Convex Lens: 20 cm Focal Length, Object at 40 cm
A convex lens with f = 20 cm placed 40 cm from an object forms a real image at 40 cm on the other side.
How to use this tool
- Enter focal length f, object distance dₒ and image distance dᵢ in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your image distance dᵢ and the full breakdown beneath it.
When an object sits at exactly twice the focal length, the lens produces a real, inverted image of equal size at twice the focal length on the other side.
Frequently asked questions
- What is the sign convention?
- Using the standard convention: distances are positive on the same side as the outgoing light (real image), negative on the same side as the incoming light (virtual image). Converging lenses have f > 0; diverging lenses f < 0.
- What happens when the object is at the focal point?
- The image distance becomes infinite (parallel outgoing rays) — no focused image forms.