Page 16 - Topic 3

A transmission diffraction grating is a small piece of glass or plastic that consists of a large number of thin, parallel,
equally spaced slits. These slits are much closer than the double slits used in two-slit interference.They are created
using precision machinery that scratches very fine
parallel lines on the plate. The untouched space
between the slits serve as the slits.

The slits are so close, they are usually measured in the
number of slits per mm.

The path difference between the adjacent rays is
represented by the side AN of ∆ABN alongside.

So = sin

∴ = sin

ℎ = sin

Since light is parallel it needs to be focused by a convex lens or even the lens of the eye. Light rays pass through the
slit without deviation and interfere constructively to produce a bright line at the centre.

Between the maxima the intensity drops to almost zero. As you move away from the central maxima the intensity
drops to zero. This is due to the large number of slits very close together. One slit may be out of phase with another
slit far away. (e.g.) light from slit 1 may be out of phase with slit 51 and slit 2 out of phase with slit 52 and so on ∴
destructive interference

Note

• the central maxima has the greatest intensity

• intensity decreases as we move away from the central maxima

• distance between maxima increases

EXAMPLES