Discovery Science WavesSp2011 w Chapters 17 & 18

Key Terms w Wave- a disturbance that transmits energy through matter or space w Medium- the matter through which a wave travels w Mechanical wave- a wave that requires a medium through which to travel w Electromagnetic wave- caused by a disturbance in electric and magnetic fields. Does not require a medium

Waves transfer energy w Ability to do work w waves carry energy w bigger the wave, the more energy it carries w Energy may spread out as the wave travels w Wave spread out in spheres that get bigger as the wave moves from the center

Transverse and longitudinal waves w Transverse waves include all electromagnetic [radio, radar, i.r., visible, u.v., X-rays] w have perpendicular motion  w longitudinal waves are sound waves. w Have parallel motion 

Wave properties w A wave is a pattern in the value of some quantity which is changing at every point of space.

Wave Properties w Crest- Highest point of a transverse wave w Trough- the lowest point of a transverse wave w Amplitude- greatest distance that particles in a medium move from their normal position when a wave passes w The amplitude is the height of the wave. w The wavelength is the distance from one wave top, or crest, to the next.

amplitude wavelength

Wave properties w Frequency refers to how many waves are made per time interval. This is usually described as how many waves are made per second, or as cycles per second. w Period- Time required for one full wavelength to pass a certain point. w w

Frequency- Period Equation

Wave speed w The speed at which a wave passes through a medium w Wave speed, wavelength, and frequency are related by the equation: v = f λ w The speed of a wave depends upon the material it is traveling through.

Electromagnetic Wave w Click here for simulation Click here for simulation w Sketch the two parts

Electromagnetic Spectrum ffffffffffffffffff RO YG B I V

Doppler effect w Whenever relative motion exists between a source of sound and a listener, the frequency of the sound is as heard by the listener is different compared to the frequency when there is no relative motion

The Doppler Effect w Heard an ambulance go by recently? Remember how the siren's pitch changed as the vehicle raced towards, then away from you? First the pitch became higher, then lower. Originally discovered by the Austrian mathematician and physicist, Christian Doppler ( ), this change in pitch results from a shift in the frequency of the sound waves w As the ambulance approaches, the sound waves from its siren are compressed towards the observer. The intervals between waves diminish, which translates into an increase in frequency or pitch (f f f f f ) w As the ambulance recedes, the sound waves are stretched relative to the observer, causing the siren's pitch to decrease. By the change in pitch of the siren, you can determine if the ambulance is coming nearer or speeding away. If you could measure the rate of change of pitch, you could also estimate the ambulance's speed.

Doppler Effect w By analogy, the electromagnetic radiation emitted by a moving object also exhibits the Doppler effect. w The radiation emitted by an object moving toward an observer is squeezed; its frequency appears to increase and is therefore said to be blue-shifted. w In contrast, the radiation emitted by an object moving away is stretched or red-shifted. w As in the ambulance analogy, blue-shifts and red-shifts exhibited by stars, galaxies and gas clouds also indicate their motions with respect to the observer.  f

Doppler Effect w Blue shifted = higher frequency = higher pitch. w Red shifted = lower frequency = lower pitch. w Electromagnetic Spectrum ffffffffffffffffff

The Doppler Effect In Astronomy w Click here for video clipvideo clip w In astronomy, the Doppler effect was originally studied in the visible part of the electromagnetic spectrum.electromagnetic spectrum w Today, the Doppler shift, as it is also known, applies to electromagnetic waves in all portions of the spectrum. Also, because of the inverse relationship between frequency and wavelength, we can describe the Doppler shift in terms of wavelength. w Radiation is red-shifted when its wavelength increases, and is blue-shifted when its wavelength decreases. w A red-shift occurs when the source and observer are moving away from each other. w A blue-shift occurs when the source and observer are moving towards each other.

Here is what the spectrum of a star that is moving toward us might look like (Blue-shift)

Here is what the spectrum of a star that is moving away from us might look like (red-shifted)

Gravitational Red-shift w According to General Relativity, the wavelength of light (or any other form of electromagnetic radiation) passing through a gravitational field will be shifted towards redder regions of the spectrum. w To understand this gravitational red-shift, think of a baseball hit high into the air, slowing as it climbs. w Einstein's theory says that as a photon fights its way out of a gravitational field, it loses energy and its color reddens. w Gravitational red-shifts have been observed in diverse settings.

Wave interactions w Reflection- the bouncing back of a wave as it meets a surface or boundary Reflection w Diffraction- the bending of a wave as it passes an edge or an opening. Animation 2 DiffractionAnimation 2 w Refraction- the bending of waves as they pass from one medium to another. What’s critical angle? Refraction w Interference- the combination of two or more waves that exists in the same place at the same time. Interference w w Why is the sky blue and sunsets red? Why is the sky blue and sunsets red?

Angle of Incidence The angle at which a ray of light approaches a surface, reflective or not, is called the angle of incidence. It is measured from an imaginary line perpendicular to the plane of the surface in question to the incoming ray of light. Angle of Reflection Once the light has reflected from a reflective surface, the angle at which the light departs from the surface is called the angle of reflection. This angle is also measured from a perpendicular to the reflecting surface to the departing ray of light. When light reflects from a surface, the angle of reflection is always equal to the angle of incidence.

Standing Waves w Standing waves are non-traveling vibrations of certain wavelength and frequency which occur on a medium of certain size w The size of the medium controls the wavelengths of the standing waves w the way that the medium is held at its ends, either fixed or open, controls the wavelengths of the standing waves. w

Node and antinodes w Node- medium does not move and have no vibration w Antinode- form where the crests of the original waves line up with crests of the reflected waves so that complete constructive interference occurs w Point of maximum vibration