hecture I
Physical Optics
Chapter 1: Jsterference of Two Beams of Light
1-1: Hygens Principle:
When waves pass through an aperture, or past the
edge of an obstacle,they always spread to some extent
into the region which is not directly exposed to the oncoming
waves. This phenomenon is called diffraction. In order to
explain the bebending of light, Higgon's nearly ttree centuries
ago proposed the rule that each point on a wave frout may
be regarded as a new source of waves. This principle
has very far-reaching applications and wil be used later
in discussing the diffraction of light, but we shell
Cousider hore ouly avery simple proof of its correctness.
In this Fiqure
A let a set of plane waves approach the barrier AB
from the left, and let the barrier centain an opening S of
width somewhat smaller then the wavelength. At all points except
5 the waves will be either reflected or absorbed, but S' wil
be free to produce a disturbance behind the Screen.lt is found
experimentally, in agreement with the above prenciple,that the
waves spread out from s'in the form of semicircles.
Huygen's principle as shown in Fig. I can be illustrated
very suceessfully with water waves. An cure lamp on the floor,
with aglass bottomed tray ofr tank above it, will cast
shadows of waves on a wwhite ceiling.
If the experiment in frig.t be performed. with light, one
would naturally expect, from the fact that light generally
travels in straight lines, that merely a narrow patch of
loght would appear at D. Howerer if the slit is made
very narrow, an appreciable broading of this pateh
is observed, its breadth increasing as the slit is further
narrowed. This remarkable evidence that light does not
always travel in straight line was mentioned at the very
beginning of this section.
when the screen CE is replaced by photographic plate,the
light is most intense in the forward direction, but its intensity
decreases slowly as the angleincreases.If the slit is small coCompared
with the wavelength of light, the intensity does not come to Zero
even when the angle of observation becomes 90.
1.2: Young's Experiment.
The original experiment perfermed by Young is shown
in Fiqure 2), Sunlight was first allowed to pass througha
pinhle I and then,at a considerable distance away,through
two pinhole S, and S. The two sets of spherrical waves
emerging from the two holes inter fered with each other
in such a way as to form a symmetrical pattern of vorying
intensity on the screen AC. Since this carly experiment was performed,it has been found convenient to replace the pinholes
by narrow slits and to use a source giving monochromatic
light,ie., light of a single waveleng th. In place of spherical
wave fronts, represented equally well in two dimensions by
the same fiqure (2.