Introduction
We know that a convex lens converge a parallel beam of light to its focus “f”. At focus the
intensity is maximum. If we reflect the beam backward after passing through the focusing lens,
we will get maximum reflected intensity when the reflecting object is at the focus of the lens.
This is the property of the system which we utilize in this experiment. We detect object’s range
from the intensity of the reflected light. If the reflecting intensity is registered maximum at the
detector, then at that moment the object will be at the focus point of the lens system. The object
can be any solid. But for different materials, the intensity peak is different.
Procedure
I took a He-Ne laser and switched it on. Then I aligned it. I took a lens that focused the beam at
its focus. I placed a beam splitter at the focus in such a position that it divided the intensity into
half, half of which passed straight and half is deviated away from the detector. This is necessary
because the laser intensity is large and the range detection experiment will be not very accurate
with large intensities because the detector will become saturated for all reflected intensities. Thus
no range detection would be possible. Then the half straight going beam is collimated by double
lens system. Then this collimated beam is focused by another lens to its focal point “f”. This “f”
is used in range specific detection of the objects. In our experiment f=16 cm. thus the reflected
intensity will be maximum at this position. I got the following setup.
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