Glossary
Brief definitions of terms that appear throughout the experiments and supply pages. Each entry has a stable anchor, so pages can link straight to it — for example glossary.html#diffraction.
Amplitude
The maximum displacement of a wave from its rest position. A wave’s intensity (the power it carries) is proportional to the square of its amplitude, which is why adding two equal waves in step makes light four times as bright, not twice.
Brewster’s angle
The angle of incidence at which light reflected from a non-metallic surface is completely polarized parallel to that surface. For glass it is about 56°, for water about 53°. It is the principle behind polarized sunglasses, which cut horizontally polarized glare.
Coherence
A fixed, steady phase relationship between two or more waves. Only coherent waves produce a stable interference pattern; light from a laser is highly coherent, which is why it makes such crisp fringes in the double-slit experiment.
Constructive interference
What happens when two waves meet in step — crest on crest — so their amplitudes add to make a larger wave. In the double slit this produces the bright fringes, wherever the path difference is a whole number of wavelengths.
Destructive interference
What happens when two waves meet exactly out of step — crest on trough — so they cancel. This produces the dark fringes, wherever the path difference is an odd number of half-wavelengths.
Dielectric
An electrical insulator that can be polarized by an electric field but does not conduct a current. Castor oil is a dielectric with a relatively high permittivity, which is why it supports strong induced dipoles in the pearl-chains experiment.
Diffraction
The bending and spreading of a wave as it passes an edge or through a gap whose size is comparable to its wavelength. Diffraction is why light can reach into the geometric shadow of an obstacle, and why a single narrow slit spreads a laser beam into a band.
Diffraction grating
A surface ruled with many closely spaced parallel slits or lines. Because it combines the light from thousands of slits, a grating spreads light into much sharper, more widely separated orders than a double slit — the basis of the spectroscope.
Electric dipole
A pair of equal and opposite charges separated by a small distance — or any charge arrangement that behaves like one. A conducting sphere in an electric field develops an induced dipole as its electrons redistribute, which is what drives chain formation in pearl chains.
Electric field
The region around a charge in which another charge feels a force, and the vector describing that force per unit charge (volts per metre). A uniform field between two parallel plates is \(E = V/d\), the voltage divided by the gap.
Huygens’ principle
The rule, due to Christiaan Huygens, that every point on a wavefront acts as a source of secondary spherical wavelets; the envelope of those wavelets forms the wavefront an instant later. It explains both diffraction and why each slit in the double slit acts as a fresh source of waves.
Interference
The addition of two or more overlapping waves by superposition, producing a pattern of regions that reinforce (bright) and cancel (dark). Interference is the definitive signature that something is behaving as a wave.
Malus’s law
The rule that light of intensity \(I_0\) passing through a polarizer emerges with intensity \(I = I_0\cos^2\theta\), where \(\theta\) is the angle between the light’s polarization and the filter’s transmission axis. See the three-polarizer experiment.
Monochromatic
Light of a single wavelength — one pure colour. A laser is very nearly monochromatic, which (together with its coherence) makes it ideal for interference and diffraction measurements.
Path difference
The extra distance one wave travels compared with another before they meet. When the path difference is a whole number of wavelengths the waves arrive in step (constructive); when it is an odd number of half-wavelengths they cancel (destructive).
Permittivity
A measure of how much a material reduces an electric field set up inside it. A higher permittivity (a larger dielectric constant) lets a dielectric support stronger induced dipoles for a given applied field.
Photon
A single quantum of light — the smallest indivisible packet of electromagnetic energy. Remarkably, the interference fringes of the double slit build up even when photons are sent through one at a time.
Polarization
The orientation of a transverse wave’s oscillation. For light it is the direction in which the electric field vibrates. Unpolarized light contains all orientations at once; a polarizer selects one. See Malus’s law and Brewster’s angle.
Rayleigh scattering
The scattering of light by particles much smaller than its wavelength. It is stronger for shorter wavelengths (blue), which is why the daytime sky is blue, and it partially polarizes skylight.
Superposition
The principle that when two waves overlap, the total displacement at each point is simply the sum of the individual displacements. Superposition is the mechanism behind all interference.
Transverse wave
A wave whose oscillation is perpendicular to its direction of travel. Light is a transverse wave in the electromagnetic field, and it is this sideways character that makes polarization possible; sound, a longitudinal wave, cannot be polarized.
Wavefront
A surface joining all the points of a wave that share the same phase — for example all the crests of a ripple at one instant. Far from a source, wavefronts are nearly flat (plane waves).
Wavelength
The distance between successive crests of a wave, written \(\lambda\). For visible light it ranges from about 400 nm (violet) to 700 nm (red), and it is exactly this quantity you measure in the double-slit experiment.