Waves - types of waves

Question 1

what is a mechanical wave? + examples

Answer 1

they are propagating oscillations
they are vibrations which pass through a substance
eg. sound waves, seismic waves, waves on a string

Question 2

what is an electromagnetic wave? + examples

Answer 2

they are oscillating electric and magnetic fields that progress through space without need for a substance. the vibrating electric field makes a vibrating magnetic field which makes a vibrating electric field further out etc.
eg. radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays

Question 3

what are progressive waves?

Answer 3

waves that move outwards from the source

Question 4

what are longitudinal waves? + examples

Answer 4

they are waves in which the direction of vibration of particles is parallel to the direction of travel of the wave (the direction of energy transfer)
eg. primary seismic waves, sound waves, compression waves on a slinky

Question 5

on a slinky with compression waves being formed, what are the 2 states of the coils of the slinky?

Answer 5

• compression - when the coils are close together
• rarefraction - when the coils are further apart

Question 6

what are transverse waves?

Answer 6

they are waves in which the direction of vibration of particles is perpendicular to the direction of travel of the wave (the direction of energy transfer)
eg. secondary seismic waves, electromagnetic waves, waves on a string/wire

Question 7

what types of waves can be polarised?

Answer 7

transverse waves only - longtitudinal waves can’t be polarised

Question 8

what does it mean if a wave is plane polarised compared to it being unpolarised?

Answer 8

• plane polarised = if vibrations of the waves stay in one plane only
• unpolarised = if vibrations of particles in the waves change from one plane to another (there is an infinite number of ways for vibrations to be perpendicular to direction of energy transfer)

Question 9

what mediums can waves travel through?

Answer 9

• longitudinal - always mechanical waves so always need a medium
• transverse - sometimes mechanical, sometime electromagnetic, so it depends it can sometimes travel through a vacuum eg. light

Question 10

how can transverse waves be polarised?

Answer 10

using a polarising filter - its a slit which the wave passes through - the orientation of the slit must be parallel to the direction of vibrations of the particles

Question 11

what is the effect of use of a polarising filter(s) on unpolarised light?

Answer 11

• with one filter - intensity reduces by 50%
• with a second filter too - if the slits are parallel intensity stays 50% of original (maximum intensity)
• if slits are perpendicular - no light will be transmitted (minimum intensity)
• any other angles of orientation of the slits - some light will be transmitted

Question 12

what is a use of polarisation?

Answer 12

• polarising sunglasses - they reduce the glare of light reflected by water/glass
• reflected light is polarised and intensity reduced as it passes through the polarising sunglasses

Question 13

what is the displacement of a wave?

Answer 13

the distance of a particle of a wave from the equilibrium position (m)

Question 14

what is the amplitude of a wave?

Answer 14

the maximum displacement of a vibrating particle from the equilibrium position - eg. the max height of the crest of a transverse wave

Question 15

what is the wavelength of a wave?

Answer 15

the smallest distance between 2 adjacent vibrating particles with the same displacement and velocity at the same time (eg. between 2 crests)

Question 16

what is a wave cycle?

Answer 16

it is from one maximum displacement to the next maximum displacement

Question 17

what is the period of a wave?

Answer 17

the time taken for one complete wave to pass through a fixed point

Question 18

what is the frequency of a wave?

Answer 18

the number of complete waves passing through a fixed point per second (Hz)

Question 19

what is the equation to find the time period of a wave?

Answer 19

T = 1 / f
time period = 1 / frequency

Question 20

the higher the frequency of a wave…

Answer 20

… the shorter the wavelength (the wave crests are closer together)

Question 21

what is the wave speed equation?

Answer 21

c = fλ
wave speed = frequency × wavelength

Question 22

what is phase difference?

Answer 22

the fraction of a cycle between 2 particles vibrating at the same frequency
it is measured in radians

Question 23

what are the different types of phase difference?

Answer 23

• in phase - when the waves are oscillating in time with each other - full wavelength apart - phase diff will be an even no of π
• perfect antiphase - when the 2 particles have opposite velocities and displacements - half a wavelength apart - phase diff is an odd no of π
• phase difference doesn’t care about the amplitudes of 2 waves

Question 24

how do you find phase difference in radians if 2 points on a wave are seperated by a horizontal distance d?

