Conceptual Physics WAVES

Review Transverse waves – particles move _________ to the direction of energy transfer. Longitudinal waves – particles move ________ to the direction of energy transfer. Electromagnetic (light) waves – Do/Don’t need a medium through which to travel. Mechanical waves – Do/Don’t need a medium through which to travel.

Check for Understanding A point is marked on a string. The string is then vibrated so that a transverse wave travels to the right as shown. In which directions does point P move? Up & down Left & right Right only Left only

Reading graphs of waves

Displacement vs. Position graphs Amplitude is the same Crest to crest = wavelength

Displacement vs. Time graphs Amplitude is the same Crest to crest = period

Longitudinal waves – Pressure vs. Distance Pressure = force per area = 𝐹 𝐴 Amplitude is the same Compression to compression = wavelength

Check Your Understanding What is the amplitude, period & frequency of the wave below?

Sound & Light Electromagnetic waves are light waves

Amplitude Loudness Smaller amplitude = less energy = quieter Sound Light Loudness Smaller amplitude = less energy = quieter Larger amplitude = more energy = louder Brightness Smaller amplitude = less energy = dim Larger amplitude = more energy = bright

Frequency Pitch Lower frequency = lower pitch Sound Light Pitch Lower frequency = lower pitch Higher frequency = higher pitch “Color” Low frequency = red High frequency = purple

The Electromagnetic Spectrum

Use this to fill in your EM spectrum Use this to fill in your EM spectrum. Sketch a wave with small wavelengths on the left & large ones on the right.

Review for Understanding An increase in what wave property would increase the volume of a sound? Wavelength Speed Amplitude Frequency A decrease in what wave property would make a light redder? Wavelength Speed Amplitude Frequency *BONUS: what would you increase to make the light redder?

Superposition

Interference Occurs when two waves meet while traveling along the same medium Causes the medium to take on a shape that results from the net effect of the two individual waves upon the particles of the medium Consider two pulses of the same amplitude traveling in different directions along the same medium; each displaced upward 1 unit at its crest and has the shape of a sine wave When they are completely overlapped, the resulting shape of the medium would be an upward displaced pulse with an amplitude of 2 units.

Constructive Interference A type of interference that occurs at any location along the medium where the two interfering waves have a displacement in the same direction. Upward or downward

Destructive Interference Occurs at any location along the medium where the two interfering waves have a displacement in the opposite direction Only momentary Equal amplitudes not necessary

The Principle of Superposition When two waves interfere, the resulting displacement of the medium at any location is the algebraic sum of the displacements of the individual waves at that same location.

Check Your Understanding Two pulses with amplitudes of 5 units and 2 units move towards each other on a stretched string as shown: If the pulses have the same speed and are the same distance from point P, which diagram best shows the resultant pulse when both pulses meet at point P?

Superposition Can Create Standing Waves Produced as the result of the repeated interference of two waves of identical frequency while moving in opposite directions along the same medium All standing wave patterns consist of nodes and antinodes. Nodes - points of no desplacement Anti-nodes - points along the medium which oscillate back and forth between a large positive displacement and a large negative displacement Check out this gif

Superposition Can Create Standing Waves The distance from one node to the next if half a wavelength; 𝑑= λ 2 The distance from one node to the next antinode is one quarter of a wavelength 𝑑= λ 4 Open ends of pipes have antinodes (air particles can move a lot) Closed ends have antinodes (particles of the material can’t move) Harmonics

Check Your Understanding Several positions along the medium are labeled with a letter. Categorize each labeled position along the medium as being a position where either constructive or destructive interference occurs.

“Oh, I’m not a zebra. I’m the Doppler Effect.

Sheldon Cooper explains the Doppler Effect Click the photo for a link to Sheldon’s explanation.

The Doppler Effect The effect produced by a moving source of waves in which there is an apparent upward shift in frequency for observers towards whom the source is approaching and an apparent downward shift in frequency for observers from whom the source is receding.

Astronomical Implications Astronomers use the information about the shift in frequency of electromagnetic waves produced by moving stars in our galaxy and beyond in order to derive information about those stars and galaxies. The belief that the universe is expanding is based in part upon observations of electromagnetic waves emitted by stars in distant galaxies. Galaxies are clusters of stars that typically rotate about some center of mass point. Electromagnetic radiation emitted by such stars in a distant galaxy would appear to be shifted downward in frequency (a red shift) if the star is rotating in its cluster in a direction that is away from the Earth. On the other hand, there is an upward shift in frequency (a blue shift) of such observed radiation if the star is rotating in a direction that is towards the Earth.

Check Your Understanding A student starts across the crosswalk as their teacher is driving towards them. The teacher honks a warning. Compared to what the teacher hears, to the student, the pitch of the horn is: A. The same B. Lower C. Higher D. There’s no way to tell