SNELL’S LAW (RELATING ANGLE
OF INCIDENCE AND ANGLE OF REFRACTION)
Let
us perform an activity to understand this concept.
Aim:
Obtaining a relation between angle of incidence and angle of
refraction.
Materials
required: A plank, white chart, protractor, scale, small black
painted plank, a
semi circular glass disc of thickness nearly 2cm, pencil
and laser light.
Procedure
Take a wooden
plank which is covered with white chart. Draw two perpendicular lines, passing
through the middle of the paper as shown in the figure. Let the point of
intersection be O. Mark one line as NN which is normal to the another line
marked as
MM. Here MM represents the line drawn along the interface of two media and NN represents the normal drawn to this line at ‘O’.
Take a protractor
and place it along NN in such
way that its
centre coincides with O. Then mark the angles from 00 to 900 on
both sides of the
line NN. Repeat the same on the other side of the line NN.
The angles should be indicated on the curved line.
Now place a
semi-circular glass disc so that its diameter coincides with the interface line
(MM) and
its centre
coincides with the point O. Point a laser light along NN in such a way that the
light propagates from air to glass through the interface at point O and observe
the path of laser light coming from other side of disc. (If you cannot observe
the path of laser light put a black-coloured plank against the curved line and
observe the light point and imagine the light path).
Is there any
deviation?
Send Laser light
along a line which makes150 (angle of incidence)
with NN and see
that it passes through point O. Measure its corresponding
angle of
refraction, by observing laser light coming from the other side
(Circular side) of
the glass slab. Note these values in table . Do the
same for the
angles of incidence such as 200,300,400,500
and 600 and note
the corresponding angles of
refraction.
Find sin i, sin r
for every ‘i’ and ‘r’ and evaluate sin i/ sin r for every
incident angle ‘i’. Finally, we will get the
ratio sin i / sin r as a constant.
This ratio gives the refractive index of
circular glass slab we used in the above activity.
Therefore according to Snell’s law
µ=sin
i/sin r
if
you consider two media with different refractive indices then here ‘µ’ is
nothing but refractive index of second medium with respect to first medium.
µ2/µ1=sin
i/sin r
ð µ1 sin(i)=µ2
sin(r).
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