Properties of Waves Flashcards

All 37 cards in this deck

What do all waves transfer?

Energy and information, without transferring matter.

A cork floats on water as a water wave passes. How does the cork move?

Up and down, at right angles (perpendicular) to the direction the wave travels — it does not travel along with the wave.

True or false? When a sound wave travels across a room, air moves from the source to your ear.

False. The air particles only oscillate backwards and forwards about fixed positions; the wave transfers energy, not matter.

Define the frequency of a wave and state its unit.

The number of waves passing a point each second; measured in hertz (Hz).

Define the wavelength of a wave.

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

What is the amplitude of a wave?

The maximum displacement of a point on the wave from its undisturbed (rest) position.

What is a wavefront?

A line joining points on a wave that are all at the same point in their oscillation (e.g. all the crests); it is perpendicular to the direction of travel.

In a transverse wave, in which direction do the oscillations happen?

Perpendicular (at right angles) to the direction the wave travels.
e.g. water waves, electromagnetic waves, seismic S-waves

In a longitudinal wave, in which direction do the oscillations happen?

In the same direction as (parallel to) the direction the wave travels.
e.g. sound waves, seismic P-waves

What are compressions and rarefactions in a longitudinal wave?

Compressions are regions where the particles are squashed closer together; rarefactions are regions where they are spread further apart.

How does a vibrating tuning fork prong make a sound wave travel through air?

It makes the air vibrate in the same direction as the wave travels, producing compressions and rarefactions.

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

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

What does the prefix G (giga) mean in a frequency such as 2.45 GHz?

$\times 10^9$, so 2.45 GHz $= 2.45 \times 10^9$ Hz.

What equation links the period $T$ and the frequency $f$ of a wave?

$T = 1/f$ (period in seconds, frequency in Hz).

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

wave speed (m/s) = distance (m) ÷ time (s), i.e. $v = x \div t$

What are the steps to measure the speed of sound in air over a measured distance?
e.g. two students 100 m apart, one firing a starting pistol

1. Measure the distance between them: 100 m
2. Time from seeing the flash/smoke to hearing the bang
3. Use $v = x \div t$ to get the speed of sound

Why do stopwatch timings for sound travelling 100 m vary so much between students?

Reaction time is large compared with the travel time, and is different for each student.

Give one improvement to measuring the speed of sound in air with a stopwatch, and its effect.

Use a much longer distance, or use an electronic timer (microphones/data logger) — this reduces the impact of reaction time and gives a more measurable time.

From which line are the angle of incidence and the angle of refraction measured?

The normal — the line drawn at 90° to the boundary.

True or false? A wave travelling along the normal into a different substance changes direction.

False. It carries straight on, with no change of direction.

True or false? For light entering glass, the angle of refraction is directly proportional to the angle of incidence.

False. The angle of refraction increases as the angle of incidence increases, but not in proportion.

What causes a wave to refract at a boundary?

A change in the speed of the wave as it enters a different substance.

Which way does a wave bend when it slows down on entering a new substance?

Towards the normal.

What happens to the wavelength of a wave that slows down as it crosses into a new substance?

The wavelength decreases (the frequency stays the same).

Name the four things that can happen to a wave when it meets a different substance.

It can be absorbed, transmitted, refracted or reflected.

What happens to the speed and wavelength of a wave when it is reflected at a boundary?

Both are unchanged; the wave bounces off at an equal angle.

True or false? A substance absorbs, transmits and reflects all wavelengths in the same way.

False. How much a substance absorbs, transmits, refracts or reflects varies with wavelength.

Give two reasons why light is less intense after passing through a glass block.

Some light is reflected at the edges/surfaces of the block, and some light is absorbed by the glass.

How can a stopwatch be used to find the frequency of ripples on water?

Count the number of waves passing a fixed point in a measured time; frequency = number of waves ÷ time.

Why is a stroboscope used when measuring the wavelength of ripples in a ripple tank?

Its flashes make the wave pattern appear frozen, so the wavelength can be measured with a ruler.

In the core practical, how is the frequency of the waves known accurately (e.g. waves on a string or ripples made by a dipper)?

It is read from the signal generator driving the vibration generator or dipper.

Why should a student repeat the timing and take a mean when measuring the speed of ripples across a ripple tank?

To reduce the effect of random errors in the timing, giving a more reliable value for the speed.

What is the period of a wave?

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

To measure the wavelength of sound, two microphones start side by side in front of a speaker and one is moved away until their oscilloscope traces line up again. What does the distance between the microphones then equal?

One wavelength of the sound wave.

In a ripple tank, why measure the distance across several wavelengths (e.g. 10) and divide, rather than measure one wavelength?

One wavelength is very short, so measuring a longer distance and dividing reduces the uncertainty and gives a more accurate wavelength.

When finding the speed of sound in a metal rod clamped at its centre and tapped at one end, how is the wavelength of the peak frequency related to the rod's length?

The wavelength is double the length of the rod.

When a wave meets a boundary at an angle, how does the size of its change in speed affect how much it refracts?

A larger change in speed makes the wave bend through a larger angle.