How Does a Boy Scout Use Eyeglasses to Start a Fire?

[floridianfisher]

Homework Statement

A boy scout starts a fire by using a lens from his eyeglasses to focus sunlight on kindling 5.2 cm from the lens. The boy scout has a near point of 15.2 cm. When the lens is used as a simple magnifier, give values for the following conditions. (Hint: Assume a "normal" eye.)
(a) What is the maximum magnification that can be achieved

(b) What is the magnification when the eye is relaxed

Homework Equations

1/p + 1/q = 1/f
Mmax= 1+ 25cm/f

The Attempt at a Solution

I used p=+5.2 and q= -(15.2)-((1/2)5.2)=-17.8
then 1/5.2 - 1/17.8 +1/f
f= 7.34
so, Mmax= 1+ 25/7.34= 4.406 but that is wrong?!?
can anyone help?

 

[chaoseverlasting]

You have to put everything in meters... you're using cm... check your units...

 

[Doc Al]

Homework Equations

1/p + 1/q = 1/f
Mmax= 1+ 25cm/f

That last equation assumes a near point of 25cm, but you are given a different near point to use.

(No problem using cm everywhere, as long as you are consistent.)

 

[floridianfisher]

So i used the same equation except for Mmax I used 1+ 15.2/7.346= 3.068 but still the answere is not right?

 

[floridianfisher]

Please help soon as I do not have much time left to solve this problem.
Thanks

 

[Doc Al]

So i used the same equation except for Mmax I used 1+ 15.2/7.346= 3.068 but still the answere is not right?

What's the focal length of the lens?

A boy scout starts a fire by using a lens from his eyeglasses to focus sunlight on kindling 5.2 cm from the lens.