Waves - Applied science Physics

Waves move energy from one place to another.  In a progressive wave the wave front moves through the medium. There are two types of waves, transverse and longitudinal.

Transverse waves are waves where the displacement of the particles in the medium is perpendicular to the direction the wave is travelling in.

Longitudinal waves are waves where the displacement of the particles in the the same direction as the wave is travelling in.

Amplitude (A) is the maximum displacement of a particle in a wave from its equilibrium position. It is measured in metres (m).

Frequency (f) is the number of complete waves passing a point in one second. It is measured in hertz (Hz).

Wavelength (?) is the distance between two identical points on a wave (i.e. one full wave). It is measured in metres (m).

Wave speed (c) is measured in metres per second (ms^-1).

Wave speed (c), frequency (f) and wavelength (l) are linked together in the following equation.

• c = wave speed (m s^-1)
• f = frequency (Hz)
• ? = wavelength (m)

Points on a wave which are always travelling in the same direction, rising a falling together, are in phase with each other.

Points on a wave which are always traveling in opposite directions to each other, one is rising while the other is falling, are in antiphase with each other.

If we measure the distance travelled by two waves and then compare those distances, any difference in the distances travelled is called the path difference. Path difference is measured in metres (m).

Longitudinal waves are waves where the displacement of the particles in the the same direction as the wave is travelling in. For example sound waves.

Transverse waves are waves where the displacement of the particles in the medium is perpendicular to the direction the wave is travelling in. For example water waves.

Transverse waves can be polarised but longitudinal waves cannot. Unpolarised light is a mixture if waves in different planes. When this light is passed through a polaroid material only light waves in one plane are transmited and the light is now polarised.

Polaroid sunglasses are popular with fishermen they reduce glare by blocking the reflected polarised light from the waters surface

Superposition of waves When two waves pass through the same point they combine together to either constructively interfere with each other or destructively interfere with each other before passing on past each other and continuing their separate journeys.

Constructive interference The two waves are in phase with each other and constructively interfere to give a wave of greater amplitude.

Destructive interference The two waves are  out of phase (anti-phase) with each other and destructively interfere to give a wave of zero amplitude.

Laser light is a source of coherent monochromatic light.

Coherence – two waves are coherent if the phase difference between them is constant. For this to be the case they must have the same frequency.

Monochromatic – means having only one wavelength of light present.

When laser light passes through a slit it is diffracted. If there are two slits present the light will diffract at both slits.

If a screen is placed on the other side of the slits from the laser an interference pattern is seen. It produces a series of bright and dark fringes.

w = fringe spacing in metres (m)
l = wavelength of the light in metres (m)
D = distance between the double slits and the screen in metres (m)
s = slit separation in metres (m)

Diffraction happens when a wave hits an obstacle or gap, diffraction is greatest when the gap is about the same size as the wavelength of the wave. The waves bend round the object or spread out when they pass through the gap, this is called diffraction.

When monochromatic laser light is shone through a narrow single slit a diffraction pattern is produced consisting of light and dark fringes. It produces a wide central bright fringe. The other bright fringes get dimmer as you move away from the centre.

A diffraction grating is a piece of glass with lots of closely spaced parallel lines on it each of which allows light to pass through it, this is a transmission diffraction grating.

Diffraction gratings are used in spectrometers. The diffraction grating splits up the light into a spectra

d = grating spacing in metres (m)
J = angle of diffraction
n = order number
l = wavelength on the light in metres (m)

This equation can be rearranged to

If there are N lines per metre on a diffraction grating then d can be calculated using

Diffraction gratings are used in the spectral analysis of light from stars.