Waves flashcards

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.

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.

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.

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

A wave with a frequency of 4 Hz

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

2 kHz with a wavelength of 17 cm

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

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

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.

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

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

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.