# Real and virtual images

The images formed by a lens can be:

• upright or inverted (upside down compared to the object)
• magnified or diminished (smaller than the object)
• real or virtual

A is an image that can be projected onto a screen. A appears to come from behind the lens.

To draw a :

1. Draw a ray from the object to the lens that is parallel to the principal axis. Once through the lens, the ray should pass through the principal focus.
2. Draw a ray which passes from the object through the centre of the lens.

Some ray diagrams may also show a third ray.

## Convex lenses

The type of image formed by a lens depends on the lens used and the distance from the object to the lens.

### Example - A camera or human eye

Cameras and eyes contain convex lenses. For a distant object that is placed more than twice the from the lens, the image is:

• inverted
• diminished
• real
Ray diagram for an object placed more than two focal lengths away from a convex lens

### Example – A projector

Projectors contain convex lenses. For an object placed between one and two focal lengths from the lens, the image is:

• inverted
• magnified
• real
Ray diagram for an object placed between 2F and F from a convex lens

In a film or data projector, this image is formed on a screen. Film must be loaded into the projector upside down so the projected image is the right way up.

### Example - Magnifying glass

A magnifying glass is a convex lens used to make an object appear much larger than it actually is. This works when the object is placed at a distance less than the focal length. The image is:

• upright
• magnified
• virtual
Ray diagram for an object placed less than one focal length from a convex lens

Only the person using the magnifying glass can see the image. The image cannot be projected onto a screen because it is a virtual image.

## Concave lenses

Concave lenses always produce images that are:

• upright
• diminished
• virtual

### Example - Peep hole lenses

Peep holes are set into doors so the occupant can identify a visitor before opening the door.

Ray diagram for an object viewed through a concave lens

For an object viewed through a concave lens, light rays from the top of the object will be and will on the other side of the lens. These rays will appear:

• from the same side of the principal axis, meaning the image will be upright
• further from the principal axis, so the image will be larger than the object