The document discusses Newton's rings experiment and diffraction. Newton's rings experiment demonstrates interference between light reflected from a curved lens surface and a flat glass surface. The position of constructive and destructive interference can be used to test the accuracy of a lens. The document also provides questions about deriving an equation for fringe positions and calculating fringe spacing and wavelength from a double-slit experiment.

1. Diffraction
Dr. Ehab Hegazy

2. 2
Newton’s Ring
The interference effect is due to the combination of ray 1, reflected from the flat plate,
with ray 2, reflected from the curved surface of the lens. Ray 1 undergoes a phase
change of 180° upon reflection, whereas ray 2 undergoes no phase change. Because
there is no path difference and the total phase change is due only to the 180° phase
change upon reflection, the contact point at O is dark.
n = 1 because the film is air.
Darkness if 2t = ml (m = 0, 1, 2, …..)
Brightness if 2t = (m + ½)l (m = 0, 1, 2, …..)
(Newton’s rings can be used to test the accuracy
with which a lens has been ground)

3. Questions
1. Derive from scratch the equation for the position of constructive
interference fringes on the screen above the center of the central
bright fringe in terms of the variables L, d, n, λ and θ.

4. 2- A laser beam (λ = 632.8 nm) is incident on two slits
0.200 mm apart. How far apart are the bright
interference fringes on a screen 5.00 m away from the
double slits?

5. 3- A Young’s interference experiment is performed with
monochromatic light. The separation between the slits is
0.500 mm, and the interference pattern on a screen 3.30 m
away shows the first side maximum 3.40 mm from the center
of the pattern. What is the wavelength?

9. Diffraction
 Huygen’s principle requires
that the waves spread out
after they pass through slits
 This spreading out of light
from its initial line of travel is
called diffraction

10. Diffraction
 the process by which a beam of light or other system of
waves is spread out as a result of passing through a
narrow aperture or across an edge, typically accompanied
by interference between the wave forms produced.

11. Diffraction, 2
 A single slit placed between a distant light
source and a screen produces a diffraction
pattern
 It will have a broad, intense central
band
 The central band will be flanked by a
series of narrower, less intense
secondary bands
 Called secondary maxima
 The central band will also be flanked
by a series of dark bands
 Called minima

12. Diffraction Grating
 a plate of glass or
metal ruled with very
close parallel lines,
producing a spectrum
by diffraction and
interference of light.