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

To draw a ray diagram:

  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 convex 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 focal length from the lens, the image is:

  • inverted
  • diminished
  • real
Three light rays extend out from the object arrows and pass through a convex lens. These cross on the other side of the lens under a smaller blue arrow.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
A ray diagram. A red arrow represents an object with 3 green arrows moving from it which travel through a convex lens and create an image represented by a blue arrow under the principal axis.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
A ray diagram. The image represented by a blue arrow has two dotted lines leading to an object and a convex lens. Green arrows extend from the object and travel below the principal axis.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.

A ray diagram showing an object viewed through a concave lens. The object emits 3 rays, the one at the top of the lens refracts, the middle straightens and the bottom carries on straight.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 refracted and will diverge 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