Question

Make the curvature radius 0.6 , the refractive index 1.5, and the diameter 0.6 . Place the lamp so that the source of light is 120  from the middle of the lens (use the ruler).
The focal length of the lens is ____________, and the focal plane is ___________ from the lens.
     120 cm / 120 cm
     60 cm / 120 cm
     60 cm / 60 cm
     30 cm / 120 cm
     60 cm / 180 cm

For this lens, when the object is 120 cm away, the image is the same distance on the other side of the lens.

 

 

 

 

Part H

Now, move the lamp so that it is 90  from the center of the lens.
How far from the lens is the image?
     60 cm
     90 cm
     180 cm
     120 cm
     160 cm

Answer 1

(G) The object distance is equal to the image distance if the object is placed at 2 f distance. Therefore, \begin{aligned} 2 f &=120 cm \\ f &=60 cm \end{aligned} Focal plane is the plane which passing through the focal point and perpendicular to the principle axis of the lens or mirror. The distance between the center of the lens and focal plane is equal to the focal length of the lens that is 60 cm. (H) The focal length of the lens is f=60 cm The object distance is u=-90 cm The thin lens equation is, \begin{aligned} 1/f &=1/v-1/u \\ 1/v &=1/f+1/u v \\ v &=f u/f+u \\ &=(60 cm)(-90 cm)/(60 cm)+(-90 cm) \\ &=180 cm \end{aligned}