What happens when an object travels at supersonic speeds?

25.9 The Doppler Effect As a wave source approaches, an observer encounters waves with a higher frequency. As the wave source moves away, an observer encounters waves with a lower frequency.

25.9 The Doppler Effect Imagine a bug jiggling its legs and bobbing up and down in the middle of a quiet puddle. The crests of the wave it makes are concentric circles, because the wave speed is the same in all directions. If the bug bobs in the water at a constant frequency, the wavelength will be the same for all successive waves. The wave frequency is the same as the bug s bobbing frequency.

25.9 The Doppler Effect The bug maintains the same bobbing frequency as before. However, an observer at B would encounter a higher frequency if the bug is moving toward the observer. Each wave crest has a shorter distance to travel so they arrive more frequently. Each crest has to travel farther than the one ahead of it due to the bug s motion.

25.9 The Doppler Effect Sound The Doppler effect causes the changing pitch of a siren. When a firetruck approaches, the pitch sounds higher than normal because the sound wave crests arrive more frequently. When the firetruck passes and moves away, you hear a drop in pitch because the wave crests are arriving less frequently. Note: The change in loudness is not the Doppler Effect! It is the shift in frequency!

25.9 The Doppler Effect Police use the Doppler effect of radar waves to measure the speeds of cars on the highway. Radar waves are electromagnetic waves. Police bounce them off moving cars. A computer built into the radar system compares the frequency of the radar with the frequency of the reflected waves to find the speed of the car.

25.9 The Doppler Effect Light The Doppler effect also occurs for light. When a light source approaches, there is an increase in its measured frequency. When it recedes, there is a decrease in its frequency.

25.9 The Doppler Effect Increasing frequency is called a blue shift, because the increase is toward the high-frequency, or blue, end of the spectrum. Decreasing frequency is called a red shift, referring to the low-frequency, or red, end of the color spectrum. Distant galaxies show a red shift in their light. A measurement of this shift enables astronomers to calculate their speeds of recession.

25.9 The Doppler Effect think! When a source moves toward you, do you measure an increase or decrease in wave speed?

25.9 The Doppler Effect think! When a source moves toward you, do you measure an increase or decrease in wave speed? Answer: Neither! It is the frequency of a wave that undergoes a change, not the wave speed.

Reflection and Refraction Reflection is where a wave strikes an object and bounces off. When a wave strikes a flat reflective surface it will reflect back at the same angle that it struck the surface at. This is called

The Law of Reflection The law of reflection states that the angle of incidence is equal to the angle of reflection on a flat reflective surface.

Examples of the Law of Reflection:

In addition to reflection a wave can experience refraction. Refraction is the bending of a wave as it enters a different medium, due to a change in speed.

Diffraction Diffraction is the bending of a wave, around a barrier. Ex.: Light in the storage room partially illuminating the classroom.

Sometimes 2 or more waves may come in contact with each other and overlap to form a new wave. This is called interference As these waves overlap, they can either multiply and enhance each other, or cancel each other out.

Constructive interference occurs when the crests and troughs coincide and produce a larger amplitude (greater overall combined energy)

Examples of constructive interference: 2 people trampolining at the same time 2 or more people singing together (in sync)

Destructive interference occurs when the crests of one wave coincide with the troughs of another, creating a smaller amplitude.

Many locations, such as auditoriums and modern stadiums are specifically designed to produce constructive interference.

Waves created through and along the crust of the earth by shifting or breaking tectonic plates are called seismic waves. These waves are a special type of Surface waves

These waves are comprised of both transverse and compressional waves, and can create very damaging earthquakes.

The San Andreas Fault line in California and the New Madrid Fault in SE Missouri are 2 locations where plates come together, and are therefore more likely to slide or break.

Underwater earthquakes can often produce tsunamis, giant ocean waves.