Topic 4 - Waves

Question 1

Describe what waves do

Answer 1

Waves transfer energy and information without transferring matter

Question 2

Define frequency

Answer 2

The number of waves that pass a single point per second

Question 3

Give evidence for how waves work

Answer 3

With water and sound waves, it is the wave and not the water or air itself that travels, therefore waves don’t transfer matter

Question 4

Define amplitude

Answer 4

Distance from equilibrium line to the maximum displacement (crest or trough)

Question 5

Define wavelength

Answer 5

Distance between the same points on two consecutive waves

Question 6

Define period

Answer 6

The time taken for a whole wave to completely pass a single point

Question 7

What is the equation for wave velocity?

Answer 7

velocity = frequency x wavelength

Question 8

Define wavefront

Answer 8

An imaginary surface representing points of a wave that are at the same point

Question 9

Describe longitudinal waves

Answer 9

-Vibrations are parallel to the direction of travel
-Have compressions and rarefactions
-Examples: Sound, Seismic P waves

Question 10

Describe transverse waves and give examples

Answer 10

-Vibrations are at right angles to the direction of travel
-Has peaks and troughs
-Examples: electromagnetic, seismic S and water waves

Question 11

What are the equations for wave speed?

Answer 11

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

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

Question 12

Describe how to measure the velocity of sound in air

Answer 12

-Make a noise around 50m from a solid wall and record the time for the echo to be heard, then use speed = distance/time
or
Have 2 microphones connected to a cataloguer at a large distance apart, and record the time difference between a sound passing from one to the other, then use speed = distance/time

Question 13

How do you calculate depth or distance from time and wave velocity?

Answer 13

distance = speed x time
If there is an echo divide distance by 2

Question 14

Describe the effects of reflection of waves at material interfaces

Answer 14

Waves will reflect off a flat surface - this is called specular reflection.
The smoother the surface the stronger the reflected wave is.
Rough surfaces scatter the light in all directions so they appear matt not reflective (diffuse reflection).
Angle of incidence = angle of reflection
-Light will reflect if the object is opaque and is not absorbed by the material. The electrons will absorb the light energy then reemit it as a reflected wave.

Question 15

Describe the effects of refraction of waves at material interfaces

Answer 15

Waves pass from one medium to another. If passing into a more optically denser medium (e.g. air to glass) the wave will be refracted at the boundary and will change direction to bend toward the normal.
-Speed decreases
-Wavelength decreases
-Energy of a wave is constant and energy is directly linked to frequency of a wave. So, if frequency is constant and speed decreases, wavelength must also decrease.

Question 16

Describe the relationship between refraction and speed

Answer 16

Change in speed causes refraction. The more an object refracts, the more it has sped up. If it refracts less/bends towards the normal it has slowed down.

Question 17

Describe the effects of transmission of waves at material interfaces

Answer 17

Waves will pass through a transparent material. The more transparent, the more light will pass through the material. It can still refract, but the process of passing through the material and still emerging is transmission.

Question 18

Describe the effects of absorption of waves at material interfaces

Answer 18

If the frequency of light matches the energy levels of the electrons the light will be absorbed by the electrons and not reemitted.
-They will be absorbed and then reemitted over time as heat, so that particular frequency has been absorbed.
-If a material appears green, only green light has been reflected, and the rest of the frequencies in visible light have been absorbed

Question 19

Describe the effect of wavelength on substances

Answer 19

Different substances may absorb, transmit, refract or reflect waves in ways that vary with wavelength.
For instance, glass transmits and refracts visible light and reflects UV

Question 20

Explain how the human ear works

Answer 20

-Sound waves enter the ear canal.
-The eardrum is a thin membrane, sound waves cause it to vibrate.
-Vibrations are passed on to tiny bones which amplify them.
-Vibrations are passed on to the liquid inside the cochlea.
-Tiny hairs inside the cochlea detect these vibrations and create electrical signals called impulses. (Each hair is sensitive to a different sound frequency)
-Impulses travel along neurones in the auditory nerve to reach the brain

(This is a form of the process of converting wave disturbances between sound waves and vibrations in solids)

Question 21

Why does the process of converting wave disturbances between sound waves and vibrations in solids only work over a limited frequency range?

Answer 21

The higher the frequency, the more energy the wave has - which would damage cells in the ear more quickly, and would not be able to work effectively long-term.
This, and the fact that we have evolved not needing to hear very high or low frequencies, means the ear only works for a limited frequency range

Question 22

Describe simply the process of converting wave disturbances between sound waves and vibrations in solids

Answer 22

A sound wave causes changes in the pressure on the surface of a solid which cause particles in the solid to vibrate.

Question 23

What is ultrasound?

Answer 23

Sound with frequencies greater than 20,000Hz (top of the human hearing range)

Question 24

What is infrasound?

Answer 24

Sound with frequencies less than 20Hz (bottom of the human hearing range)

Question 25

Describe the main use of ultrasound

Answer 25

-Foetal scanning: It's used to image the foetus, allowing measurements to be made to check that it is developing normally. It has a small amplitude so is low energy. This makes it safer, as no damage is done to any living cells. It's non-invasive and not harmful.

Question 26

Describe the main use of infrasound

Answer 26

The vibrations caused by earthquakes are seismic waves. -P-waves are longitudinal and can pass through solids and liquids. -S-waves are transverse, only pass through solids and move slower. On the opposite side of the Earth to an earthquake, only P waves are detected, suggesting the core of the earth is liquid - hence no S waves can penetrate.

Question 27

Describe the core practical to investigate the suitability of equipment to measure the speed, frequency and wavelength of a wave in a solid

Answer 27

-Using the first finger and thumb on one hand, hold a metal rod at its middle point. -Hit one end of the rod with a hammer. -Hold a smartphone with an app which measures frequency near the end of the rod. -Record the peak frequency from the phone app. -Measure the length of the metal rod and record the length. -Calculate the speed of the sound waves in the rod using the frequency and wavelength recorded.

Question 28

Describe the core practical to investigate the suitability of equipment to measure the speed, frequency and wavelength of a wave in a liquid

Answer 28

-Set up the ripple tank with about 5 cm depth of water. -Adjust the height of the wooden rod so that it just touches the surface of the water. -Switch on the lamp and motor and adjust until low frequency waves can be clearly observed. -Measure the length of a number of waves then divide by the number of waves to record wavelength. It may be more practical to take a photograph of the card with the ruler and take your measurements from the still picture. -Count the number of waves passing a point in ten seconds then divide by ten to record frequency. -Calculate the speed of the waves using: wave speed = frequency × wavelength.