In the standard sign convention for lenses, what is the sign of the image distance di for a real image?

• A. Positive
• B. Negative
• C. Zero
• D. Undefined

Answer: (A)

In this sign convention, distances measured in the direction of the light’s travel are taken as positive. For a lens, the image distance di is the distance from the lens to the image along the optical axis. A real image forms on the opposite side of the lens from the object, where the light actually converges to a point. That location is to the right of the lens (in the usual left-to-right setup), so di is a positive distance. Virtual images would form on the same side as the object and have negative di. Zero would imply the image sits at the lens, which isn’t the case for a real image, and undefined isn’t applicable here. Hence, di is positive for a real image.