waves

Question 1

what do waves transfer

Answer 1

energy and information but not matter

Question 2

Describe evidence that with water and sound waves it is the
wave and not the water or air itself that travels

Answer 2

(waves transfer energy and information but the particles stay in the same place)
- water ex = drop a twig into pool of water, ripples form and move across waters surface, ripples don’t carry the water (or twig with them though)
- sound = strum a guitar string and create a sound wave, the wave travels to your ear but it doesn’t carry the air away from the guitar (this would = vacuum)

Question 3

Define and use the terms frequency and wavelength as applied
to waves

Answer 3

-wavelength = length of a full cycle of a wave (eg from crest to crest)
- frequency = number of completed cycles of the wave passing a certain point per second (measured in hertz 1hz = 1 wave per second)

Question 4

Use the terms amplitude, period, wave velocity and wavefront
as applied to waves

Answer 4

• amplitude = displacement from the rest position to a crest or trough
• period = number of seconds it takes for one full cycle (period= 1/frequency)
• wave velocity = wave speed = frequency * wavelength
• wavefront = An imaginary surface representing points of a wave that are at the same point in their cycle

Question 5

what is a transverse wave

Answer 5

vibrations are perpendicular (/) to the direction the wave travels
ex electromagnetic waves, S-waves, ripples and waves in water

Question 6

what is a longitudinal wave

Answer 6

vibrations are parallel to the direction the wave travels (-)
ex sound waves, seismic waves

Question 7

equation relating wave speed, frequency and wavelength

Answer 7

wave speed = frequency * wavelength
(m/s) (Hz) (m)

Question 8

equation relating wave speed, distance and time

Answer 8

wave speed = distance / time
(m/s) (m) (s)

Question 9

Describe how to measure the velocity of sound in air

Answer 9

1. attach a signal generator to a speaker you can generate sounds with a specific frequency
2. use two microphones and an oscilloscope to find wavelength
3. set up oscilloscope, so the detected waves at each microphone are shown as separate waves
4. both microphones next to speaker, slowly move one away until two waves are aligned on display but have moved exactly one wavelength apart
5. measure distance between microphones to fin one wavelength
6. use formula wave speed = frequency * wavelength to find speed to sound waves passing through air - frequency is whatever you set the signal generator to in the first place

Question 10

describe how to measure the ripples on water surfaces

Answer 10

1. signal generator attached to dipper of a ripple tank (can create waves at a set frequency)
2. dim lights and turn on strobe light (see wave pattern made by shadows of the wave crests on the screen below tank)
3. alter frequency of the strobe light until wave pattern on screen appears to freeze and stop moving (happens when frequency of the waves and the strobe light are equal - waves appear not to move as they are being lit at the same point in their cycle each time)
4. distance between each shadow line is equal to one wavelength, measure distance between lines that are 10 wavelengths apart to find average wavelength
5. use wave speed = frequency * wavelength to find speed of waves

Question 11

Explain how waves will be refracted at a boundary in terms of
the change of direction and speed

Answer 11

1. waves travel at different speeds in materials with different densities - so when a wave crosses a boundary between materials it changes speed
2. if the wave hits the boundary at an angle this change of speed causes a change of direction - refraction
3. greater the change in speed = more the wave bends (changes direction)
4. if it bends towards the normal if it slows down, if bends away from the normal id it speeds up

Question 12

Recall that different substances may
refract waves in ways that vary with
wavelength

Answer 12

how much a wave refracts depends on its wavelength (as the frequency of of a wave doesn’t change between boundaries)
example EM waves with shorter wavelength bend more

Question 13

what happens when a wave meets a boundary between two materials (a material interface) * absorbed

Answer 13

the wave is absorbed by the 2nd material - wave transfers energy to the materials energy stores
- ex used for microwaves as energy is transferred to a thermal energy store which leads to heating

Question 14

what happens when a wave meets a boundary between two materials (a material interface) * transmit

Answer 14

wave is transmitted through the second material - the wave carries on travelling through the new material (often leads to refraction)
- ex communications, lenses of glasses and cameras

Question 15

what happens when a wave meets a boundary between two materials (a material interface) * reflect

Answer 15

wave is reflected - where incoming ray is neither absorbed or transmitted but instead is sent back away from second material
- ex how echoes are created

Question 16

Core Practical: Investigate the suitability of equipment to
measure the speed, frequency and wavelength of a wave in a
solid and a fluid

Answer 16

• oscilloscope and ripple tank methods as well as
  !use peak frequency to find the speed of waves in solids
  1. can find the speed of waves in solid by measuring frequency of sound waves produced when you hit the object
  2. measure and record the length of the metal rod
  3. set up apparatus making sure to secure rod at its centre
• apparatus includes: rod, clamps, elastic bands, microphone to computer, hammer
  4. tap the rod with a hammer, write down peak frequency displayed on computer
  5. repeat this 3 times to get an average peak frequency
  6. wave speed = frequency * wavelength where wavelength is twice the length of the rod (calculates speed of the wave)

Question 17

investigating refraction practical (in rectangular glass blocks)

Answer 17

1. place rectangular glass block on piece of paper and trace around it, use ray box to shine ray of light at the middle of one side of the block
2. trace incident ray and the emergent ray on other side of block, remove block, straight line join up incident and the emergent ray to show the path of the refracted ray through block
3. draw normal at point where light ray entered block, use protractor to measure the angle between the incident ray and the normal and the angle between refracted ray and normal
4. do same for point where ray emerges from block
5. repeat 3 times keeping angle of incidence as the ray enters the block the same
6. calculate average
   - ray of light should bend towards normal as it enters the block as air has a low] optical densities so ray will almost always slow down
   - should see ray of light bend away from normal as it leaves the block as light ray speeds up as it leaves the block and travels through the air
   - all EM waves can be refracted this experiment uses visible light so you can see ray being refracted as it travels through block

Question 18

what is the normal

Answer 18

an imaginary line perpendicular (right angles) to the point where the incoming wave hits the boundary

Question 19

what is the angle of incidence

Answer 19

angle between incoming (incident) ray and the normal

Question 20

what is the angle of refraction

Answer 20

angle between refracted ray and normal

Question 21

what are ray diagrams

Answer 21

can be used to show path that a wave travels, rays are straight lines that are perpendicular to wavefronts