Properties of Waves Flashcards

All 55 cards in this deck

What do all waves transfer?

Energy and information, without transferring matter.

True or false? A water wave carries the water across the pond with it.

False. The water (matter) stays in place and only oscillates; the wave transfers energy.

What does a floating cork show about a water wave passing under it?

It bobs up and down about the same place, showing the water itself does not travel with the wave.

What evidence shows that air does not travel along with a sound wave?

Dust (or a flame) in front of a loudspeaker vibrates back and forth about a fixed position rather than being blown away.

What is a wavefront?

A line joining all the points on a wave that are at the same point in their oscillation, e.g. a line along a crest; it is at right angles to the direction of travel.

Define the frequency of a wave, and give its unit.

The number of complete waves passing a point per second, measured in hertz (Hz).

Define the wavelength of a wave, and give its unit.

The distance from one point on a wave to the same point on the next wave (e.g. crest to next crest), measured in metres (m).

Define the amplitude of a wave.

The maximum distance a point on the wave moves from its undisturbed (rest) position, measured in metres.

Define the period of a wave, and give its unit.

The time taken for one complete wave to pass a point, measured in seconds (s).

True or false? The amplitude of a wave is the vertical distance from a crest to a trough.

False. That distance is twice the amplitude; amplitude is measured from the rest position to a crest (or trough).

How do you calculate the frequency of a wave from its period?

e.g. period $= 0.2$ s

1. Use $f = 1 \div T$
2. Substitute: $1 \div 0.2$
3. $f = 5$ Hz

What is a transverse wave?

A wave in which the oscillations are at right angles to the direction of energy transfer, e.g. water waves, electromagnetic waves, seismic S-waves.

What is a longitudinal wave?

A wave in which the oscillations are parallel to (along) the direction of energy transfer, e.g. sound waves and seismic P-waves.

What is a compression in a longitudinal wave?

A region where the particles are squashed close together (higher pressure).

What is a rarefaction in a longitudinal wave?

A region where the particles are spread further apart (lower pressure).

Which seismic waves are longitudinal and which are transverse?

P-waves are longitudinal; S-waves are transverse.

True or false? Sound waves in air are transverse waves.

False. Sound waves are longitudinal — the air particles vibrate along the direction the wave travels.

State the wave equation linking wave speed, frequency and wavelength, with units.

wave speed (m/s) $=$ frequency (Hz) $\times$ wavelength (m), i.e. $v = f\lambda$

How do you calculate the frequency of a wave from its speed and wavelength?

e.g. speed $= 300$ m/s, wavelength $= 2$ m

1. Rearrange $v = f\lambda$: $f = v \div \lambda$
2. Substitute: $300 \div 2$
3. $f = 150$ Hz

What is $v = f\lambda$ rearranged to make wavelength the subject?

$\lambda = v \div f$

What is 1 GHz in hertz?

$1 \times 10^{9}$ Hz (1 MHz $= 1\times10^{6}$ Hz, 1 kHz $= 1000$ Hz).

True or false? If the frequency of a wave doubles while its speed stays the same, the wavelength doubles.

False. The wavelength halves, because $v = f\lambda$ and $v$ is constant.

Which units must be used in $v = f\lambda$ to give a speed in m/s?

Frequency in hertz (Hz) and wavelength in metres (m).

State the equation linking wave speed, distance and time, with units.

wave speed (m/s) $=$ distance (m) $\div$ time (s)

What are the steps to measure the speed of sound in air?

e.g. two students standing 100 m apart

1. Measure the distance between them with a tape measure: 100 m.
2. One bangs cymbals; the other starts a stopwatch on seeing the bang and stops on hearing it: about 0.3 s (repeat and mean).
3. $v = x \div t = 100 \div 0.3 \approx 330$ m/s

What are the steps to measure the speed of ripples on a water surface by timing?

e.g. a ripple crossing a tank 0.5 m long

1. Mark a start line and a finish line a measured distance apart: 0.5 m.
2. Time one ripple (wavefront) travelling between them: 1.0 s (repeat and mean).
3. $v = x \div t = 0.5 \div 1.0 = 0.5$ m/s

True or false? Human reaction time is a major source of error when timing sound over a short distance.

