[LargeFormat] will a darkslide skim across a pond from the s

[Michael Briggs]

On 02-Dec-01 Dave Southwood wrote:
> 1)i have a basic knowledge of the physics which govern the passage of light
> through air, coatings and then into glass. what i would like to know, please,
> is what formula determines the maximum angle of incidence which light must
> have to the surface of the lens before it reflects, and does not penetrate
> and refract?

For a light ray traveling from air to glass, the only angle at which no light
enters the glass is 90 degrees--perfectly parallel to the surface.  Of
course, close to 90 degrees the amount reflected is high and the amount
refracted is low.   (The angle of incidence is measured from the normal or
perpendicular to the surface.)

The case of light traveling from glass to air is quite different.  Past
Brewsters angle total internal refraction occurs and no light is refracted. 
This explains the reflection you can see through prisms at certain angles.
Brewsters angle i (angle of incidence) is given by tan i = n (index of
refraction).

The laws for the fraction of light reflected and refracted, as a function of
polarization, are called Fresnel's laws of reflection.  These are described in
Applied Photographic Optics by Sidney Ray.   They are derived from Maxwell's
Equations in any physics textbook on electrodynamics.   Ray also has an
excellent chapter on optical coatings.
 
> i know that the curvature of the lens affects things. how is this accommodated
> in the calculation of the aforementioned angle?

On the scale of the wavelength of light, all lenses are flat--just like the
earth appears flat (neglecting local features) on the scale of a person.  So you
measure angles from the normal to the surface of the lens at the point that the
light ray intersects the surface of the lens.

--Michael