The Thin Lens Equation
Examples using lenses
A Converging/Convex Lens
• In convex lenses,
▫ f is positive
▫ do is always positive
▫ di can be positive or negative
A Diverging/Concave Lens
• In concave lenses,
▫ f is negative
▫ do is always positive
▫ di is negative
The Equation
1
1
1


f do di

Sample Problem
• Calculate di
Sample Problem
f = 17 cm
do = 48 cm
di = ?
1
1 1


f do di
1
1 1


17 48 di
1
1
1


di 17 48
1
48
17


di 816 816
1
31

di 816
31di  816
di  26.3cm
Calculating Percent Error
You can check the accuracy of your drawings by comparing your measurements
to your calculations.
•Theoretical is the image distance you calculated using the thin lens equation
•Experimental is the image distance you measured on your scale ray diagram
theoretical  exp erimental 
%error  
x100%


theoretical
•If you get a negative percent – just ignore the negative sign.
The Thin Lens Equation
• The equation:
1
1 1


f do di
• To use the thin lens equation, you need to follow this
sign convention:
• Object distances (do) are always positive
• Image distance (di):
 are positive for real image (image is opposite side of lens
as object)
 Are negative for virtual images (when the image is on the
same side of the lens as object)
• The focal length (f) is positive for converging lenses and
negative for diverging lenses.
Practice
• Complete Questions 4-11 on the Lenses Problem
Set.