Waves flashcards
12 cards on Waves: the key points, worked examples and the slips to watch. Swipe through them, or print them.
Card 1 of 12
Waves
Transverse and longitudinal waves
What is the difference between a transverse wave and a longitudinal wave, with an example of each?
Answer it in your head, then turn the card over.
Transverse and longitudinal waves
Key points
- Transverse: the oscillations are at right angles to the direction of energy transfer.
- Ripples on water and every electromagnetic wave, light included, are transverse.
- Longitudinal: the oscillations are parallel to the direction of energy transfer. Sound in air is longitudinal.
- A longitudinal wave has compressions, where particles are squashed together, and rarefactions, where they spread out.
Watch out
Sound is longitudinal, not transverse: its particles move back and forth along the direction the wave travels.
Waves
Waves carry energy, not matter
What does a wave transfer, and what happens to the material it travels through?
Answer it in your head, then turn the card over.
Waves carry energy, not matter
Key points
- A wave transfers energy from place to place, but not the material it passes through.
- A cork on a pond bobs up and down as ripples pass: it does not move along with them.
- In a sound wave, the air particles only vibrate back and forth about one spot.
- Sound needs particles to vibrate, so it cannot cross a vacuum. Light needs no medium.
Watch out
The water or air does not travel with the wave: only the energy does.
Waves
Amplitude and wavelength
What are the amplitude and the wavelength of a wave, and how do you measure them on a diagram?
Answer it in your head, then turn the card over.
Amplitude and wavelength
Key points
- Amplitude: the maximum displacement of a point on the wave from its rest position.
- A bigger amplitude carries more energy: a louder sound or a brighter light.
- Wavelength (λ): the distance from one point on a wave to the same point on the next, such as crest to crest. It is in metres.
Watch out
Amplitude runs from the middle line to a peak, not peak to trough; a wavelength runs peak to the next peak.
Waves
Frequency and period
What are the frequency and the period of a wave, and how are they linked?
Answer it in your head, then turn the card over.
Frequency and period
Key points
- Frequency: the number of complete waves passing a point each second, in hertz (Hz).
- Period: the time for one complete wave to pass a point, in seconds.
- They are reciprocals: a higher frequency means a shorter period.
Formula
T = 1 ÷ f: period (s) = 1 ÷ frequency (Hz)
A wave with a frequency of 4 Hz
T = 1 ÷ 4 = 0.25 s
Four waves pass each second, so each takes a quarter of a second.
Watch out
Change kilohertz to hertz first: 1 kHz is 1000 Hz.
Waves
The wave speed equation
How do you calculate the speed of a wave from its frequency and wavelength?
Answer it in your head, then turn the card over.
The wave speed equation
Key points
- Wave speed is how fast the energy moves through the medium.
- Frequency in hertz and wavelength in metres give the speed in m/s.
- The same equation works for sound, water waves and electromagnetic waves.
Formulae
v = fλ: wave speed (m/s) = frequency (Hz) × wavelength (m)
λ = v ÷ f and f = v ÷ λ
2 kHz with a wavelength of 17 cm
2 kHz = 2000 Hz and 17 cm = 0.17 m
v = 2000 × 0.17 = 340 m/s
Watch out
2 × 17 = 34 forgets the units: change kHz to Hz and cm to m before you multiply.
Waves
Measuring the speed of water waves
How do you measure the speed of water waves in a ripple tank?
Answer it in your head, then turn the card over.
Measuring the speed of water waves
Key points
- Required practical: count the waves passing a point in 10 seconds, and divide by 10 for the frequency.
- Measure across 10 waves and divide by 10 for the wavelength: one wave is too small to measure well.
- Then work out the speed with v = fλ.
30 waves in 10 s; 10 waves span 50 cm
f = 30 ÷ 10 = 3 Hz
λ = 50 ÷ 10 = 5 cm = 0.05 m
v = 3 × 0.05 = 0.15 m/s
Watch out
Measure many waves and divide, rather than timing or measuring just one: it cuts the error.
Waves
Refraction
What is refraction, and which way does light bend as it enters and leaves glass?
Answer it in your head, then turn the card over.
Refraction
Key points
- Refraction: a wave changes direction as it passes into a new material at an angle, because its speed changes.
