Waves flashcards

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What do waves transfer?
Waves transfer energy (and information) from place to place without transferring matter. The particles of the medium only oscillate about fixed positions.
What is the difference between transverse and longitudinal waves?
In transverse waves, oscillations are at right angles to the direction of travel (light, water). In longitudinal waves they are parallel to it (sound), forming compressions and rarefactions.
Define amplitude, wavelength, frequency and period.
Amplitude: maximum displacement from the rest position. Wavelength: distance between matching points on adjacent waves. Frequency: waves per second (Hz). Period: time for one wave, T = 1 ÷ f.
State the wave equation and give the unit of each quantity.
v = f × λ. v is wave speed (m/s), f is frequency (Hz), λ is wavelength (m).
A wave has frequency 50 Hz and wavelength 0.20 m. What is its speed?
v = fλ = 50 × 0.20 = 10 m/s.
A sound wave has period 0.004 s. What is its frequency?
f = 1 ÷ T = 1 ÷ 0.004 = 250 Hz.
What happens when a wave is reflected? State the law.
The wave bounces off a surface. The angle of incidence equals the angle of reflection, both measured from the normal.
What is refraction and why does it happen?
Refraction is a change in wave direction when it crosses a boundary at an angle, because its speed changes. The frequency stays the same; the wavelength changes.
Describe how a light ray bends entering and leaving a glass block.
Entering glass it slows and bends towards the normal. Leaving glass it speeds up and bends away from the normal. If it enters along the normal it does not bend.
State the equation for refractive index.
n = sin i ÷ sin r, where i is the angle of incidence in air and r is the angle of refraction in the material. Both angles are measured from the normal.
Light enters glass at 40° and refracts at 25°. Find the refractive index.
n = sin 40° ÷ sin 25° = 0.643 ÷ 0.423 = 1.52.
What is total internal reflection and what two conditions are needed?
All the light is reflected back inside a denser material. The light must travel from the denser to the less dense medium, and the angle of incidence must be greater than the critical angle.
How is the critical angle linked to refractive index?
sin c = 1 ÷ n. For glass with n = 1.5, sin c = 0.667, so c = 42°. Optical fibres and prisms use total internal reflection.
What is the Doppler effect?
When a wave source moves relative to an observer, the observed frequency and wavelength change. A source moving towards you gives a higher frequency and shorter wavelength; moving away gives a lower frequency and longer wavelength.
Name the regions of the electromagnetic spectrum in order of increasing frequency.
Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. Wavelength decreases along this order.
What do all electromagnetic waves have in common?
They are transverse, travel at 3.0 × 10⁸ m/s in a vacuum, and transfer energy. Different types have different frequencies and wavelengths.
Give one use of each: radio waves, microwaves, infrared.
Radio waves: broadcasting and communications. Microwaves: cooking and satellite signals. Infrared: remote controls, grills and thermal imaging.
Give one use of each: visible light, ultraviolet, X-rays, gamma rays.
Visible light: seeing and optical fibres. Ultraviolet: sterilising water, security marking. X-rays: medical imaging of bones. Gamma rays: sterilising equipment, treating cancer.
Why are ultraviolet, X-rays and gamma rays dangerous?
They carry high energy and are ionising: they can damage cells and DNA, causing cell mutation, burns and cancer. Exposure should be limited.
What are the dangers of microwaves and infrared?
Microwaves heat water in body cells and can cause internal heating. Infrared can cause skin burns.
(Paper 2) What range of frequencies can humans hear?
Roughly 20 Hz to 20 000 Hz (20 kHz). Sound above this is ultrasound; below is infrasound.
How does sound travel and why can it not travel through a vacuum?
Sound is a longitudinal wave: particles vibrate and pass on compressions and rarefactions. It needs particles, so it cannot travel through a vacuum. It is fastest in solids.
(Paper 2) How do pitch and loudness relate to a sound wave?
Higher frequency means higher pitch. Greater amplitude means louder sound.
How can echoes be used to find the distance to a reflecting surface?
Send out a pulse of sound and measure the time t for the echo to return. The pulse travels there and back, so distance = speed × t ÷ 2.
An echo from a cliff returns after 0.60 s. Sound speed is 340 m/s. How far away is the cliff?
Total distance = 340 × 0.60 = 204 m there and back, so the cliff is 204 ÷ 2 = 102 m away.
What is a wavefront, and how do wavefronts show reflection and refraction?
A wavefront is a line joining points on a wave that are in step. Reflected wavefronts keep the same wavelength and bounce off at equal angles; when a wave enters a slower medium the wavefronts bunch closer (shorter wavelength) and change direction.
Describe how to investigate refraction of light using a glass block.
Place the block on paper and draw round it. Shine a narrow ray in at an angle, mark points along the incident and emergent rays, then remove the block and join the marks. Draw the normal and measure the angles i and r.
What are the colours of the visible spectrum, in order of increasing frequency?
Red, orange, yellow, green, blue, indigo, violet. Red has the longest wavelength and lowest frequency; violet has the shortest wavelength and highest frequency.
How can you measure the speed of water waves in a ripple tank?
Use a stroboscope to make the pattern appear still and measure λ with a ruler, and find the frequency from the strobe or dipper rate. Then calculate v = fλ.
(Paper 2) How do you measure the speed of sound in air?
Stand a known distance from a wall and clap; time the echo with a stopwatch or two microphones and a timer. Speed = distance travelled ÷ time. Repeat and average.

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