Homework Help: Need to clear confusion on a simple lense problem (solution is worked out )

1. Apr 24, 2010

Need to clear confusion on a simple lense problem (solution is worked out :D)

1. The problem statement, all variables and given/known data

The following thin lenses are made of glass
of index of refraction 1.7 and are in air with
index of refraction 1.
Find the focal length of a lens with r1 =
22 cm and r2 = 8 cm.

2. Relevant equations

1/f = (n-1)[(1/R1)-(1/R2)]

3. The attempt at a solution

n of glass = 1.7
n of air = 1

R1 =22 cm

R2 = 8 cm

So, I plug it in the above equation

1/f = (1.7 - 1) (1/22 - 1/8)

f = -17.95918367 cm

I need to clear up few concepts.

1) Is f negative because this lens in Diverging (is it diverging? I was expecting the question to mention it! :D)

2) is my working correct? My answer should be negative? Its kind of wierd! :)

3) How do I figure out from the question as to which side is convex and which side is concave?

Thanks! :)

2. Apr 24, 2010

Stonebridge

Re: Need to clear confusion on a simple lense problem (solution is worked out :D)

The sign convention is:
R1 is the radius of curvature of the lens surface closest to the light source,
R2 is the radius of curvature of the lens surface farthest from the light source
R1 is positive if the first surface is convex, and if R1 is negative the first surface is concave. The signs are reversed for the back surface of the lens: if R2 is positive the back surface is concave, and if R2 is negative the back surface is convex.
The focal length f is positive for converging lenses, and negative for diverging lenses.