Question #43145

A point source of white light is placed on the principal axis of a thin converging lens at a distance of $20 \text{ cm}$ from its optical centre. The focal lengths of the lens for blue and red light are $f_b = 20 \text{ cm}$ and $f_r = 25 \text{ cm}$, respectively. An opaque screen containing two narrow slits separated by $d = 0.5 \text{ mm}$ is placed just behind the lens (assume the distance between the lens and the slits is negligible), with its plane perpendicular to the principal axis. The midpoint between the two slits is located at a distance $y_0 = 1.5 \text{ mm}$ from the principal axis. An observation screen is placed at a distance $D = 2.0 \text{ m}$ from the slit plane. Find the distance on the observation screen between the central maximum of the interference pattern for blue light and the central maximum for red light.