A long narrow horizontal slit is positioned 1 mm above a horizontal...

In Lloyd's mirror experiment, the interference occurs between the direct light from the slit and the reflected light from the mirror.

The virtual image of the slit formed by the mirror acts as the second coherent source. The distance between the real slit and its virtual image is the effective slit separation $d$.

$d = 2h = 2 \times 1 \text{ mm} = 2 \text{ mm} = 2 \times 10^{-3} \text{ m}$

The distance to the screen is $D = 1.0 \text{ m}$ and the wavelength of light is $\lambda = 700 \text{ nm} = 700 \times 10^{-9} \text{ m}$.

The fringe-width is given by the formula:

$\beta = \dfrac{\lambda D}{d}$

Substituting the values:

$\beta = \dfrac{700 \times 10^{-9} \times 1.0}{2 \times 10^{-3}}$

$\beta = 350 \times 10^{-6} \text{ m}$

$\beta = 0.35 \text{ mm}$