Why Was My Calculation of a -4.2 Diopter Lens's Focal Length Rejected?

[sp1974]

The problem reads "What is the focal length (in meters) of a -4.2 diopter lens"



The formula equation I have is = 1/f to get the diopter power



-4.2 = 1/f and solve for f I get -.2381

This is a homework problem where the solutions are entered on line. I entered the solution above and it didn't accept. Any hints/help?

 

[Doc Al]

Looks right to me. But online systems are often fussy about things like significant figures: Try rounding it off to 2 sig figs. (Some systems insist on 3 sig figs.)

 

[sp1974]

Ok thanks. I know threwout the two semisters rounding has never been a factor but will try. Worst case if I use all my attempts I show teacher and get the 1 point for the problem. Thanks!