A parallel beam of light of wavelength 620 nm is focused by a...

Given:
Wavelength of light, $\lambda = 620 \text{ nm} = 620 \times 10^{-9} \text{ m}$
Diameter of the lens, $D = 8.0 \text{ cm} = 8.0 \times 10^{-2} \text{ m}$
Focal length of the lens, $f = 20 \text{ cm} = 20 \times 10^{-2} \text{ m}$

The radius of the central bright spot (Airy disk) formed by a lens is given by:
$r = \dfrac{1.22 \lambda f}{D}$

The diameter of the central bright spot is:
$d = 2r = \dfrac{2.44 \lambda f}{D}$

Substituting the given values:
$d = \dfrac{2.44 \times 620 \times 10^{-9} \times 20 \times 10^{-2}}{8.0 \times 10^{-2}}$

$d = \dfrac{2.44 \times 620 \times 10^{-9} \times 20}{8.0}$

$d = 2.44 \times 1550 \times 10^{-9} \text{ m}$

$d = 3782 \times 10^{-9} \text{ m}$

$d = 3.782 \times 10^{-6} \text{ m}$

Rounding to two significant figures, we get:
$d \approx 3.8 \times 10^{-6} \text{ m}$