- Light slows down as it enters glass or water, and bends towards the normal.
- As it leaves the glass into air it speeds up, and bends away from the normal.
- Meeting the boundary along the normal, it changes speed but not direction.
Why a straw looks bent in water
Light from the straw refracts as it leaves the water into the air.
Your brain traces the rays back in straight lines, so the straw seems higher than it is.
Watch out
Slowing down bends light towards the normal, and speeding up bends it away. Don't swap them.
Waves
Absorbing, transmitting and reflecting
What can a material do to an electromagnetic wave that reaches it, and what decides which happens?
Answer it in your head, then turn the card over.
Absorbing, transmitting and reflecting
Key points
- A material can absorb, transmit or reflect electromagnetic waves, as well as refract them.
- Which happens depends on the wavelength of the wave.
- Glass transmits visible light but absorbs much of the ultraviolet.
- The body's soft tissue transmits X-rays, but bone absorbs them.
Watch out
Absorbed energy is not lost: absorbing a wave transfers its energy to the material, often warming it.
Waves
Physics onlyReflection and ray diagrams
What is the law of reflection, and why does a mirror give a clear image but paper does not?
Answer it in your head, then turn the card over.
Reflection and ray diagrams
Key points
- The angle of incidence equals the angle of reflection.
- Both are measured from the normal: a line at right angles to the surface where the ray hits it.
- A smooth surface reflects light in one direction (specular reflection), giving a clear image.
- A rough surface, like paper, scatters light in many directions (diffuse reflection).
Drawing a reflected ray
Draw the normal, dashed, at 90° to the mirror where the ray hits.
Measure the angle of incidence from the normal: say 35°.
Draw the reflected ray at 35° on the other side of the normal.
Watch out
Measure from the normal, not the mirror: from the surface you get 90° minus the right angle.
Waves
The electromagnetic spectrum
What do all electromagnetic waves have in common, and what is their order?
Answer it in your head, then turn the card over.
The electromagnetic spectrum
Key points
- All electromagnetic waves are transverse and travel at the same speed in a vacuum: 3 × 10⁸ m/s.
- Longest wavelength first: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
- Along that order the wavelength shortens, and the frequency and energy rise.
- It is a continuous spectrum, and our eyes detect only the visible part.
A radio wave of 100,000,000 Hz
λ = v ÷ f = 300,000,000 ÷ 100,000,000 = 3 m
Watch out
Gamma rays are not faster than radio waves: in a vacuum every electromagnetic wave has the same speed.
Waves
Uses of electromagnetic waves
What is each part of the electromagnetic spectrum used for?
Answer it in your head, then turn the card over.
Uses of electromagnetic waves
Key points
- Radio: TV and radio broadcasts. Microwaves: satellite signals, mobile phones and cooking.
- Infrared: heaters, cooking, thermal imaging cameras and remote controls.
- Visible light: seeing, photography and fibre optic communication.
- Ultraviolet: energy-efficient lamps and security marks. X-rays and gamma: medical imaging and treatment.
Why bones show up on an X-ray
X-rays pass through soft tissue, but bone absorbs them.
So bones leave a pale shadow on the image.
Watch out
TV remote controls use infrared, and satellites use microwaves: keep the neighbouring parts of the spectrum apart.
Waves
Dangers of electromagnetic waves
Which electromagnetic waves are harmful to the body, and why?
Answer it in your head, then turn the card over.
Dangers of electromagnetic waves
Key points
- Ultraviolet, X-rays and gamma rays carry enough energy to damage cells and the DNA inside them.
- Ultraviolet can age the skin early and cause skin cancer. Sunscreen absorbs some of it.
- X-rays and gamma rays are ionising: they can change genes and cause cancer.
- Radiation dose, in sieverts, measures the risk of harm, so hospitals keep X-ray doses low.
Watch out
The harm rises with frequency: ultraviolet, X-rays and gamma are the dangerous end, not radio waves.
Print these cards
Four cards to an A4 sheet, each the size of a standard A6 revision card.
One-sided
Each card prints on one face: the question at the top, the notes under it.
Two-sided
Questions on the front, notes on the back. Print double-sided and flip on the long edge, then cut along the lines.
Put it into practice
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