Why Are Reflected Rays Often Ignored in Lens Studies?

[E3Phy]

Hi,
I am wondering why reflected rays are not considered with lenses? If a Ray strikes a surface another is reflected off that striking point; however, this is not added when studying lenses, only refracted rays are considered.
Is there any law which gives the ratio of reflected and refracted rays with lenses?

Cheers.

 

[Ibix]

The reflection is very dim, so isn't really useful and you usually ignore them. If you get into optical engineering you can certainly consider reflections. You generally want to make sure they're absorbed somewhere in the system because they typically annoy the user by giving ghost images - google "lens flare" for the most common example of these.

 

[DaveE]

I am wondering why reflected rays are not considered with lenses?

Maybe because they aren't always pertinent to the person doing the analysis. If there's a reason to consider them, they can be analyzed.

Is there any law which gives the ratio of reflected and refracted rays

Yes. Absolutely. Although it can be quite a chore. Normally you would minimize reflection in a conventional lens, or maximize it for mirror lenses (like most modern telescopes). The other rays are, hopefully, insignificant and don't require detailed analysis. However, there are optical systems where it does matter.

This is all based on the basic classical E&M concept of reflection and transmission coefficients.

https://en.wikipedia.org/wiki/Fresnel_equations

 

[E3Phy]

Thank you all for your answers; just wanted to make sure I was not missing anything; I'll study the given links further

 

[Gleb1964]

why reflected rays are not considered with lenses?

A good optical design would considered a parasite reflections and a scattered light. Most optical soft are intended for optimizing the main light flow passing the optical system, but there are special modes to study secondary reflections and scattering. However it is more difficult level as demanding to have a good models of the coatings and baffles. Good design should at least consider that the parasite reflection should not focus close to the image plane to avoid host images.
The most difficult is the management of the scattered light which demands exact model of lenses, mechanics, baffles, coatings on optical and mechanical surfaces in combination with a different combination of the objects in the object space.

 

[sophiecentaur]

Thank you all for your answers; just wanted to make sure I was not missing anything; I'll study the given links further

Those reflections are always there and they're the bane of the designer's life. The reflections are from the surfaces (and there may be six in a three element , high quality lens. The surfaces of the lens elements are not just spherical and the secondary images occur all over the place.