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#### CHAPTER 10 WAVE OPTICS

When can we consider the light beam to be parallel beam in single slit experiment?

• We can consider the two or more paths from the slits to the screen as parallel only if the screen on which the fringes are formed is at a large distance, i.e. a << D.

• Alternatively, this situation can be obtained by placing a convex lens after the slit and putting the screen at its focus.

Advantages of lens

• The lens does not introduce any extra path differences in a parallel beam.

• This arrangement gives more intensity than placing the screen far away.

Angular size and size on the screen

• The separation of the central maximum from the first null of the diffraction pattern is by an angle $\frac{\lambda }{a}$. This size on the screen will be $\frac{f\mathrm{\lambda }}{a}$.