Question

It is important in this experiment that the spacing d be small. One reason is that the laser beam is narrow and we have to have the beam fall on both slits. But Thomas Young did the experiment in 1800 with an ordinary light source, not a laser. Give another reason why d has to be small. [What would your interference pattern look like if we were able to use d = 1 cm?]

Answer 1

As we know that the distance between two successive bright fringes on the screen is given by:
. h = 2 L / kd

Let us assume we have green light, wavelength = 550 * 10^-9 m meters, and an observation screen L=1 meter from the slit screen. This gives K =1.1*10⁷ . If we assume a slit separation of 1 mm,

this results in a fringe separation of    h = 0.5 mm.

If we decrease the separation of the slits to 0.5 mm, we increase the fringe separation to 1 mm. It is clear, though, that the slits must be very closely spaced together in order to observe sizable fringes; there is another coherence-related reason to have closely-spaced slits.

So ultimatly d should be small.