The thin lens equation relates which quantities?

• A. Wavelength and frequency
• B. Focal length to lens diameter
• C. The speed of light in air to that in glass
• D. Object distance, image distance, and focal length

Answer: (D)

The main idea here is how a lens forms an image by linking distances to its focal length. In the thin-lens model, the imaging relationship is 1/f = 1/do + 1/di, where do is the object distance (how far the object is from the lens), di is the image distance (how far the image forms from the lens), and f is the focal length (a measure of how strongly the lens bends light). This means you can determine where the image will be or how far the object must be to produce an image at a certain location, once you know the focal length.

That’s why the option linking object distance, image distance, and focal length is the best fit—the equation directly ties all three quantities together. The other ideas touch on wavelength and frequency (wave properties), lens diameter (affects light gathering and the f-number, not this distance-based relation), or speeds of light in different media (about refraction at interfaces, not the straight-line imaging of the thin-lens model).