# Images Formed by Lenses

### Definitions

• Focal point (F) It is the point at which a beam of parallel rays converges upon or appears to diverge from. It is also known as the principal focus of the lens.
• Focal length is the distance from the centre of the lens to the focal point.
• Centre of curvature (C): It is the center of the sphere from which the lens was cut. The center of curvature for a convex lens is located on the side opposite to the object, while for a concave lens, it is located on the same side as the object.

## Images formed by Convex Lens

Position of object Position of image Nature of image
At infinityAt focusReal, diminished, inverted
At 2FAt 2FReal, same size and inverted
Between 2F and FBeyond 2FReal, enlarged, inverted
At FNo imageRefracted rays are parallel
Between F and lensOn the same side as the objectVirtual, enlarged and upright

### Uses of Convex Lens

• Used in magnifying glass.
• Used to correct the vision of long-sighted persons.

### Images formed by Concave Lens

Position of object Position of image Nature of image
At infinityAt focusVirtual, diminished, upright
Between infinity and FBetween the object and the lensVirtual, diminished, upright

### Uses of Concave Lens

• Used in spy holes on doors to check the visitors.
• Used to correct the vision of short-sighted persons.

### Real Images

• Real Image is an image which can be projected onto a screen.
• Real images are always located in front of the mirror and behind the lens.
• Real images are always inverted and never upright.
• Concave lenses, Convex mirrors and plane mirrors always form virtual images and can never form real images.

### Virtual Images

• Virtual Image is an image which cannot be projected onto a screen.
• Virtual images are always located in behind the mirror and in front of the lens.
• Virtual images are always upright and never inverted.