Physics & Astronomy Glossary

A

angle of incidence

The angle between a ray which is incident on a surface, and the normal to that surface.

angle of reflection

The angle between a ray which is reflected from a surface, and the normal to that surface.

angle of refraction

The angle between the ray which is refracted at a surface and the normal to that surface.

angular frequency

The rate of change of an angle per unit time. The standard units for angular frequency are radians/second. Angular frequency is typically denoted by ω, the Greek letter omega. Angular frequency is related to the frequency, f, by ω = 2πf.

anode

The positive pole of an electric battery, as opposed to the anode. In an electrical device with two electrodes separated by a potential difference, the anode is the positively charged electrode. Electric current flows from the anode to the cathode.

B

birefringence

A material is birefringent if the index of refraction differs for incident electromagnetic waves with different polarizations. Examples of birefringent media include calcite (CaCO₃), quartz (SiO₂), and ice (H₂O).

Brewster's angle

An electromagnetic wave, incident on a surface can be broken down into two polarization states. One in which the electric field is polarized perpendicular to the plane of incidence, and another which is polarized parallel to the plane of incidence. Brewster's angle is the angle of incidence at which only the component of the electric field polarized normal to the incident plane is reflected. Hence, if unpolarized light is reflected of a surface at Brewster's angle, the reflected light is completely polarized. Brewster's angle occurs when the angle between the reflected and refracted rays are 90 degrees. Named for Sir David Brewster who discovered it empirically.

C

cathode

The negative pole of an electric battery, as opposed to the anode. Electric current flows from the anode to the cathode.

D

diffraction

Diffraction, the deviation of light from rectilinear propagation, is a characteristic of wave phenomena which occurs when a portion of a wave front is obstructed in some way. When various portions of a wave front propagate past some obstacle, and interfere at a later point past the obstacle, the pattern formed is called a diffraction pattern.

dispersion

The dependence of the velocity of a wave on the frequency of the wave is known as dispersion. A media in which waves of different frequencies propagate at different speeds is said to be dispersive.

E

F

Fermat's principle

Fermat's principle of least time states that in traveling from point A to point B light follows the path which takes the least time.

frequency

The number of oscillations or vibrations per unit time. In SI units frequency is measured in inverse seconds or Hertz, abbreviated Hz. Frequency is the he inverse of the period T, f=1/T.

G

group velocity

The speed of the modulated signal. v_g = dω/dk

H

Huygens' Principle

Each point on a propagating wave can be considered as the source of a small wave, or wavelet which propagates at the same speed as the original wave. The new wave front for the propagating wave can be constructed by drawing a line tangent to the edges of these wavelets.

I

index of refraction

The index of refraction is the ratio of the speed an electromagnetic wave in vacuum to the speed of the electromagnetic wave in a particular medium. typically denoted by the lowercase n. n = c/v. The index of refraction is a property of the medium in question, and the incident electromagnetic wave. In general, they index of refraction may depend on on the frequency of the incident wave, and on its polarization. A media in which the index of refraction depends on the frequency of the electromagnetic wave is said to be dispersive. A media in which the index of refraction depends on the polarization of the incident wave is said to be birefringent

irradiance

The intensity of a light wave, commonly denoted by the letter I. The irradiance is the power per unit area carried by an electromagnetic wave. The time averaged value of the magnitude of the Poynting vector.

J

K

L

M

Mach number

The ratio of the speed of an object to the speed of sound.

monochromatic

Light of a single wavelength.

N

normal

A line which is perpendicular to a surface

O

P

period

The time to complete a cycle.

phase constant

The constant φ in a simple harmonic wave: A(x,t) = A_m sin(kx-ωt+ φ).

phase velocity

The velocity of a carrier wave in a modulated signal.

Poynting Vector

The Poynting vector represents the flux of energy density per unit time through a unit area. Commonly denoted by the letter S. The Poynting vector is proportional to the cross product of the electric and magnetic fields: S = (1/μ₀) E × B. The time averaged value of the Poynting vector is the irradiance. Named after John Henry Poynting

plane of incidence

The plane defined by the incident, reflected and refracted rays. These rays lie in a plane known as the plane of incidence.

Q

R

S

SI (International System of) units

The seven basic units of the SI system are

Quantity	Unit	Abbreviation
Length	meter	m
Time	second	s
Mass	kilogram	kg
Temperature	kelvin	k
Current	ampere	A
Luminous Intensity	candela	cd
amount of substance	mole	mol

simple harmonic wave

Also known as harmonic waves. Any wave form can be broken down into a combination of simpler waves which are sinusoidal. These sines and cosines are known as simple harmonic waves.

T

U

V

W

wavelength

The distance between to successive peaks of a periodic wave. Wavelength is typically denoted by λ, the Greek letter lambda.

wavenumber

The wave number is typically denoted by the letter k. It is related to the wavelength by: k = 2π/λ. In a simple harmonic wave, the wave number is the coefficient which multiplies the position. A(x,t) = A_m sin(kx-ωt+φ).

X

Y

Z