Physics - Geometrical Optics Question with Solution | TestHub

Focal Length of a Zoom Lens: Figure shows a simple version of a zoom lens. The converging lens has focal length f1, and the diverging lens has focal length f2 = -|f2|. The two lenses are separated by a variable distance d that is always less than f1. Also, the magnitude of the focal length of the diverging lens satisfies the inequality |f2| > (f1 - d). To determine the effective focal length of the combination lens, consider a bundle of parallel rays of radius r0 entering the converging lens. It can be shown that the radius of the ray bundle decreases to r0' = r0(f1 - d)/f1 at the point that it enters the diverging lens. Final image I' is formed a distance s2' = |f2|(f1 - d)/(|f2| - f1 + d) to the right of the diverging lens.

If the rays that emerge from the diverging lens and reach the final image point are extended backward to the left of the diverging lens, they will eventually expand to the original radius r0 at some point Q. The distance from the final image I' to the point Q is the effective focal length of the lens combination; if the combination were replaced by a single lens of focal length f placed at Q, parallel rays would still be brought to a focus at I'. The effective focal length is given by f = f1|f2|/(|f2| - f1 + d).

Assume that f1 = 12.0 cm, f2 = -18.0 cm, and the separation d is adjustable between 0 and 3.0 cm.