A slide projector is used to project an image of a waterfall from a slide onto a

Question

A slide projector is used to project an image of a waterfall from a slide onto a screen which is 7.20 m from the slide. The slide is 3.70 cm from the center of the lens. Show a sketch

(a) What is the focal length of the lens being used? ANS: +3.68 cm

(b) Could you make out the image of the waterfall if you stood in front of the screen and looked directly into the incoming light from the projector? Why or why not? ANS: No – it’s a real image, b/c?

(c) How much larger than the slide is the image of the slide on the screen? ANS:194

(d) Is the image upright or inverted? ANS: inverted

e) Why are slides inserted upside-down into the slide projectors?

Explanation / Answer

given data

   object distance s = 3.70 cm = 0.037 m, and
   image distane s'= 7.20 m

a) focal length?

   from the relation 1/f = 1/s + 1/s'

           1/f = 1/0.037+ 1/7.2
           f = 0.0368 m = 3.68 cm

b)
if you stood in front of the screen and looked directly into the incoming light from the projector,
image of the water fall will not be seen because rays of your face or head will fall on the screen so giving your head image.
           OR

   if you stood below the focal length the image will not be appear

c) from the lateral magnification formula m= - s'/s = - 7.2/0.037= -194

      
   so 194 times larger than the slide is the image of the slide on the screen

d)   Formed image is inverted because the magnification is negative so image is inverted.

e)   because of inverted images will form on screen so the slides will be inserted upside-down into the slide projectors.

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