In a cinema, a picture 2.5 cm wide on the film is projected to an image 3.0 m wide on a screen which is 18 m away.The focal length of the lens is about:

Question

Quizplus · Accepted Answer

We can use the thin lens formula:

1/f = 1/o + 1/i

where f is the focal length of the lens, o is the object distance, and i is the image distance.

First, we need to convert the meters to centimeters:

3.0 m = 300 cm 
18 m = 1800 cm

Next, we can use similar triangles to find the object distance, o:

o / 2.5 cm = 1800 cm / 300 cm

o = 15 cm

Now we can plug in the values we have:

1/f = 1/o + 1/i

1/f = 1/15 cm + 1/300 cm

1/f = 21/300 cm

f ≈ 14.3 cm

The closest answer choice to this value is D) 15 cm.

In a Cinema, a Picture 2