QUESTION 6 In a film projection apparatus, it is desired to produce pictures 12

Question

QUESTION 6

In a film projection apparatus, it is desired to produce pictures 12 feet wide on a screen 50 feet from the lens; the size of the picture on the film is one inch wide. What must be the focal length of the projecting lens used?

      
A) 144 inches

      
B) 50 inches

      
C) 25 inches

      
D) 4.138 inches

QUESTION 7

Why is starlight such a good light source for measuring the focal length of a lens?

      
A) Because it is a bright source

      
B) Because it is far away

      
C) Because it produces white light

      
D) It is not a good source

Explanation / Answer

given height of image = 12feet

height of object = 0.0833 ft

12/0.0833 = -50/u

u = -0.347 ft

we know that

1/v+1/u = 1/f

v = 50ft

f = 0.349 ft

or

f = 4.138 inches

7)

B) Because it is far away

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