75.  A nearsighted person cannot read a sign that is more than 5.2 m from his eyes. To deal with this problem, he wears contact lenses that do not correct his vision completely, but do allow him to read signs located up to distances of 12.0 m from his eyes. What is the focal length of the contacts?

 

So to read distances up to 12.0 m away, means the object distance he needs is 12.0 m.  Since he can see without lenses up to 5.2 m that makes the image distance created by an object has to be 5.2 m.  But we need a virtual image that will not actually form in space and will be upright, so we must use -5.2 m as our image distance.  So now we can use the lens equation and find the focal length.

 

 

 

 

 

Please send any comments or questions about this page to ddonovan@nmu.edu

This page last updated on July 7, 2018