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6_Ch 25 College Physics ProblemCH25 Optical Instruments

# 6_Ch 25 College Physics ProblemCH25 Optical Instruments -...

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342 CHAPTER 25 25.5 The exposure time is being reduced by a factor of 2 1 1256 s 1 132 s 8 t t = = Thus, to maintain correct exposure, the intensity of the light reaching the film should be increased by a factor of 8. This is done by increasing the area of the aperture by a factor of 8, so in terms of the diameter, ( ) 22 21 48 4 DD π = or 8 = . The new f -number will be () ( ) 1 2 2 1 -number 4.0 -number 1.4 88 8 f ff f D D == = or 1.4 f 25.6 (a) The intensity is a measure of the rate at which energy is received by the film per unit area of the image , or 1 image IA and x . Consider an object with horizontal and vertical dimensions y hh as shown at the right. If the vertical dimension intercepts angle θ , the vertical dimension of the image is y hq ′ = , or y ′ ∝ . Similarly for the horizontal dimension, x ′ ∝ , and the area of the image is 2 image x y A ′′ = q . Assuming a very distant object,
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