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 Spanish - Chinese The Optics Book - Interferences & Difraction Written by:Tim
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In this section:

The Optics Book

Interference (2nd part)
Interference (3rd part)
Difraction

10. Polarization
11. Quantic Optics

## Interference (continuation)

Figure 12 show a transparent film of uniform thickness illuminated by monochromatic light of wavelength l from a point source S. The eye is positioned so that a particular incident ray I from the source enters the eye as ray r1, after reflection from the front surface of the film at a. The incident ray also enters the film at a as a refracted ray and is reflected from the back surface of the film at b; it then emerges from the front surface of the film at c and also enters the eye as ray r2. The geometry of Fig 12 is such that r1 and r2 are parallel. Having originated in the same point source, they are also coherent and so are capable of interfering. Because these two rays have traveled over paths of different lengths, have traversed different media, and have suffered different kinds of reflection at a and b, there is a phase difference between them. The intensity perceived by the eye, as the parallel rays from the region ac of the film enter it, is determined by this phase difference.

 FIGURE 12 Credits: Halliday David

For near-normal incidence (q1» 0 in Fig. 12) the geometrical path difference for the two paths from S is close to 2d. We might expect the resultant wave reflected from the film near a to be an interference maximum if the distance 2d is an integral number of wavelengths. This statement must be modified for two reasons.