# Fresnel equations

Partial transmission and reflection amplitudes of a wave travelling from a low to high refractive index medium.

The Fresnel equations (or Fresnel conditions), deduced by Augustin-Jean Fresnel , describe the behaviour of light when moving between media of differing refractive indices. The reflection of light that the equations predict is known as Fresnel reflection.

## Overview

When light moves from a medium of a given refractive index n1 into a second medium with refractive index n2, both reflection and refraction of the light may occur. The Fresnel equations describe what fraction of the light is reflected and what fraction is refracted (i.e., transmitted). They also describe the phase shift of the reflected light.

The equations assume the interface is flat, planar, and homogeneous, and that the light is a plane wave.

## Definitions and power equations

Variables used in the Fresnel equations.

In the diagram on the right, an incident light ray IO strikes the interface between two media of refractive indices n1 and n2 at point O. Part of the ray is reflected as ray OR and part refracted as ray OT. The angles that the incident, reflected and refracted rays make to the normal of the interface are given as θi, θr and θt, respectively.

The relationship between these angles is given by the law of reflection:

$\theta_\mathrm{i} = \theta_\mathrm{r}$

and Snell's law:

$\frac{\sin\theta_\mathrm{i}}{\sin\theta_\mathrm{t}} = \frac{n_2}{n_1}$

The fraction of the incident power that is reflected from the interface is given by the reflectance R and the fraction that is refracted is given by the transmittance T.[1] The media are assumed to be non-magnetic.

The calculations of R and T depend on polarisation of the incident ray. Two cases are analyzed:

1. The incident light is polarized with its electric field perpendicular to the plane containing the incident, reflected, and refracted rays, i.e. in the plane of the diagram above. The light is said to be s-polarized, from the German senkrecht (perpendicular).
2. The incident light is polarized with its electric field parallel to the plane described above. Such light is described as p-polarized.

The reflectance for s-polarized light is

$R_\mathrm{s} = \left|\frac{n_1\cos\theta_{\mathrm{i}}-n_2\cos\theta_{\mathrm{t}}}{n_1\cos\theta_{\mathrm{i}}+n_2\cos\theta_{\mathrm{t}}}\right|^2 =\left|\frac{n_1\cos\theta_{\mathrm{i}}-n_2\sqrt{1-\left(\frac{n_1}{n_2} \sin\theta_{\mathrm{i}}\right)^2}}{n_1\cos\theta_{\mathrm{i}}+n_2\sqrt{1-\left(\frac{n_1}{n_2} \sin\theta_{\mathrm{i}}\right)^2}}\right|^2$,

while the reflectance for p-polarized light is

$R_\mathrm{p} = \left|\frac{n_1\cos\theta_{\mathrm{t}}-n_2\cos\theta_{\mathrm{i}}}{n_1\cos\theta_{\mathrm{t}}+n_2\cos\theta_{\mathrm{i}}}\right|^2 =\left|\frac{n_1\sqrt{1-\left(\frac{n_1}{n_2} \sin\theta_{\mathrm{i}}\right)^2}-n_2\cos\theta_{\mathrm{i}}}{n_1\sqrt{1-\left(\frac{n_1}{n_2} \sin\theta_{\mathrm{i}}\right)^2}+n_2\cos\theta_{\mathrm{i}}}\right|^2$.

The second form of each equation is derived from the first by eliminating θt using Snell's law and trigonometric identities.

As a consequence of the conservation of energy, the transmission coefficients are given by [2]

$T_s = 1-R_s\,\!$

and

$T_p = 1-R_p\,\!$

These relationships hold only for power coefficients, not for amplitude coefficients as defined below.

If the incident light is unpolarised (containing an equal mix of s- and p-polarisations), the reflection coefficient is

$R = \frac{R_s+R_p}{2}\,\!$

For common glass, the reflection coefficient at θi = 0 is about 4%. Note that reflection by a window is from the front side as well as the back side, and that some of the light bounces back and forth a number of times between the two sides. The combined reflection coefficient for this case is 2R/(1 + R), when interference can be neglected (see below).

The discussion given here assumes that the permeability μ is equal to the vacuum permeability μ0 in both media, embodying the assumption that the material is non-magnetic. This is approximately true for most dielectric materials, but not for some other types of material. The completely general Fresnel equations are more complicated.

For low-precision applications where polarization may be ignored, such as computer graphics, Schlick's approximation may be used.

