Physics - Waves Flashcards
Waves
What is a wave?
How is a wave created?
A wave transfers energy from one place to another without any overall transfer of matter.
In order to make a wave something needs to vibrate. We call the thing that vibrates a medium. This can be either a collection of particles or an electromagnetic field.
What evidence is there that it is
the wave that travels and not
matter?
When a sound wave travels through air the air particles do not travel from one place to another – if they did there would be an absence of air along the path and all of it would accumulate at the end! When a water wave travels across the ocean towards land the water particles don’t all travel towards
the land. If they did the ocean would have an absence of water!
In both these examples the particles simply vibrate about a fixed position and do not travel.
What is a transverse wave?
What waves are transverse?
In a transverse wave vibrations are at right angles (perpendicular) to the direction of energy transfer
(direction of the wave). These vibrations cause peaks and troughs. Waves on the surface of water (ripples) and all electromagnetic radiation such as visible light are examples of transverse waves.
What is a longitudinal wave?
What waves are longitudinal?
In a longitudinal wave vibrations are in the same direction (parallel) as the direction of energy transfer (direction of the wave). These vibrations cause compressions and rarefactions.
Sound is an example of a longitudinal wave.
What is a compression and
rarefaction in a longitudinal wave?
When a longitudinal wave travels through a substance, sometimes the particles are close together
(compression) and sometimes they are far apart (rarefaction)
What is the definition of
frequency? What is its unit?
The frequency of a wave is defined as the number of vibrations per second (or the number of waves per second). The unit is Hz (Hertz)
What is the definition of time period?
Time period is defined as the time for one vibration (or the time for one wave to pass a point). The unit is s (seconds).
What is the definition of amplitude?
The amplitude is the maximum displacement of a vibration from its rest position.
What is the definition of
wavelength?
The definition of wavelength is the distance from a point on a wave to the same point on the next wave (peak to peak for example). The unit is m (metres).
What is the relationship between time period and frequency?
Time period =
1/frequency
T = 1/f
Time period (seconds, s), Frequency (Hertz, Hz)
NB: this equation will be given to you in an exam on the equation sheet
What is wave speed?
Wave speed is the speed at which energy is transferred through the medium.
How do you measure the speed of sound in air?
Person one should stand a large distance away from person two and make an obvious visible sound. The distance between them (at least 100m) should be measured with a tape measure. Person two should start their stop clock when they see the sound being made and stop it when they hear the sound. The
speed of sound can then be calculated from speed = distance / time.
What is the relationship between wave speed, frequency and
wavelength?
Wave speed = frequency × wavelength
v = fλ
Speed (metres per second, m/s), Frequency (Hertz, Hz), Wavelength (metres, m)
How can you measure the speed of ripples on water surface?
Using a ripple tank you can measure the distance across ten waves using a ruler and then divide by 10 to find one wavelength. You can read the frequency from the signal generator that is driving the ripple tank making the waves. The speed can then be calculated using speed = frequency x wavelength.
How would you determine the
speed of waves on a string?
Stretch a string over a pulley using masses. Use a frequency generator to generate a wave in the string.
Measure the wavelength when a clear wave is shown and note the frequency on the generator. Use the wave speed equation to determine the speed.
To vary the wave speed, the frequency of the generator can be varied and/or the tension in the string can be varied by changing the masses.
What is the difference between
diffuse and specular reflection?
Specular reflection happens when light is incident on smooth surfaces. Parallel rays of incident light are reflected in a predictable manner and remain parallel to each other upon reflection. Diffuse reflection happens on rough surfaces. Parallel rays of light do not remain parallel to each other on
reflection and are scattered in different directions.
What is the law of reflection?
The angle of incidence equals the angle of reflection
What is a virtual image? Why is it different to a real image?
A virtual image is one from which the light rays appear to come but don’t actually come from the image e.g. the image seen in a mirror. A real image is one where the light rays do come from.
What is refraction and why does it happen?
Refraction is the change in speed of a wave when it reaches the boundary between two different materials. It happens because materials have different optical densities. It causes the wave to change direction.
What happens to frequency and wavelength when a wave
undergoes refraction?
When a wave undergoes refraction its frequency doesn’t change. This is because the source of the wave doesn’t change at all.
If a wave refracts because it speeds up then its wavelength will increase.
If a wave refracts because it slows down then its wavelength will decrease.
What effects does the refraction of light lead to?
Refraction of light at water/air boundaries causes objects to appear closer to the water surface than they actually are. Refraction is the reason why a straw in a glass of water looks broken.
What happens to parallel rays of light when they enter a convex lens?
In a convex lens parallel rays from a distant source of light are made to converge at a point known as the focal point, a distance called the focal length from the centre of the lens.
What do objects viewed through convex lenses look like?
The image formed by a convex lens depends on where the object is and the focal length of the lens.
To accurately draw lens diagrams:
1. Ray from top of object horizontal to lens then down through F on other side.
2. Ray from top of object directly through the centre of the lens.
At this point the rays should cross and this is where the “top” of the image is.
A third ray can be drawn down through F nearest the object (if possible) then horizontal through the lens
to confirm the position of the image.
What happens to parallel rays of light when they enter a concave lens?
In a concave lens parallel rays of light are made to diverge away from the lens. The focal point is on the same side of the lens as the object.
