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.
. If the incident beam is parallel, the image of the converging lens will act as an object for the diverging lens. This point will be
Options:
Answer:
Solution:
The parallel beam is focused by the converging lens at its focal plane, forming a real image. Before the rays can meet there, they are intercepted by the diverging lens, so that point acts as a virtual object for the diverging lens. Hence (A): real image for the converging lens, virtual object for the diverging lens.
