http://www.ck12.org Chapter 21. Physical Optics
FIGURE 21.13
Oil in the street.FIGURE 21.14
to the wavelength of the light in the oil. Recall that the wavelength in air is longer than the wavelength in oil since
the index of refraction of air is less than oil.
We may, therefore, write as the condition for constructive interference:
2 d=mλ
And the condition for destructive interference:
2 d=
(
m+^12)
λ,m= 0 , 1 , 2 ,...We can see that constructive and destructive interference occur for specific wavelengths of light. If the path difference
changes, for example, as we move farther away from the perpendicular condition we assumed, then light of different
wavelength will constructively and destructively interfere. This is one reason that you see different colors when
changing your line of sight (you move your head!). Another factor influencing interference effects occurs when the
thickness of thin layer changes, which is a most probable event.
Phase changes
When a beam of light is reflected from a medium with a greater index of refraction than the medium in which it
travels, it undergoes a phase change of 180◦upon reflection, as shown in Figure 21.15 and Figure 21.16. The effect
is analogous to a wave that is traveling along a rope that is fixed at the end. The reflected wave “flips” upside down
at the fixed point. Conversely, if the wave is free to move at the end, the reflected wave reflects back without a phase
change. In our example above, both the reflections from the air-oil interface and the oil-water interface underwent a
phase change. Thus, both waves remained in phase.