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CONVEX LENSES
LAB 23-3
CC ___
CONCEPT
A convex lens is thicker in the middle of the lens than at the edge of the lens. The principal axis of
the lens is an imaginary line perpendicular to the geometric center of the lens. It extends from
both sides of the lens. On the principal axis at some distance from the lens is the focal point F of
the lens. Light rays that strike the lens parallel to the principal axis come together or converge at
this point. The exact location of the focal point of a lens depends on both the shape of the lens
and the index of refraction of the material making up the lens. Twice the focal length from the lens
is an important position designated 2F. If the lens is symmetrical, the focal point F and point 2F
are located the same distance on either side of the lens.
An equation that relates distance to the object distance and the focal length of the lens is:
1 1 1
 
d o di f
This is called the lens equation. It relates the image distance di to the object distance do and the
focal length f. This is the same equation used for mirrors.
Note that if the object is at infinity, then: 1/ do  0 , so di  f . Thus the focal length is the image
distance for an object at infinity.
For lenses:
We use the following sign conventions:
The focal length is positive for converging lenses and negative for diverging lenses.
The object distance is positive if it is on the side of the lens from which the light is
Coming, otherwise it is negative.
The image distance is positive if it is on the opposite side of the lens from where the
light is coming; if it is on the same side, di is negative. The image distance is
positive for a real image and negative for a virtual image.
The height of the image, hi, is positive if the image is upright, and negative if the image
is inverted relative to the object. (ho is always taken as positive).
The lateral magnification, m, of a lens is defined as the ratio of the image height,
m  hi / ho . So
we have
m
hi
d
 i
ho
do
For an upright image the magnification is positive, and for an inverted image it is negative.
OBJECTIVE
Investigate the position and characteristics of images produced by convex lenses.
MATERIALS
Convex lens
Cardboard screen and support
Meter stick
Light source
PHYSICSINMOTION
PROCEDURE
*Prior to the start of this lab you must have downloaded the lens simulator and have at hand hard
copies of all the possible positions an object could be placed.
PART I – Focal Length of a Convex Lens
*To locate the focal foint of a convex lens, follow the
same procedure used for concave mirrors.
*As in the diagram below, use one
meter stick, place the screen at one
end and the lens beyond it. Aim the apparatus
through an open door,
*Adjust the lens until an object comes into focus on
the screen.
*Record the lens position and the characteristic of
the image. This location is the focal point.
PART II – Images Produced By Convex Lenses.
*Set up apparatus as shown on the right.
Place a screen, lens and object on a
meter stick that is supported by two
stands.
*The light source will be placed just behind
the meter stick.
*Keep the lens in a fixed position and
move only the object and screen.
*Set the object between f and the lens, at
f, between f and 2f, at 2f, and beyond 2f.
*Record the position of the image and
its characteristics in the data table.
DATA
POSITION
OF OBJECT
POSITION
OF IMAGE
TYPE OF IMAGE
(real or Virtual)
MAGNIFICATION
(larger/smaller)
DIRECTION OF
IMAGE
(inverted/erect)
LARGE
DISTANCE
BETWEEN
A&F
AT F
BETWEEN
F&C
DATA ANALYSIS
1. Summarize the characteristics of images formed by the convex lens.
PHYSICSINMOTION
AT C
BEYOND
C