Principles of the Optics


9. Interference of Light
When the wave lies on top of the other, it sometimes reinforces or weakens each
other. We call this phenomena as Interference of light.

y@The Reinforcing Model z

In case waves are on the same phase, such as peak to peak or
trough to trough, waves are reinforced.

y@The Offsetting Model z

In case waves lie on the clear opposite, waves are weaken
each other.

*1 The phenomena in beautifully shining of soap-bubbles will be created as result of
interference of light which reflects each other on the boundaries of thin surfaces
of soap-bubbles.
*2. We adhere the thin films on the glass in order to weaken reflection of light from
the glass. The lens adhered films is called as coating lens. With this thin films,
yellow and green colors of light goes off because of interference of light but red
and purple colors, which are at both ends within visible area, reflect a little.
With this phenomena, coating lens like camera lens can be seen in color of blue
and purple.
*3. If you pile up numbers of films, you can reduce reflection of light in widespread
area ( which film arrangement is known multi-coating.).
In case of eyeglasses, green color is often reflected.
Please see different reflection ratio depending on the film coating as under.

@@@@@Single coating @@@@@@@@@@@@Multi-coating

Reflection ratio

@@@@Blue @Green @@Red @@@@@@@Blue@@@Green@@ Re‚„

10. Diffraction ;
It is called as diffraction of wave in such phenomena that waves in proceeding turn
towards into the shadow area of obstacle, We have the phenomena in diffraction of
wave even through usual experiences in daily life , for example, waves in the port go
into behind of breakwaters or we can often listen to whispering behind of concrete
wall. However, it is difficult for us to see diffraction of light in daily life since it is very
minute comparing to other visible waves.
y The phenomena in Diffraction of Light z
‡@ You can see a ring around the electric light in a big bath of hot spring in which it is full of steam. This phenomena comes out with diffraction of light from the electric light through small particles of steam.

‡A The ring around the sun or the moon is created by reflection or refraction with fine particles of ice crystal in the cirrostratus. There is similar phenomena which is called as Corona ( a small ring inside with purple color and outside with red color). This phenomena is taken place by diffraction of light coming out from uniform size water drops of clouds when it covers the sun or the moon. This is the same phenomena mentioned above in item ‡@.

11. Polarization of Light:
Oscillation of light on the same surface including the proceeding direction is called
as Polarization of light. We can understand the polarization of light using a polarizing

The wave can not go through if the bars are in perpendicular position.
The wave can go through if the bars are in horizontal position.

y Polarizing Phenomena z
‡@ We sometime can not see the letters on the black board because of shinning. If we look at the board with adjustment of the optical axis in such occasion, we can see the letters well on the board as result of absorbing almost of reflected light from the

‡A Obstructing the reflected light from the water by turning the polarizing plate in front of the camera, we can take the photograph of a fish in the water. It is same when you take photographs through the windows.

‡B In case of a compact disk, laser light goes and back on the same track but tracks for
forth and back shall be changed, in such case polarizing phenomena is used.


Polarizing beam splitter


1/4 wave plate


11.Polarizing beam splitter


1/4 wave plate

* It will be discussed later on the work of each element.

To be continuedc..

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