Decreasing the focal length of a convex lens, all else equal, would:

• A. It would cause the lens to produce only real images.
• B. Make the lens stronger.
• C. It would cause the lens to produce only virtual images.
• D. Make the lens weaker.

Answer: (B)

Decreasing the focal length increases the lens’s convergence power, so the lens becomes stronger. In optics, lens power is P = 1/f (with f in meters); a shorter focal length means a larger P, meaning the lens bends incoming light more and can bring it to focus more sharply. The image type (real or virtual) depends on the object’s distance from the lens relative to the focal length, via the lens equation 1/f = 1/do + 1/di. Shortening f changes where the image forms, but it doesn’t by itself force all images to be real or all to be virtual. It simply makes the lens stronger. The other statements do not describe this effect correctly: a real-image/virtual-image outcome depends on object distance, not just f, and “weaker” is the opposite of what happens when f decreases.