Waves and Optics

Properties of waves

LensesUltrasound

Sound

DiffractionWhen waves pass through a narrow gap they spread out.Most diffraction occurs when width of gap = wavelengthImportant for satellite dishes

The longitudinal P (push pull) waves travel faster than the transverse S (shake) waves. P waves can also go through the liquid core of the Earth, unlike the S waves.

As the frequency increases so does the pitch of a note.We can hear from 20Hz to 20kHz (the upper threshold of human hearing). Frequencies above 20kHz are ultrasound.

Used for industrial cleaning where dirt is broken up by the high frequencies and breaking up kidney stones.

Pre-natal scanning – the waves are reflected from the fetus. – no damage to the fetus and shows up soft tissue.

Sound is caused by longitudinal vibrations of the particles of air, liquid or solid, in a backwards and forwards motion., causing compressions and rarefactions.

A louder sound has larger amplitude.

RadioLong wave 500m radio waves are reflected from the ionosphere. AM waves have their amplitude modulated by the sound.

MicrowavesAre absorbed by water in food but pass through the atmosphere to satellites.

Infra-red (IR)Heat is emitted by all hot objects including ourselves. Shiny surfaces reflect IR but dark surfaces absorb IR.

Object beyond 2F Object between F and 2F

Object inside F

Seismic waves

InterferenceWhen waves cross over, if two crests arrive in phase, we have constructive interference. The path difference must be an even number of half wavelengths.But when a crest meets a trough we have destructive interference.

PolarisationA Polaroid filter can produce plane-polarised waves where the vibrations are in one plane only.

The Electromagnetic Spectrum

VisibleIn a laser all the waves are in phase – used to read CDs.

Ultra-violetAbsorbed by ozone, sunblock and the skin.

X-rayAbsorbed by bone not tissueElectrons hit a metal

GammaCome from the nucleus. Absorbed by lead.

all travel at c, through a vacuum as a transverse wave.

wavespeed = frequency x wavelength

Refractive index = speed of light in vacuumSpeed of light in medium

n = sin i sin r

If the angle of incidence is more than the critical angle then there is total internal reflection - used in optical fibres.

Sin c = nr

ni