List of photonics equations

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This article summarizes equations in the theory of photonics, including geometric optics, physical optics, radiometry, diffraction, and interferometry.


Geometric optics (luminal rays)[edit]

Main article: Geometrical optics

General fundamental quantities[edit]

Quantity (common name/s) (Common) symbol/s SI units Dimension
Object distance x, s, d, u, x1, s1, d1, u1 m [L]
Image distance x', s', d', v, x2, s2, d2, v2 m [L]
Object height y, h, y1, h1 m [L]
Image height y', h', H, y2, h2, H2 m [L]
Angle subtended by object θ, θo, θ1 rad dimensionless
Angle subtended by image θ', θi, θ2 rad dimensionless
Curvature radius of lens/mirror r, R m [L]
Focal length f m [L]
Quantity (common name/s) (Common) symbol/s Defining equation SI units Dimension
Lens power P m−1 = D (dioptre) [L]−1
Lateral magnification m dimensionless dimensionless
Angular magnification m dimensionless dimensionless

Physical optics (EM luminal waves)[edit]

Main article: Physical optics

There are different forms of the Poynting vector, the most common are in terms of the E and B or E and H fields.

Quantity (common name/s) (Common) symbol/s Defining equation SI units Dimension
Poynting vector S, N W m−2 [M][T]−3
Poynting flux, EM field power flow ΦS, ΦN W [M][L]2[T]−3
RMS Electric field of Light Erms N C−1 = V m−1 [M][L][T]−3[I]−1
Radiation momentum p, pEM, pr J s m−1 [M][L][T]−1
Radiation pressure Pr, pr, PEM W m−2 [M][T]−3


Main article: Radiometry
Visulization of flux through differential area and solid angle. As always is the unit normal to the incidant surface A, , and is a unit vector in the direction of incident flux on the area element, θ is the angle between them. The factor arises when the flux is not normal to the surface element, so the area normal to the flux is reduced.

For spectral quantities two definitions are in use to refer to the same quantity, in terms of frequency or wavelength.

Quantity (common name/s) (Common) symbol/s Defining equation SI units Dimension
Radiant energy Q, E, Qe, Ee J [M][L]2[T]−2
Radiant exposure He J m−2 [M][T]−3
Radiant energy density ωe J m−3 [M][L]−3
Radiant flux, radiant power Φ, Φe W [M][L]2[T]−3
Radiant intensity I, Ie W sr−1 [M][L]2[T]−3
Radiance, intensity L, Le W sr−1 m−2 [M][T]−3
Irradiance E, I, Ee, Ie W m−2 [M][T]−3
Radiant exitance, radiant emittance M, Me W m−2 [M][T]−3
Radiosity J, Jν, Je, J W m−2 [M][T]−3
Spectral radiant flux, spectral radiant power Φλ, Φν, Φ, Φ

W m−1 (Φλ)
W Hz−1 = J (Φν)
[M][L]−3[T]−3 (Φλ)
[M][L]−2[T]−2 (Φν)
Spectral radiant intensity Iλ, Iν, I, I

W sr−1 m−1 (Iλ)
W sr−1 Hz−1 (Iν)
[M][L]−3[T]−3 (Iλ)
[M][L]2[T]−2 (Iν)
Spectral radiance Lλ, Lν, L, L

W sr−1 m−3 (Lλ)
W sr−1 m−2 Hz−1 (Lν)
[M][L]−1[T]−3 (Lλ)
[M][L]−2[T]−2 (Lν)
Spectral irradiance Eλ, Eν, E, E

W m−3 (Eλ)
W m−2 Hz−1 (Eν)
[M][L]−1[T]−3 (Eλ)
[M][L]−2[T]−2 (Eν)


Luminal electromagnetic waves[edit]

Physical situation Nomenclature Equations
Energy density in an EM wave
  • = mean energy density
For a dielectric:
Kinetic and potential momenta (non-standard terms in use) Potential momentum:

Kinetic momentum:

Cononical momentum:

Irradiance, light intensity
  • = time averaged poynting vector
  • I = irradiance
  • I0 = intensity of source
  • P0 = power of point source
  • Ω = solid angle
  • r = radial position from source

At a spherical surface:

Doppler effect for light (relativistic)

Cherenkov radiation, cone angle
  • n = refractive index
  • v = speed of particle
  • θ = cone angle
Electric and magnetic amplitudes
  • E = electric field
  • H = magnetic field strength
For a dielectric

EM wave components Electric


Geometric optics[edit]

Physical situation Nomenclature Equations
Critical angle (optics)
  • n1 = refractive index of initial medium
  • n2 = refractive index of final medium
  • θc = critical angle
Thin lens equation
  • f = lens focal length
  • x1 = object length
  • x2 = image length
  • r1 = incident curvature radius
  • r2 = refracted curvature radius

Lens focal length from refraction indices

Image distance in a plane mirror
Spherical mirror
  • r = curvature radius of mirror
Spherical mirror equation

Image distance in a spherical mirror

Subscripts 1 and 2 refer to initial and final optical media respectively.

