Evans SPH 3U1

Physics Grade 11

Unit 3: Light and Optics

Note 9: Calculations with Lenses

Reference: Chapter 11.9

Thin lens Formula:

Magnification,m:

where:

d_o is the distance between the object and the lens

d_i is the distance between the image and the lens

f is the focal length

h_i is the height of the image

h_o is the height of the object

Sign Conventions:

h_o    height of object must be positive

d_o  distance to object must be positive

f     positive for convex lens, negative for concave

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

d_i     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

Quiz