Thin Lens Equation - Practice Problems

This image shows a converging lens. The distance between the center of the lens and the focal point is 10 cm. Therefore, this lens has a focal length of ____________.

• + 10 cm

• - 10 cm

• + 20 cm

• - 20 cm

This image shows a diverging lens. The distance between the center of the lens and the focal point is 10 cm. Therefore, this lens has a focal length of ____________.

• + 10 cm

• - 10 cm

• + 20 cm

• - 20 cm

A diverging lens has a focal length of -10 cm.  If an object is held 40 cm away from the lens, the object distance will be _______. 
 $Hint:\ \ \ \ \ f_{_{\ }}=\ -10\ cm\ \ \ \ \ \ \ \ \ \ d_o\ =\ 40\ cm$  

   $\frac{1}{f}=\frac{1}{d_o}+\frac{1}{d_i}$  

• - 8 cm

• + 8 cm

• 30 cm

• - 50 cm

Suppose an object is held 60 cm away from a converging lens. As a result, an image is produced 30 cm on the other side of the lens. From this data, the focal length of the lens can be calculated to be _______. 
 $Hint:\ \ \ \ \ d_{o_{\ }}=\ 60\ cm\ \ \ \ \ \ \ \ \ \ d_i\ =\ 30\ cm$  

   $\frac{1}{f}=\frac{1}{d_o}+\frac{1}{d_i}$  

• 60 cm

• 90 cm

• 20 cm

A converging lens with a focal length of +20 cm is used to create an image of an object that is held 70 cm away from the lens.  From this data, the image distance can be calculated to be _______. 
 $Hint:\ \ \ \ \ f_{\ }=\ 20\ cm\ \ \ \ \ \ \ \ \ \ d_o\ =\ 70\ cm$  

   $\frac{1}{f}=\frac{1}{d_o}+\frac{1}{d_i}$  

• 16 cm

• 28 cm

• - 50 cm

• 42 cm