The lens formula relating the image distance, p , to the focal length, f , and object distance, s , is
The magnification of a lens is
For the projector in the previous problem, the image is ____________ times as large as the object.
A) 0.001
B) 1.01
C) 50
D) 100
E) 101

Question

The lens formula relating the image distance, p , to the focal length, f , and object distance, s , is 
The magnification of a lens is 
For the projector in the previous problem, the image is ____________ times as large as the object.
A) 0.001 
B) 1.01 
C) 50 
D) 100 
E) 101

Quizplus · Accepted Answer

Given the lens formula: $\frac{1}{f}=\frac{1}{s}+\frac{1}{p}$
Applying the values given, we get: $\frac{1}{f}=\frac{1}{80}+\frac{1}{180}=\frac{7}{720}$
Solving for $f$, we get: $f=103$ mm

Also, given magnification of lens, $m=\frac{p}{s}=\frac{d_i}{d_o}$
Where, $d_i$ is the image distance and $d_o$ is the object distance

Solving for $d_i$, we get: $d_i=md_o=\frac{1}{11}d_o+d_o$
Given that $d_o=20mm$, we get: $d_i=\frac{21}{11}	imes20mm=381/11\ mm$

The image size is the ratio of image distance to object distance: $\frac{d_i}{d_o}=\frac{381}{11	imes20}=\frac{381}{220}\approx\boxed{1.73\approx100\%}$ (approximately) or option D.

The Lens Formula Relating the Image Distance, P , to the Focal