True. That is why a long distance is used and the timing is repeated and averaged.

What is the approximate speed of sound in air?

About 330–340 m/s.

What equation gives the depth of water from the time taken for an echo to return?

depth $=$ (wave speed $\times$ time) $\div 2$

What are the steps to calculate depth from an echo time?

e.g. sound at 1500 m/s, echo returns after 2 s

1. Total distance travelled $= v \times t = 1500 \times 2 = 3000$ m.
2. Halve it, because the wave travels down and back: $3000 \div 2$.
3. Depth $= 1500$ m

True or false? The time measured for an echo is the time for the wave to travel to the reflecting surface.

False. It is the time for the wave to travel there and back, so the distance found must be halved.

Name the four things that can happen to a wave at a boundary between two materials.

Reflection, refraction, transmission and absorption.

What happens to a wave that is reflected at a material interface?

It bounces back off the boundary and stays in the first material.

What happens to a wave that is transmitted at a material interface?

It passes through the boundary and carries on travelling in the second material.

What happens to a wave that is absorbed by a material?

Its energy is transferred to the material's energy stores (often heating it), so the wave is not transmitted.

True or false? When a wave is absorbed by a material its energy is destroyed.

False. The energy is transferred to the material (usually to its thermal energy store).

What is refraction of a wave?

The change in direction of a wave as it crosses a boundary between two materials, caused by a change in its speed.

What causes a wave to refract at a boundary?

A change in the wave's speed in the new material.

Which way does a wave bend when it slows down on entering a new material at an angle?

Towards the normal.

Which way does a wave bend when it speeds up on entering a new material at an angle?

Away from the normal.

What is the normal at a boundary?

A construction line drawn at right angles (90°) to the boundary at the point where the wave meets it.

True or false? A wave travelling along the normal changes direction when it crosses a boundary.

False. Its speed changes but it carries straight on with no change of direction.

Which property of a wave stays the same when it is refracted?

The frequency, because it is set by the source.

What happens to the wavelength of a wave when it slows down on entering a new material?

The wavelength decreases, because frequency is unchanged and $v = f\lambda$.

What makes a wavefront change direction when it meets a slower material at an angle?

One end of the wavefront enters and slows down before the other end, so the wavefront swings round.

True or false? A wave can be refracted without its speed changing.

False. Refraction is caused by the change in speed at the boundary.

How do the absorption, transmission, refraction and reflection of waves depend on wavelength?

They vary with wavelength — the same substance can affect different wavelengths in different ways.

How does a red filter act on the different wavelengths of white light?

It transmits red light and absorbs the other wavelengths.

Why can a glass prism split white light into a spectrum?

Different wavelengths are refracted (bent) by different amounts in the glass.

True or false? A material that is transparent to visible light must transmit all other wavelengths too.

False. Glass transmits visible light but absorbs much of the infrared and ultraviolet.

What equipment is used to produce and observe waves in a fluid in the core practical?

A ripple tank with a motorised dipper or bar (driven by a signal generator) and a lamp above to project the ripples onto a screen.

What are the steps to find the speed of ripples in a ripple tank using $v = f\lambda$?

e.g. ripples with 50 waves passing a point in 10 s

1. Frequency $=$ waves counted $\div$ time $= 50 \div 10 = 5$ Hz.
2. Measure 5 wavelengths on the screen and divide by 5: $\lambda = 0.04$ m.
3. $v = f\lambda = 5 \times 0.04 = 0.2$ m/s

What equipment is used to make and measure waves in a solid in the core practical?

A signal generator and vibration generator attached to a stretched string (with masses over a pulley); the frequency is read from the signal generator.

What is the main difficulty in measuring the wavelength of ripples in a ripple tank, and one improvement?

The ripples move too fast to measure clearly; use a strobe light (or film them and freeze the frame) to make the pattern appear still.

True or false? Measuring the length of ten wavelengths and dividing by ten reduces the percentage error in the wavelength.

True. The measured length is larger, so the uncertainty in the ruler reading is a smaller proportion of it.