Topic 2- Waves And The EM Spectrum

Question 1

Waves

Answer 1

• Transfer energy and information in the direction they are travelling
• The amplitude of a wave is the displacement from the rest position to the crest or trough
• The wavelength is the length of a full cycle of wave (from crest to crest)

Question 2

Wave Frequency

Answer 2

• Number of complete cycles in a wave at a certain point per second
• Measured in hertz (Hz)
• 1 hertz equals 1 wave per second

Question 3

The period of a wave

Answer 3

• The number of seconds it takes for one full cycle

- Period = 1 divided by frequency

Question 4

Transverse Waves

Answer 4

• Vibrations are perpendicular to the direction it travels
• Examples of transvere waves :
• All EM waves
• S-waves
• Ripples in water

Question 5

Longitudinal Waves

Answer 5

• Vibrations are parallel to the direction it travels
• Examples are sound waves and p-waves
• They squash up and stretch out making compressions and rarefactions

Question 6

Wave Speed

Answer 6

Distance divided by time

Frequency x Wavelength (m)

Question 7

Measuring the speed of sound

Answer 7

• Use an oscilloscope
• By attaching a signal generator to a speaker, sounds can be generated with a specific frequency
• To find a wavelength, use two microphones and an oscilloscope

Question 8

Practical to find wavelengths

Answer 8

• Set up the oscilloscope so that the detected wave by each microphone are shown as separate waves
• Start with both microphones next to the speaker, gradually move one away until both waves are aligned and one wavelength apart
• Measure the distance between the microphones to find one wavelength
• The speed can then be found as the frequency is whatever the generator is set to

Question 9

Practical to measure the speed of water ripples

Answer 9

• Use a signal generator attached to a dipper of a ripple tank to create water waves at a set frequency
• Dim the lights and turn on a strobe light, the wave crests will appear as shadows
• Alter the frequency until the shadow stops moving, this means that the frequency of the light and water waves are the same, the shadows do not move because they water waves are being lit at the same time in their cycle
• The distance between each shadow line is equal to one wavelength
• Calculate the speed

Question 10

Speed of waves in solids

Answer 10

• Use peak frequency
• Measure the frequency of sound waves produced when they hit the object
• When an object is struck, the wavelengths are the same as the sound waves produced by the action

Question 11

Practical to work out of speed of waves in a solid

Answer 11

• Measure and record the length of a rod
• Tether a rod to a clamp with elastic bands
• Use a microphone to record sound waves
• Tap the rod with a hammer, record the peak frequency recorded by the microphone
• Repeat this three times to work out an average
• Calculate the speed where the distance is twice the length of the rod

Question 12

Boundaries at waves

Answer 12

Waves can be transmitted, absorbed and reflected at boundaries

A boundary is the space between two materials (a material interface)

Question 13

How waves act at boundaries

Answer 13

• The wave is absorbed by the second material, the waves transfers to the material’s energy store, the energy is transferred to thermal energy store which leads to heating
• The wave is transmitted through the second material, the wave carries on travelling through the new material. This leads to refraction. This is used i communications and glasses
• The wave can be reflected, this is where the ray is neither absorbed nor transmitted, instead it is sent back, this is how an echo works

Question 14

Refraction

Answer 14

• Waves travel at different speeds in materials at different densities. So when a wave crosses a boundary, it changes speed
• If the wave hits a boundary at an angle, the change in speed causes a change in direction
• If the waves is travelling perpendicular, then there will be change in speed but it will not be refracted
• The greater the speed, the greater the change in direction
• The wave bends towards the normal if it slows down at the boundary, and away if it speeds up

Question 15

Refraction of EM waves

Answer 15

• Light is slower in denser materials
• How much an EM wave can be refracted is determined by the wavelength.
• Shorter wavelengths bend more leading to the wavelengths spreading out more (dispersion) such as a white light becoming a spectrum
• The frequency of the wave stays the same as it crosses a boundary, so the wavelength and the wave speed are inversley proportional

Question 16

Ray Diagrams

Answer 16

• Shows the path that the wave travels
• First draw the boundary and the normal which should form a perpendicular cross
• Draw an incident ray that meets the normal at the boundary
• The angle between the ray and the normal is the angle of incidence
• Draw the refracted ray on the other side of the boundary. If the first material is denser than the first, then it will bends towards the normal
• The angle of refraction will be smaller than the incidence
• If the second is less optically dense, the angle of refraction will be larger

Question 17

Boundaries at waves

Answer 17

Waves can be transmitted, absorbed and reflected at boundaries

A boundary is the space between two materials (a material interface)

Question 18

How waves act at boundaries

Answer 18

• The wave is absorbed by the second material, the waves transfers to the material’s energy store, the energy is transferred to thermal energy store which leads to heating
• The wave is transmitted through the second material, the wave carries on travelling through the new material. This leads to refraction. This is used i communications and glasses
• The wave can be reflected, this is where the ray is neither absorbed nor transmitted, instead it is sent back, this is how an echo works

Question 19

Refraction

Answer 19

• Waves travel at different speeds in materials at different densities. So when a wave crosses a boundary, it changes speed
• If the wave hits a boundary at an angle, the change in speed causes a change in direction
• If the waves is travelling perpendicular, then there will be change in speed but it will not be refracted
• The greater the speed, the greater the change in direction
• The wave bends towards the normal if it slows down at the boundary, and away if it speeds up

Question 20

Refraction of EM waves

Answer 20

• Light is slower in denser materials
• How much an EM wave can be refracted is determined by the wavelength.
• Shorter wavelengths bend more leading to the wavelengths spreading out more (dispersion) such as a white light becoming a spectrum
• The frequency of the wave stays the same as it crosses a boundary, so the wavelength and the wave speed are inversley proportional

Question 21

Ray Diagrams

Answer 21

• Shows the path that the wave travels
• First draw the boundary and the normal which should form a perpendicular cross
• Draw an incident ray that meets the normal at the boundary
• The angle between the ray and the normal is the angle of incidence
• Draw the refracted ray on the other side of the boundary. If the first material is denser than the first, then it will bends towards the normal
• The angle of refraction will be smaller than the incidence
• If the second is less optically dense, the angle of refraction will be larger