Thin lens problem

1. Feb 7, 2005

nemzy

A transparent photographic slide is placed in front of a converging lens with a focal length of 2.24 cm. The lens forms an image of the slide 11.4 cm from the slide.
(a) How far is the lens from the slide if the image is real?

(b) How far is the lens from the slide if the image is virtual?

I got the wrong answer, and i dont know why.

This is how i attempted part a:

a) focus=2.24 cm
q= 11.4 cm (positive, since the image is real)
1/p + 1/q = 1/f solve for p

b) i did same process except p = -11.4 cm (negative since virtual image)

But yet still got the wrong answer

What am i doing wrong? thanks

2. Feb 8, 2005

ehild

q is not 11.4 cm. q should be the distance of the slide from the lens, and 11.4 cm is said to be the distance between the slide and its image.
ehild