PHYSICS

light coming through the opening behaves as a single point
source, so that the light emerges in all directions, instead of just
passing straight through the slit.
Consider light waves coming from various portions of the slit.
The character of the diffraction pattern depends on the phase
difference between the waves (fig. 21.12).
The conditions for minima are:
asinn = ms, where m = 1, 2, 3, ... (21.9)
The conditions for maxima are:
. (2m+I)A
astns 8 = 2 ' where m = 1,2,3, '" (21.10)
21.3.6. Measurement of Mean Erythrocyte Size
Thomas Young, in 1813, was the first to apply the principle of
diffraction to the measurement of spherical objects such as human
erythrocytes. The princi ple of the erythrocytometer depends
on the fact that the size of the diffraction spectrum varies with
the size of the red cells and its distance from the light source. The
device consists of two cylinders, one of which telescopes into the
other. The outer cylinder can be moved up or down thus varying
the distance between the top of the outer cylinder and the
bottom of the inner cylinder. A metal disk is inserted across the
bore of the inner cylinder. The disk has a small central aperture to
emit a beam of light producing the diffraction spectrum. At the
top of the outer cylinder is a slot for holding a slide containing a
thin film of blood. The distance of the film from the light source
is adjusted until the inner ring of the spectrum directly overlays
the inner circle of the hole. The mean size of the erythrocytes is
read directly in micrometers.
21.3.7. Diffraction Grating
A diffraction grating consists of a large number of equally
spaced parallel slits (fig. 21.13). A grating can be made by engraving
parallel lines on a glass plate using a precision machining
technique. The number (N) of lines can be varied from 600 to
ISO
Fig. 21.13. Diffraction grating
2800 l l ines/rnm. The slit spacing
(d) equals to the inverse
of this number, or:
d = I/N (21.11)
The equation for diffraction
grating describes the condition
for maxima in the interference
pattern at the angle e :
dsin8 = ms: (m = 0, 1, 2, 3, ... ) (21.12)
The diffraction grating is used in physics to determine precisely
the wavelength of a source of light.
Example. Monochromatic light from a helium-neon laser (wavelength
632.8 nm) is incident normally on a diffraction grating containing
N = 6000 lines/em. Find the angle at which one would observe the firstorder
maximum.
Solution. A slit spacing d of the diffraction grating is equal to the
inverse of the number N, or d = liN = 1/6000 m = 1.667.10- 6 m. For
the first-order maximum (m = 1), we obtain from the equation for
diffraction grating:
dsinn = A
sinO =A /d = 0.3797
o = 22.31".
21.3.8. X-Ray Diffraction and Structure of DNA
The amount of information that can be derived from the examination
of any material depends ultimately on how fine a probe
is used. For example, examination of biological tissue using an
optical microscope is limited by the wavelength of visible light
that is in the 500 nm region. The wavelength of X-rays, in contrast,
is in the 0.1 nm region, so they provide an excellent probe.
In the early 1950's, J.D. Watson and F.H.C. Crick utilized the
X- ray diffraction techniques to deduce the double helical
structure of deoxyribonucleic acid (DNA). They realized that the
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