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Fresnel Equations

Definition: equations for the amplitude coefficients of transmission and reflection at the interface between two transparent homogeneous media

Category: general optics

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Fresnel equations specify the amplitude coefficients for transmission and reflection at the interface between two transparent homogeneous media:

refraction at an interface
Figure 1: Refraction at an interface between two media.
Fresnel t_s
Fresnel r_s
Fresnel t_p
Fresnel r_p

For example, ts is the amplitude transmission coefficient for s polarization; the transmitted amplitude is that factor times the incident amplitude in that case (disregarding any phase changes for transmission in the media). n1 and n2 are the refractive indices of the two media. The corresponding propagation angles (measured against the normal direction) are θ1 and θ2 (see Figure 1).

For example, the amplitude transmission coefficient is ts for s polarization, i.e., if the electric field vector is perpendicular to the plane of incidence.

The power reflection coefficients are obtained simply by taking the modulus squared of the corresponding amplitude coefficients. For the transmission, one must add a factor (n2 cos θ2) / (n1 cos θ1) in order to take into account the different propagation angles.

Calculator for Fresnel Equations

Refractive index of medium 1:
Refractive index of medium 2:
Angle of incidence: calc
Output angle: calc (must be calculated before calculating the values below!)
Power transmissivity, s pol.: calc
Power reflectivity, s pol.: calc
Power transmissivity, p pol.: calc
Power reflectivity, p pol.: calc
R_s / R_p: calc
T_s / T_p: calc

Enter input values with units, where appropriate. After you have modified some values, click a "calc" button to recalculate the field left of it.

The calculations cannot be done in the regime of total internal reflection.

Fresnel reflectivity
Figure 2: Power reflectivity of the interface for s and p polarization, if a beam is incident from air onto a medium with refractive index 1.47 (e.g., silica at 1064 nm).

Figure 2 shows in an example case how the reflectivity of the interface depends on the angle of incidence and the polarization. The reflection coefficient vanishes for p polarization if the angle of incidence is Brewster's angle (here: ≈55.4°).

For the simplest case with normal incidence on the interface, the power reflectivity (which is the modulus squared of the amplitude reflectivity) can be calculated with the following equation:

Fresnel reflectivity

Application to Absorbing Media

Fresnel equations can also be applied to absorbing media, including metals. In that case, a complex refractive index needs to be used; the imaginary part is related to the absorbance. The reflection and transmission coefficients are then generally no more real numbers.

A difficulty occurs concerning the calculation of power transmission in cases with non-normal incidence. The above mentioned correction factor for taking into account the different propagation angles will be complex, but the power transmission factor would of course have to be a real number. The problem is related to the fact that the optical intensity is no longer constant along the wavefronts.

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See also: Fresnel reflections, reflectivity, transmissivity, refraction, total internal reflection, Brewster's angle, refractive index
and other articles in the category general optics

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