2
$\begingroup$

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?

$\endgroup$
2
  • $\begingroup$ search for "camera lens reflection" and you will see that there is reflection. $\endgroup$
    – anna v
    Commented Feb 19 at 5:46
  • $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Commented Feb 19 at 6:06

1 Answer 1

3
$\begingroup$

The ratio of power reflected from an interface to the power incident is called reflectance. For a smooth (as in not rough) interface, it can be calculated using the Fresnel equations. For a ray normally incident on a lens surface from air (or vacuum), it is given by $$ R_\text{normal} = \left(\frac{n - 1}{n + 1}\right)^2$$ where $n$ is the refractive index of the material the lens is made of. For a glass lens with $n = 1.5$, $R_\text{normal} = 4\%$.

These reflections result in an image that is dimmer than it would be without reflections, and also produce dim "stray" images at various locations in an optical system. Why aren't they considered? Probably because in an introductory class, the teacher already has their hands full trying to teach students how an image is formed, without getting into into reflections from lens surfaces. Perhaps this is no great crime, because a relatively small fraction of power is reflected at each interface, and stray reflections are often dim enough not to matter for many applications.

Reflections are often considered in practice. Transmission losses can be significant especially in optical systems consisting of many lenses. If the image brightness is important, attempts are made to reduce transmission losses using anti-reflective coatings for instance. Undesired internal reflections can also cause problems like lens flare.

$\endgroup$
1
  • 1
    $\begingroup$ Thank you for your answer; just wanted to make sure I was not missing anything; I'll study the given links further $\endgroup$
    – DisD
    Commented Feb 19 at 7:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.