To show that the intensity of the radiation on the surface is inversely proportional to the square distance of the suface from the radiation source.

1. Experiment # 6
(Inverse Square Law)
Objective:
To show that the intensity of the radiation on the surface is inversely proportional to the square
distance of the suface from the radiation source.
Apparatus:
Fig6.1 (Thermal radiation heat exchanger unit)
Procedure:
 Initial position: Distance from heat source(X) = 80mm.
 The power control was set to widen the position and the heaterwas allowed
approximately 5 minutes to reach a stable temperature prior to starting yhe experiment.
 The radiometer reading(R) and the distance from the heat source (X) were recorded for a
number of positions of the radiometer along the horizontal track.
 The radiometer was allowed approximately 2 minutes to stabilize after being moved to
each new position.
 Repeat the experiment for 700, 600, 500, 400, or 300.
 The logaritm values (log10) of the data taken were calculated.
 A log-log plot of radiation reading against distance was generated.
Specifications of apparatus:
a) Light source:
Consist of bulb rated at 60 Watt, 24 VDC to produce a white light using an
acrylic filter.
b) Heated surface:

2. Consist of a heating A1 plate rated at 216 Watt, 24VDC, Matt Black
finish.
c) Target plate:
Consist of three polished, Gray and Matt Black with bolt on temperature
sensor.
d) Aperture:
Consist of two plates with insulation onone side of the surface. The
insulated surface will be facing the heated surface during experiment.
e) Plastic Filter:
Consist of three type of density known as clear, medium and dark plate.
f) Radiometer:
Range: 0-2000 W/𝑚2
.
g) Light meter:
Range: 0-3333 Lux.
Overall dimensions:
Height: 550mm.
Width: 320mm
Depth: 1250mm.
General Requirements:
The equipment should be installed on a firm level work surface. The location should be
remote from the heaters or darkened room for light experiments. If a darkened room is
not avaialbe the apparatus should be located in subducd lighting away from direct
sunlight.
Electrical:
240 VAC, 1-phase, 50/60HZ.

3. Observation:
Distance (X)
(mm)
Radiometer Reading
(W/𝒎 𝟐
)
ln (x)
(mm)
ln (R)
(W/𝒎 𝟐
)
800 58.4 6.684 4.067
700 76.7 6.551 4.339
600 105.5 6.396 4.658
155.5 6.214 5.046
250.2 5.991 5.522
473.5 5.703 6.160
Graph:
Result:
The reding obtained, a graph of ln ( R ) against ln (X) is in a straight line that having a negative
slope of 1.6196 which is approximately -2, therefore, verifying the inverse square relationship
between distance and radiation intensity that satisfy the equation.
Intensity ∝
1
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒2
The inverse squre law sating that the intensity of the radiation at a location is inversely
proportional to thee squre of its distance from the source of radiation. It is prove that the intensity
of radiation on a surface is inversely proportional to the square of the distance of the surface
from the radiation source according to the experimental results and graph.