Inverse Square Law problems
I1 ? d 2 ?
=? ?
I 2 ? d1 ?
2
Draw an illustration for each problem and show your work.
1. What is the intensity of the x-ray beam at 72-in if the original x-ray intensity was
360mR at 36-in?
2. An exposure at 40 inches has an intensity of 160mR. What distance would be
needed to reduce the intensity to 100 mR?
3. If the intensity of the x-ray beam is 400mR at 3 feet, how far would you have to
move to reduce the intensity to /f the original?
4.
An exposure at 42 inches is 100mR. What distance should be used to reduce the
intensity to 50 mR?
5.
The intensity at 50 inches is 75mR. What is the intensity at 40 inches?
6.
The intensity at 30 inches is 50 mR. What is the intensity at 72 inches?
The Inverse Square Law compares distance
with the intensity of the x-ray beam.
? Inverse relationship
? As the distance increases, the intensity
decreases
? As the distance decreases, the intensity
increases
??1
=
??2
??2
??1
2
Letàtry a few problems
If the exposure at 35 inches is 50mR,
what is the exposure at 45 inches?
??1
??2 2
?
=
??2
??1
50????
45 2
?
=
??
35
? 61,250=2,025X
? 30mR = X
If the exposure is 100mR at 40 inches,
how much would the distance need to be
to obtain an exposure of 75mR?
??1
?
=
??2
100
?
=
75
??2 2
??1
?? 2
40
? 160,000=75X2
? X2 = 2,133
?X=
2,133 = 46 inches

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