WEBVTT
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This is College Physics Answers
with Shaun Dychko.
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We want to know how far from
the camera the film must be
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given the camera lens has
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a 35.0 millimeter focal length
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and the flower that we are taking
a photo of is
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75.0 centimeters away.
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So the flower is the object so this is
the object distance, in other words
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and we multiply by 10 millimeters for
every centimeter in order to have
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common units between our two distances
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so we'll choose to have everything
in millimeters say.
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And we know that the lens
equation says that
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1 over the object distance plus
1 over the image distance
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equals 1 over the focal length
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and it's the image distance
we are trying to find
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that's the distance where the film
should be away from the lens
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because the image needs to be
placed on the film.
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Okay!
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So we have 1 over object distance
subtracted from both sides here
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and that gives us 1 over image
distance on the left
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and then raise both sides to
the exponent negative 1
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and on the left side, this gives
us image distance
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and that equals
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1 over f minus 1 over d o all to
the power of negative 1.
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So that's 1 over 35.0 millimeters—
focal length—
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minus 1 over 750 millimeters—
object distance—
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and take the reciprocal of that difference
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and you get 36.7 millimeters
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is the distance from the lens to the film.