# Angle Of Deviation - An Optics Problem, help needed in optics

1. Apr 5, 2009

### omega360

hello,
how can i find out the angle of deviation for plano-concave lenses.
thanks

2. Apr 5, 2009

### Staff: Mentor

Could you please provide us some background on plano-concave lenses, and how rays are traced through them? What do you think the relevant equations are for the "angle of deviation?"

Is this homework or coursework?

3. Apr 5, 2009

### Staff: Mentor

If you send (for example) a beam of parallel light rays into a lens (any lens, not just plano-concave ones), they come out at various angles, different angles for different rays. This is what makes lenses useful: they change the convergence or divervence of a whole collection of light rays.

So, you need to be more specific. Which ray do you want the angle of deviation for?

4. Apr 5, 2009

### omega360

i am working on a project related to illumination and i need to diverge rays of the sun using plano concave lens. so, i need to find out how the angle of deviation relates to the curvature of the lens and the material of the lens

please help me

5. Apr 5, 2009

### Andy Resnick

That's an odd application, but in any case, the calculation is the same for plano-convex and plano-concave. The cone angle of light 'q' (given by the numerical aperture NA = sin(q) for a lens in air) is the ratio of focal length 'f' to lens diameter 'D': NA = 2*D/f. So sin(q) = 2*D/f, and the cone angle the inverse sine of 2*D/f. The focal length is calculated from the radius of curvature of the curved face.

Just remember to keep track of the sign.

This calculation neglects aberrations, so the answer is not exact.

6. Apr 5, 2009

### omega360

Still didn't get how i could derive how much angle light would deviate after it passes through the lens. also what if the rays are parallel to the principal axis. what would be angle 'q'??

also could i know how light intensity is affected by it passing through the lens.

Last edited: Apr 6, 2009
7. Apr 6, 2009

### Andy Resnick

The deviation of a particular ray through a lens with in general depend on the ray height when it hits the lens. A ray will propogate undeviated if it passes through the center of the lens, for example.

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