These ratios are sometimes also used, following simply from other definitions of refractive index, wave phase velocity, and the luminal speed equation:



Physical situation Nomenclature Equations
Angle of total polarisation
intensity from polarized light, Malus' law
  • I0 = Initial intensity,
  • I = Transmitted intensity,
  • θ = Polarization angle between polarizer transmission axes and electric field vector

Diffraction and interference[edit]

Property or effect Nomenclature Equation
Thin film in air
  • n1 = refractive index of initial medium (before film interference)
  • n2 = refractive index of final medium (after film interference)
  • Minima:
  • Maxima:
The grating equation
  • a = width of aperture, slit width
  • α = incident angle to the normal of the grating plane
Rayleigh's criterion
Bragg's law (solid state diffraction)
  • d = lattice spacing
  • δ = phase difference between two waves
  • For constructive interference:
  • For destructive interference:


Single slit diffraction intensity
  • I0 = source intensity
  • Wave phase through apertures

N-slit diffraction (N ≥ 2)
  • d = centre-to-centre separation of slits
  • N = number of slits
  • Phase between N waves emerging from each slit

N-slit diffraction (all N)
Circular aperture intensity
Amplitude for a general planar aperture Cartesian and spherical polar coordinates are used, xy plane contains aperture
  • A, amplitude at position r
  • r' = source point in the aperture
  • Einc, magnitude of incident electric field at aperture
Near-field (Fresnel)

Far-field (Fraunhofer)

Huygen-Fresnel-Kirchhoff principle
  • r0 = position from source to aperture, incident on it
  • r = position from aperture diffracted from it to a point
  • α0 = incident angle with respect to the normal, from source to aperture
  • α = diffracted angle, from aperture to a point
  • S = imaginary surface bounded by aperture
  • = unit normal vector to the aperture
Kirchhoff's diffraction formula

Astrophysics definitions[edit]

In astrophysics, L is used for luminosity (energy per unit time, equivalent to power) and F is used for energy flux (energy per unit time per unit area, equivalent to intensity in terms of area, not solid angle). They are not new quantities, simply different names.

Quantity (common name/s) (Common) symbol/s Defining equation SI units Dimension
Comoving transverse distance DM pc (parsecs) [L]
Luminosity distance DL pc (parsecs) [L]
Apparent magnitude in band j (UV, visible and IR parts of EM spectrum) (Bolometric) m dimensionless dimensionless
Absolute magnitude


M dimensionless dimensionless
Distance modulus μ dimensionless dimensionless
Colour indices (No standard symbols)

dimensionless dimensionless
Bolometric correction Cbol (No standard symbol) dimensionless dimensionless

See also[edit]



  • P.M. Whelan; M.J. Hodgeson (1978). Essential Principles of Physics (2nd ed.). John Murray. ISBN 0-7195-3382-1. 
  • G. Woan (2010). The Cambridge Handbook of Physics Formulas. Cambridge University Press. ISBN 978-0-521-57507-2. 
  • A. Halpern (1988). 3000 Solved Problems in Physics, Schaum Series. Mc Graw Hill. ISBN 978-0-07-025734-4. 
  • R.G. Lerner; G.L. Trigg (2005). Encyclopaedia of Physics (2nd ed.). VHC Publishers, Hans Warlimont, Springer. pp. 12–13. ISBN 978-0-07-025734-4. 
  • C.B. Parker (1994). McGraw Hill Encyclopaedia of Physics (2nd ed.). McGraw Hill. ISBN 0-07-051400-3. 
  • P.A. Tipler; G. Mosca (2008). Physics for Scientists and Engineers: With Modern Physics (6th ed.). W.H. Freeman and Co. ISBN 978-1-4292-0265-7. 
  • L.N. Hand; J.D. Finch (2008). Analytical Mechanics. Cambridge University Press,. ISBN 978-0-521-57572-0. 
  • T.B. Arkill; C.J. Millar (1974). Mechanics, Vibrations and Waves. John Murray,. ISBN 0-7195-2882-8. 
  • H.J. Pain (1983). The Physics of Vibrations and Waves (3rd ed.). John Wiley & Sons,. ISBN 0-471-90182-2. 
  • J.R. Forshaw; A.G. Smith (2009). Dynamics and Relativity. Wiley,. ISBN 978-0-470-01460-8. 
  • G.A.G. Bennet (1974). Electricity and Modern Physics (2nd ed.). Edward Arnold (UK). ISBN 0-7131-2459-8. 
  • I.S. Grant; W.R. Phillips; Manchester Physics (2008). Electromagnetism (2nd Edition). John Wiley & Sons. ISBN 978-0-471-92712-9. 
  • D.J. Griffiths (2007). Introduction to Electrodynamics (3rd ed.). Pearson Education, Dorling Kindersley,. ISBN 81-7758-293-3. 

Further reading[edit]

  • L.H. Greenberg (1978). Physics with Modern Applications. Holt-Saunders International W.B. Saunders and Co. ISBN 0-7216-4247-0. 
  • J.B. Marion; W.F. Hornyak (1984). Principles of Physics. Holt-Saunders International Saunders College. ISBN 4-8337-0195-2. 
  • A. Beiser (1987). Concepts of Modern Physics (4th ed.). McGraw-Hill (International). ISBN 0-07-100144-1. 
  • H.D. Young; R.A. Freedman (2008). University Physics – With Modern Physics (12th ed.). Addison-Wesley (Pearson International). ISBN 0-321-50130-6.