Waves and EM Spectrum

Question 1

What do waves transfer?

Answer 1

energy and information in the direction that they are travelling in

Question 2

What happens when waves travel through a medium?

Answer 2

-The particles of the medium vibrates
-These particles then transfer energy and information between each other
-Overall particles stay in the same place-only energy and information transferred

Question 3

What is the amplitude of a wave?

Answer 3

The displacement from rest position to a crest or trough

Question 4

What is the wavelength?

Answer 4

-The length of a full cycle of the wave (e.g crest to crest or compression to compression)

Question 5

What is the frequency of a wave?

Answer 5

-The number of full cycles of the wave passing a certain point per second
-Measured in hertz (Hz)
-1 Hz = 1 wave per second

Question 6

What is the period of a wave?

Answer 6

-The number of seconds it takes for a full cycle of the wave to pass a point
-Period = 1 / frequency

Question 7

Transverse waves

Answer 7

-Vibrations are perpendicular to direction wave is travelling
-Waves that are transverse include:
All EM waves
S-waves
Ripples and waves in water

Question 8

Longitudinal waves

Answer 8

-Vibrations are parallel to the direction the wave travels
-waves squash up and stretch out the arrangement of particles in the medium they pass through
-Make compressions (high pressure) and rarefactions (low pressure)

Question 9

Wave speed (with equation)

Answer 9

-Tells you how quickly a wave moves through space

v (m/s or wave speed) = f (Hz or frequency) x λ (m or wavelength)

Question 10

Oscilloscope practical steps - measuring the speed of sound

Answer 10

1) Set up the oscilloscope so detected waves at each microphone are shown as separate waves
2) Start with both microphones next to speaker, then move one away until two waves are aligned but have moved one wavelength apart
3) Measure distance between microphones to find one wavelength (λ).
4) Use wave speed equation to find speed of sounds waves through the air

Question 11

Water ripples using a strobe light practical steps- Measure speed of water

Answer 11

1)Use signal generator attached to dipper of ripple tank to create a set frequency
2)Dim lights and turn on strobe lights to reveal wave pattern
3)Alter frequency of strobe light until wave pattern on the screen appears to ‘freeze’ or stop moving
4) Distance between each shadow line is equal to one wavelength. Measure the distance between 10 waves and find the average wavelength.
5) Use v = f x λ to calculate speed of waves

Question 12

What to do if the waves seen on screen are magnified? - wave ripple practical

Answer 12

-Stick a piece of tape of a known length, e.g 10cm, onto bottom of ripple tank then measure its shadow
-If shadow is longer than the tape, then the length of shadow ÷ by actual length of tape will give you the scale factor.

Question 13

Peak frequency practical - find speed of waves in solids

Answer 13

-You can find speed of waves in solid by measuring the frequency of sound waves produced when hitting object:
1) Measure and record the length of a metal rod
2) Set up apparatus making sure to secure the rod at its centre
3) Tap the rod with the hammer. Write down peak (loudest) frequency displayed by computer.
4) Repeat this three times to get a average peak frequency
5) Calculate speed of the wave using v = f x λ where λ = twice the length of the rod

Question 14

What happens when a wave meets a boundary?

Answer 14

Three things can happen:
1) The wave is absorbed - wave transfers energy to the material’s energy stores
2) The wave is transmitted - the wave travels through the material. Leads to refraction
3) Wave is reflected - when wave is ‘sent back’ away from the material

Question 15

What is refraction?

Answer 15

If a wave hits a boundary at an angle, the change of speed causes a change in direction - refraction.

Question 16

Refraction in terms of the normal

Answer 16

The wave bends towards the normal if it slows down and away from the normal if it speeds up

Question 17

Refraction in terms of an EM wave

Answer 17

-EM waves usually travel more slowly in denser materials
-Shorter EM wavelengths bend more. This can lead to dispersion (e.g white light becoming a spectrum)

Question 18

Frequency of a wave

Answer 18

-The frequency of a wave stays the same when it crosses the boundary.
-As v = f x λ, change in speed caused by change in wavelength

Question 19

Ray diagram construction - Steps

Answer 19

1) First, start by drawing the boundary between your two materials and the normal. The normal is drawn perpendicular to surface at point of incidence.
2)Draw incident ray that meets normal at boundary. Angle between ray and normal is angle of incidence (use protractor)
3) Draw refracted ray on other side of boundary
4)If second material is optically denser than first, refracted ray bends towards the normal. If less dense, refracted ray bends away from normal.

Question 20

Why do the glass block experiment in a dim room?

Answer 20

-So you can clearly see the ray.
-Ray of light must be thin, so you can easily see the middle of the ray when tracing it and measuring angles from it.

Question 21

The law of reflection is:

Answer 21

Angle of incidence = angle of reflection

Question 22

What does TIR mean?

Answer 22

-Total Internal reflection
-Can only happen when a wave travels through a dense material to wards a less dense material

Question 23

If incidence ray is less than critical angle

Answer 23

Most of the light is refracted into the outer layer, but some of it is internally reflected.

Question 24

If incidence ray is equal to critical angle

Answer 24

The ray would go along the surface (with quite a bit of internal reflection)

Question 25

If incidence ray is larger than the angle

Answer 25

No light comes out. It’s all internally reflected, i.e TIR

Question 26

What is specular reflection?

Answer 26

-Happens when waves are reflected in a single direction by a smooth surface.
-Means you get a clear reflection

Question 27

What is diffuse reflection

Answer 27

-Occurs when waves are reflected by a rough surface and waves reflect in all different directions
-Happens because normal is different for each incidence ray.
-Surface looks matte and no clear reflection

Question 28

How are sound waves caused?

Answer 28

-Sound waves caused by vibrating objects
-They pass through the surrounding medium in a series of compressions and rarefactions

Question 29

How does a sound wave travel through a solid?

Answer 29

-When the sound wave hits, the air particles hitting the object cause the particles in solid to move back and forth
-These particles hit the next particles passing the sound wave through object as series of vibrations

Question 30

How fast do sound waves travel through solid, liquid and gas?

Answer 30

Sound waves travel faster in liquids than they do in gases and faster in solids than they do in liquids.

Question 31

How does the wavelength of a sound wave change?

Answer 31

-It gets longer when the wave speeds up an shorter when it slows down.
-Echoes are just reflected sound waves

Question 32

What does a sound wave reflect off?

Answer 32

-Hard, flat surfaces
-Sound waves can’t travel in space as it’s mostly a vacuum

Question 33

How do sound waves get from the ear drum to the brain?

Answer 33

-Sound waves that reach your eardrum cause it to vibrate
-These are passed on to tiny bones called ossicles, through semicircular canals and to the cochlea.
-Cochlea turns these vibrations into electrical signals which get sent to the brain.

Question 34

What does a higher frequency mean?

Answer 34

A higher frequency sound wave has a higher pitch

Question 35

Infrasound waves

Answer 35

-Waves so low in frequency that we can’t hear them
-Waves under 20Hz
-Scientists can monitor infrasound to try and predict natural events e.g volcano erupting

Question 36

Ultrasound waves

Answer 36

-Electrical devices can be made to produce electrical oscillations of any frequency
-Can be converted into mechanical vibrations to produce sound waves beyond human hearing
-frequencies above 20 000 Hz