Answer 24

ϕ = (2πd)/λ

Question 25

what is a wave front?

Answer 25

A line or surface on which the vibrations of the medium have the same phase at all points

Question 26

what happens during reflection of a wave?

Answer 26

the angle between the reflected wavefront and the surface is the same as the angle between the incident wavefront and the surface

Question 27

what is refraction?

Answer 27

when a wavefront approaches at an angle to a boundary, they change direction and speed

Question 28

what is diffraction?

Answer 28

when waves spread out after passing through a gap or going round an obstacle - The narrower the gap, the more the waves spread out - the longer the wavelength, the more they spread out.

Question 29

what is the principle of superposition?

Answer 29

When 2 waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point

Question 30

what are the 3 (or 4) effects of superposition?

Answer 30

- crest + crest = supercrest - reinforcement - trough + trough = supertrough - reinforcement - crest + trough of equal amplitude = sero amplitude - cancellation (- crest + trough not equal amplitude = resultant called the minimum)

Question 31

what is superposition?

Answer 31

the process by which 2 waves combine into a single wave form when they overlap

Question 32

how are stationary waves formed?

Answer 32

- when 2 progressive waves with the same frequency and amplitude but moving in opposite directions superpose - often the result of reflection of a progressive wave superposing with the original wave - at the point where the 2 waves are in phase an antinode is formed, and where they are in antiphase a node is formed - no energy is transferred along the wave, it just oscillates between the kinetic and potential stores of the medium

Question 33

what is a node?

Answer 33

a point where there is no oscillation - 2 waves meet with phase diff 180 degrees/π radians

Question 34

what is an antinode?

Answer 34

a point where oscillation is at maximum amplitude - 2 waves meet in phase at that point

Question 35

what is the distance between 2 nodes/antinodes?

Answer 35

half a wavelength

Question 36

what is the phase relationship for particles on a stationary wave?

Answer 36

- all points between 2 nodes oscillate in phase - all points on opposite sides of a node oscillate in antiphase

Question 37

is there any energy transfer in a stationary wave?

Answer 37

- no energy transfer to surroundings - max energy at antinodes (since max amplitude), no/min energy at nodes (since min amplitude) - sound waves - silence at nodes, max vol at antinodes - microwave oven - no heating at nodes, max heating at antinodes

Question 38

describe the experimental set up of forming stationary waves on a string

Answer 38

apparatus: frequency generator, vibrator, pulley, weight, string 1. set the frequency generator to 0 2. increase the frequency until the forst harmonic appears 3. adjust frequency to get the antinode with maximum possible amplitude - record this frequency 4. increase tension of the string by adding mass to the end, then repeat steps 2-3 5. repeat step 4 until you have all your readings 6. repeat steps 2-5 twice to get repeats 7. find mean frequency for T using f = (1/2L)*√(T/μ) 8. plot a graph of f against √T

Question 39

how do you find the frequency of the first harmonic (given in formula book)?

Answer 39

f = (1/2l)*(√T/μ) where: f = frequency l = length of string T = tension μ = mass per unit length of string

Question 40

what is the first harmonic of a string?

Answer 40

- aka the fundamental mode of vibration - its a single loop that has a node at either end and a single antinode halfway between the 2 nodes - it is the lowest frequency that gives a pattern

Question 41

what is the second and third harmonic, etc?

Answer 41

- second harmonic - 3 nodes, 2 antinodes - occurs at 2x frequency of first harmonic - third harmonic - 4 nodes, 3 antinodes - occurs at 3x frequency of first harmonic - and so the pattern repeats

Question 42

how do stationary waves form in a pipe?

Answer 42

- resonant frequencies exist in an air-filled tube - sound waves can be reflected by an open or closed end of a pipe - closed ends form nodes - open ends form antinodes

Question 43

how do string instruments produce sound?

Answer 43

- when the stretched string is plucked, it vibrates with a mix of first and higher harmonics - the pitch is determined by the frequency being played (which depends on which harmonic is being played)

Question 44

how can you change the pitch of a string in a string instrument?

Answer 44

- to increase the pitch: raise tension or shorten the length of the string - to decrease pitch: lower tension or increase length of the string