### Special angles

At one particular angle for a given n1 and n2, the value of Rp goes to zero and a p-polarised incident ray is purely refracted. This angle is known as Brewster's angle, and is around 56° for a glass medium in air or vacuum. Note that this statement is only true when the refractive indices of both materials are real numbers, as is the case for materials like air and glass. For materials that absorb light, like metals and semiconductors, n is complex, and Rp does not generally go to zero.

When moving from a denser medium into a less dense one (i.e., n1 > n2), above an incidence angle known as the critical angle, all light is reflected and Rs = Rp = 1. This phenomenon is known as total internal reflection. The critical angle is approximately 41° for glass in air.

## Amplitude equations

Equations for coefficients corresponding to ratios of the electric field complex-valued amplitudes of the waves (not necessarily real-valued magnitudes) are also called "Fresnel equations". These take several different forms, depending on the choice of formalism and sign convention used. The amplitude coefficients are usually represented by lower case r and t.

Amplitude ratios: air to glass
Amplitude ratios: glass to air

### Conventions used here

In this treatment, the coefficient r is the ratio of the reflected wave's complex electric field amplitude to that of the incident wave. The coefficient t is the ratio of the transmitted wave's electric field amplitude to that of the incident wave. The light is split into s and p polarizations as defined above. (In the figures to the right, s polarization is denoted "$\bot$" and p is denoted "$\parallel$".)

For s-polarization, a positive r or t means that the electric fields of the incoming and reflected or transmitted wave are parallel, while negative means anti-parallel. For p-polarization, a positive r or t means that the magnetic fields of the waves are parallel, while negative means anti-parallel.[3] It is also assumed that the magnetic permeability µ of both media is equal to the permeability of free space µ0.

### Formulas

Using the conventions above,[3]

$r_s = \frac{n_1 \cos \theta_\text{i} - n_2 \cos \theta_\text{t}}{n_1 \cos \theta_\text{i} + n_2 \cos \theta_\text{t}}$
$t_s = \frac{2 n_1 \cos \theta_\text{i}}{n_1 \cos \theta_\text{i} + n_2 \cos \theta_\text{t}}$
$r_p = \frac{n_2 \cos \theta_\text{i} - n_1 \cos \theta_\text{t}}{n_1 \cos \theta_\text{t} + n_2 \cos \theta_\text{i}}$
$t_p = \frac{2 n_1\cos \theta_\text{i}}{n_1 \cos \theta_\text{t} + n_2 \cos \theta_\text{i}}$

Notice that $t_s = 1 + r_s$ but $t_p \neq 1 + r_p$.[4]

Because the reflected and incident waves propagate in the same medium and make the same angle with the normal to the surface, the amplitude reflection coefficient is related to the reflectance R by [5]

$R=\left| r \right|^2$.

The transmittance T is generally not equal to |t|2, since the light travels with different direction and speed in the two media. The transmittance is related to t by.[6]

$T=\frac{n_2 \cos \theta_\text{t}}{n_1 \cos \theta_\text{i}} \left| t \right|^2$.

The factor of n2/n1 occurs from the ratio of intensities (closely related to irradiance). The factor of cos θt/cos θi represents the change in area m of the pencil of rays, needed since T, the ratio of powers, is equal to the ratio of (intensity × area). In terms of the ratio of refractive indices,

$\rho = n_2/n_1$,

and of the magnification m of the beam cross section occurring at the interface,

$T = \rho m t^2$.

## Multiple surfaces

When light makes multiple reflections between two or more parallel surfaces, the multiple beams of light generally interfere with one another, resulting in net transmission and reflection amplitudes that depend on the light's wavelength. The interference, however, is seen only when the surfaces are at distances comparable to or smaller than the light's coherence length, which for ordinary white light is few micrometers; it can be much larger for light from a laser.

An example of interference between reflections is the iridescent colours seen in a soap bubble or in thin oil films on water. Applications include Fabry–Pérot interferometers, antireflection coatings, and optical filters. A quantitative analysis of these effects is based on the Fresnel equations, but with additional calculations to account for interference.

The transfer-matrix method, or the recursive Rouard method[7] can be used to solve multiple-surface problems.

## References

1. ^ Hecht (1987), p. 100.
2. ^ Hecht (1987), p. 102.
3. ^ a b Lecture notes by Bo Sernelius, main site, see especially Lecture 12.
4. ^ Hecht (2003), p. 116, eq.(4.49)-(4.50).
5. ^ Hecht (2003), p. 120, eq.(4.56).
6. ^ Hecht (2003), p. 120, eq.(4.57).
7. ^ see, e.g. O.S. Heavens, Optical Properties of Thin Films, Academic Press, 1955, chapt. 4.