PHYSICS

its bathing medium), there will be very little difference in amplitude
between the light which passes though the medium and that
which passes through the specimen. There will, however, be a
phase difference introduced because of the refractive index difference
between the medium and specimen. Because the eye cannot
detect differences in phase, the purpose of the interference microscope
is to convert the phase difference into an amplitude difference.
One way of doing this is to use round-the-square interference
(fig. 21.9).
Comparison slide
M] I () t () ~S1
Light sourse x { , ]I ~ y
SI
~ Eye
Fig. 21.9. The interference microscope (see text for explanation)
o B 0 c
I
Fig. 21. 10. Vector representation of background and transmitted light:
a ­ the vector DB represents the background light and OT the light
transmitted through the object; b ­ DC represents the amplitude and
phase of the light which has passed through the comparison slide; c ­
the phase of DC has been adjusted so that it is exactly out of phase
with light transmitted through the object OT; d - the background
does not appear dark as the background vector DB and comparison
vector DC are not 180 0 out of phase
148
~ c~~~~_ ~ R~.
T 0 B 0 0 B
a b c d u
/,
'1;
I
I
b
microscope
Lb
Fig. 21.12. Mechanism of diffraction of light by a narrow slit
of width a: a ­ condition of maximum; b ­ condition of minimum
149
a
The observed image arises
from the sum of two light
beams. One passes though the
object and the other traverses a
comparison slide in a manner
by which the phase and amplitude
of the comparison beam
can be adjusted. In fig. 21.10
the vector OB represents the
background light and OT the
light transmitted through the
object. As the latter has a slightly higher refractive index than the
suspending medium, OT will be directed behind OB by a small
angle (fJ). The phase of the light passing through the comparison
slide is now adjusted so that it is exactly out of phase with light
transmitted through the object. Contrast is therefore achieved and
the object appears dark in a bright surround (fig. 21.11).
21.3.5. Diffraction
Diffraction is the apparent bending of light waves around
obstacles in its path. Bending is due to Huygen's principle, which
states that all points along a wave front act as if they were point
sources. Thus, when a wave comes against a barrier with a small
opening, all but one of the effective point sources is blocked. The
b
",.l('l bsin8 = (2m +1) )../2
...
b