# What is the relation between Reflectance and Intensity?

I am just reviewing Fresnel's equations where Reflectance $$R$$ is equal to

$$R=|r|^2$$

where $$r$$ is the complex reflection coefficient. Is there an equation that relates $$R$$ to intensity or is there a reason one is related to the other?

• Until you get to very high intensities, reflectance is independent of intensity. How high is "very high"? For conventional metal-coated mirrors, "very high" means intensities requiring a pulsed laser in a lab. – garyp Mar 18 at 21:18
• Thanks for the response. I'm looking into reflectance spectroscopy and frequently they will plot intensity as a function of wavelength in place of reflectance. I'm just confused on how one just swaps between one and the other in these situations (so situations where wattage is ~5-20mW). – MathHelper123 Mar 18 at 21:32
• The reflected intensity $I_R$ is related to the intensity of the incident light $I_0$ by $I_R=RI_0$ so $I_R\propto R$. A graph of $I_R$ against wavelength will look the same as a graph of $R$ against wavelength. Is this what you meant? – John Rennie Mar 19 at 6:08
• The reflection r is complex defined through the ratio of the incoming to the The reflected field. So in terms of intensity or irradiance <S>cos(a) is the squared modulus, s pointing vector. – user591849 Mar 19 at 9:12