Evans SPH 3U1

Physics Grade 11

Unit 3: Light and Optics

Note 9: Calculations with Lenses


Reference: Chapter 11.9


Thin lens Formula:





do is the distance between the object and the lens

di is the distance between the image and the lens

f is the focal length

hi is the height of the image

ho is the height of the object


Sign Conventions:

ho    height of object must be positive

ddistance to object must be positive

f     positive for convex lens, negative for concave

hi    positive if image is upright, negative if image is inverted

di     negative if image is on same side (virtual), positive if image is on opposite side (real)


Example 1: a converging lens is being used as a magnifying glass. If the focal length of the lens is 15cm and an ant that is 10cm away is being viewed, find the location of the image and its magnification.

(you'll find di is negative 30cm which means the image is on the same side of the lens as the object. It is a virtual image. The image is larger than the object and it is right side up)


Pg. 395 #'s 70, 73, 74, 76