# Geometric optics- lenses

I was wondering why would light rays converge at a biconvex lens and diverge at a biconcave lens? Is it concerning on how light rays behave in a convex or a concave mirror?

Also, is it possible to predict or sketch how would light rays behave using the angle of incidence and angle of refraction?

• Try googling by the phrase "lens as composed of prisms", and u might get a good reason. – Wrichik Basu May 14 '17 at 13:35
• I suspect you meant biconvex and biconcave. If you truly did mean "bio" rather than "bi" as a prefix, this question is perhaps better suited for the biology SE site. – David Hammen May 14 '17 at 14:11
• Sorry. It was indeed "bi" sorry for the typo. – Czar Luc May 14 '17 at 14:20

Here is what I think,

At P1, By Snells law, Since the glass is a denser medium(as compared to air), the velocity of light is less than that in air. Therefore, the light ray deflects towards the normal.

Afterwards, at P2, Since the air is a rarer medium than glass, By Snell's Law, light gets deflected away from the normal.

In the same way you can explain that of a biconcave lens.

Hope this helps :)

• How did you know how the normal lines are oriented at each point? I actually thought about your answer but I couldn't figure out how to orient the normal lines at each point. – Czar Luc May 14 '17 at 15:36
• The surfaces of most lenses are sections of spheres. It's possible to prove that you get better focusing if the surfaces are instead paraboloids. – rob May 14 '17 at 16:01
• The normals are always taken perpendicular to the surface( at that point) . – Physicpsycho May 14 '17 at 17:03

You can show it using Snell's law of refraction, or the least time principle. However, this is not similar to convex or concave mirrors.