7.6 DIFFRACTION GRATING
A diffraction grating is an optical device
(normally made of glass plate) which
consists of a great number of narrow-
closely spaced-equidistant-parallel slits
or lines.
It is commonly characterized by the
number of line per unit length, N.
Types of diffraction grating :
• Transmission grating
(consisting slits).
• Reflection gratings
(consisting ruled fine lines).
A diffraction grating is mainly used
to obtain very sharp maxima (bright
fringes).
The condition of maximum of
diffraction by a diffraction grating :
where m = 1, 2, 3,…
Note :
To calculate the greatest
number of orders observed,
then θ → 90o.
White light actually consists of 7-colored
spectrums which move with same speed,
c = 3x108 m s-1 but have different energy
(thus, they have different wavelength,
and frequency, f.
Based on the condition :
d sinm = m
Since R > P, then R > P.
Note :
At the zeroth order (m = 0) :
all the spectrum diffracted at = 0o
(combinations of all the spectrums
form white light).
&
QUESTION 1
Calculate the number of line per mm on a diffraction
grating if the 4th order maximum is formed at an angle
of 40.0o from the central maximum when
monochromatic light of wavelength 489 nm is used.
ANSWERS
N = 328600 line / m-1
&
QUESTION 2
A laser light with a wavelength of 496 nm is projected
normally upon a diffraction grating with 4.9x105 lines
per meter.
(a) How many bright spots can be seen
(b) how can you get the number of bright spots to be
more than in (a).
ANSWERS
(a) 9
(b) …
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