3B Scientific Light Box - Manual (Page 11)

11
µ of the slab can also be found by the apparent depth method.
µ = real depth/apparent depth. (This is strictly true only when i
1
is close to 90 °)
This phenomenon is observed in day-to-day life when an underwater object appears to be closer to the
surface than it actually is.
Angle of Minimum Deviation:
Allow a single ray to strike a face of a prism and emerge from another
face. Rotate the prism until minimum deviation is reached - the position of the prism in which the
emergent ray is least deviated from the incident ray path. Mark the outline of the prism and the rays.
Denote the angle enclosed by the two sides to be A.
Then,
( )
( )
2
sin
2
sin
A
D A +
= µ
Measure
µ by the above formula and by (sin i
1
/sin r
1
) and (sin r
2
/sin i
2
). Are they the same?
Repeat the experiment by using different prisms so that different A’s are used.
• Are i
1
and r
2
equal? Are i
2
and r
1
equal (at minimum deviation)?
• Is D equal to {(i
1
+r
2
)-(i
2
+r
1
)}?
Total Internal Reflection - Semi-circular Slab: Allow a single ray to strike the semi-circular face
normally (if the ray within the slab is undeviated, then the incident ray is normal). This ray will strike
the flat face at its central point. Rotate the slab about this point until the
∠r = 90°. As ∠r approaches
90 °, it is noticed that the ray reflected from the flat face increases in intensity while the refracted ray
becomes weaker. At ∠r = 90°, the refracted ray completely disappears while the reflected ray is very
strong.
r
1
i
2
r
2
i
1
D
A
Fig. 10