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2.
M18/4/PHYSI/HP3/ENG/TZ2/XX
A student carries out an experiment to determine the variation of intensity of the light with
distance from a point light source. The light source is at the centre of a transparent spherical
cover of radius C. The student measures the distance x from the surface of the cover to a
sensor that measures the intensity I of the light.
x
C
cover
light sensor
point light source
The light source emits radiation with a constant power P and all of this radiation is
transmitted through the cover. The relationship between I and x is given by
I=
(a)
P
4π (C + x )2
This relationship can also be written as follows.
1
I
Show that K = 2
π
P
= K x + KC
[1]
.
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40EP05
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M18/4/PHYSI/HP3/ENG/TZ2/XX
(Question 2 continued)
(b)
The student obtains a set of data and uses this to plot a graph of the variation of
1
with x.
I
45
40
35
30
25
1
−1
/W 2m
I
20
15
10
5
0
–5
(i)
0
0
5
5
10
10
x / cm
15
15
20
20
25
25
Estimate C.
[2]
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40EP06
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M18/4/PHYSI/HP3/ENG/TZ2/XX
(Question 2 continued)
(ii)
Determine P, to the correct number of significant figures including its unit.
[4]
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(c)
1
Explain the disadvantage that a graph of I versus 2 has for the analysis in
x
(b)(i) and (b)(ii).
[2]
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40EP07