Classification Of Fringes

Fig.B:Fringes of constant thickness formed by wedge shaped film

Consequently, the maxima in interference pattern occur at positions x, given by Eqn.(10.3.26b). The consecutive bright fringe (dark fringes) are separated by

          .............................(4)

The difference in film thickness between the consecutive maxima is given by

          Since in this case, each fringe is the locus of all points in the film for which optical thickness is same, fringes are termed as “ Fringe of constant thickness”

Fringes of Equal chromatic Order (FECO)

These types of fringes are formed using white light. In this case, a narrow white light is incident on the film. The angle of incidence for all the light rays (different) is constant. Assuming that refractive index doesn't vary appreciably with , the angle of refraction will be same for all the wavelengths. The condition for maximum is again given by Eqn.(10.3.23) .

  Fringes of Equal chromatic Order (FECO)

These types of fringes are formed using white light. In this case, a narrow white light is incident on the film. The angle of incidence for all the light rays (different colours) is constant. Assuming that refractive index doesn't vary appreciably with , the angle of refraction will be same. The condition for maximum is again given by Eqn.(23) .

Since is same for all the wavelength of light the path difference (phase difference) depends upon . All the rays for which is same will be brought to focus (this interface) at one point. The fringes thus produced are called as fringe of equal chromatic order (FECO).