Diffraction & Interference
of Light
Chapter 19
19.1 Interference
 Diffraction Interference Incoherent
light-unsynchronized
wave fronts
 Coherent light-synchronized wave
fronts, waves are in phase
When Light Waves Interfere
 Thomas
Young (1772-1829) studies the
human eye and the voice
 This lead him to study waves
 He applied this to study wave interference
in oceans and lakes
 Later proved that light has wave
properties by producing interference
patterns
The Two-Slit Interference
Pattern
 Italian,
Francesco Maria Grimaldi
first noted that the edges of
shadows are not sharp
 He named this spreading of the
wave diffraction
 Young
used monochromatic light
(a source of only one color and
one wavelength)
 He also created coherent light from
incoherent sources.
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He placed a barrier with a very narrow single
slit in front of incoherent light.
Only coherent light will pass through the slit
because it is so narrow.
The light is then diffracted through the narrow
slit
The wave spreads out in a cylindrical shape so
that when this wave reaches the next barrier
that has two slits the waves will be coherent
These two waves will then undergo either
constructive or destructive interference and
create interference fringes
 The
dark and bright bands are
called interference fringes
 The bright bands were from
constructive interference
 The dark bands were from
destructive interference
Measuring the Wavelength
of a Light Wave
 Young
used his experiment to
determine the wavelength of light
 similar triangles


1. If L is much large
than d, then the line
segments S1P1 and
S2P1are nearly parallel
to each other and to line
segment QP1, and
triangle RS1S2 is very
nearly a right triangle.
Thus, sinθ≈λ/d.
2. If the angle θ is
small, then sinθ is very
nearly equal to the tanθ
Wavelength Equation
xd
l
L
 Where:
x is distance from central
bright band to first bright band
 l is wavelength
 L is distance from slits to screen
 d is distance between slits
 Be
careful of the units
 wavelength is nm, nanometers,
10-9
19.2 Single Slit Diffraction
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Door of a room
difference between
double and single slit
is a wider central
bright band
Similar equation
m is the position of
the of the dark band
mlL
xm 
w
 Insect’s
bodies and wings are
diffraction gratings
 Hundreds of tiny ridges
 The light hits them and
produces interference patterns
Diffraction Gratings
 plastic
or glass that has
thousands of lines per
centimeter scratched into it
 10,000 lines per cm
 10-6 m is the spacing between
lines
 Gratings
form interference patterns
similar to the double slits
 The bright bands are in the same
location, but they are narrower
 The dark regions are wider
 Individual
colors are seen easier
because they are separated
 Better measurement of
wavelengths
 The same equation is used where
d is the distance between the
lines
Grating Spectrometer
 An
instrument used to measure
wavelength with a diffraction grating
 Quality control in labs
 Uses the angle instead of L and x
 l = d sinθ