Concave Lens
Definition
A lens thinner in the middle that spreads light rays apart — always giving smaller, upright, virtual images.
Symbol: · Unit: m (focal length)
Technical definition
A diverging lens with negative focal length. Parallel rays diverge as if from the near virtual focal point. Images are always virtual, upright, and diminished regardless of object distance.
Example
Peephole door viewers use concave lenses to shrink a wide hallway view into a small upright image.
Common confusion
Concave lenses never form real images — no matter the object distance, the image is virtual and smaller. Students often assume ‘lens = can project’; only converging lenses project real images.