Topic 4 - Waves Flashcards
Describe what waves do
Waves transfer energy and information without transferring matter
Define frequency
The number of waves that pass a single point per second
Give evidence for how waves work
With water and sound waves, it is the wave and not the water or air itself that travels, therefore waves don’t transfer matter
Define amplitude
Distance from equilibrium line to the maximum displacement (crest or trough)
Define wavelength
Distance between the same points on two consecutive waves
Define period
The time taken for a whole wave to completely pass a single point
What is the equation for wave velocity?
velocity = frequency x wavelength
Define wavefront
An imaginary surface representing points of a wave that are at the same point
Describe longitudinal waves
-Vibrations are parallel to the direction of travel
-Have compressions and rarefactions
-Examples: Sound, Seismic P waves
Describe transverse waves and give examples
-Vibrations are at right angles to the direction of travel
-Has peaks and troughs
-Examples: electromagnetic, seismic S and water waves
What are the equations for wave speed?
wave speed (m/s) = frequency (Hz) x wavelength (metre)
v = f ⋅λ
wave speed (m/s) = distance (metre)/time (s)
Describe how to measure the velocity of sound in air
-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
Describe how to measure the velocity of ripples on water surfaces
-Move a pencil along the paper at the same speed as a wavefront, and measure the time taken to draw this line and the length of the line - then use speed = distance/time
-Use a stroboscope, which has the same frequency as the water waves, then measure distance between the ‘fixed’ ripples and use velocity = frequency x wavelength
How do you calculate depth or distance from time and wave velocity?
distance = speed x time
If there is an echo divide distance by 2
Describe the effects of reflection of waves at material interfaces
-Waves will reflect off a flat surface
-The smoother the surface, the stronger the reflected wave is
-Rough surfaces scatter the light in all directions, so they appear matt and not reflective
-The 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
Describe the effects of refraction of waves at material interfaces
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.
Describe the effects of transmission of waves at material interfaces
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.
Describe the effects of absorption of waves at material interfaces
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
Describe the effect of wavelength on substances
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
Explain how the human ear works
-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)
Why does the process of converting wave disturbances between sound waves and vibrations in solids only work over a limited frequency range?
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
Describe simply the process of converting wave disturbances between sound waves and vibrations in solids only work over a limited frequency range?
A sound wave causes changes in the pressure on the surface of a solid which cause particles in the solid to vibrate.
What is ultrasound?
Sound with frequencies greater than 20,000Hz (top of the human hearing range)
What is infrasound?
Sound with frequencies less than 20Hz (bottom of the human hearing range)
Describe the main use of ultrasound
-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 n to harmful.
Describe the main use of infrasound
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.
Describe the core practical to investigate the suitability of equipment to measure the speed, frequency and wavelength of a wave in a solid and a fluid
-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.
