Convex and Concave Ray Diagrams

Real and Virtual Images

Lenses can form images of the objects placed in front of them. Images can be described as:

Real – The light rays converge and meet each other at a point, which means that the Image can be projected onto a screen.

Virtual – The light rays diverge away from a point, so the light rays do not meet. However, they appear to meet behind the lens. Virtual Images can not be projected onto a screen.

Magnified – The image formed is larger than the object

Diminished – The image formed is smaller than the object

Upright – The image formed is the same way up as the object

Inverted – The image formed is upside down compared to the object – Real images are always inverted

Drawing a Ray Diagram

We can use ray diagrams to understand how lenses work and to identify the location of the image formed.

In a ray diagram, a convex lens is drawn as a vertical line with arrows pointing outwards in both directions. This is to resemble the shape of a convex lens.

Whereas, a concave lens is drawn as a vertical line with arrows pointing inwards. This is to resemble the shape of a concave lens.

Method:

1. Draw a line from the top of the object, parallel to the axis. Once the line reaches the lens, refract it through the focal point.

2. Draw a line through the focal point before the lens. Once it reaches the lens, refract the ray parallel to the axis.

  • Not all ray diagrams include this ray.

3. Draw a line from the top of the object, through the centre of the lens.

  • The image is the arrow drawn from the axis to the point at which the rays meet.

Examples of Convex Lenses

If an object is placed at a greater distance than the focal length from the lens, then the convex lens will produce a real image. However, the convex lens will produce a virtual image if the image is placed closer to the lens than the focal length.

Human eye or camera

Both cameras and eyes have convex lenses. Below is the ray diagram for a distant object which is greater than twice the focal length from the lens.

The image is:

  • Real
  • Diminished
  • Inverted

Magnifying glasses

A magnifying glass is a convex lens that produces an enlarged image. In this case, the object is less than the focal length from the lens. Below is the ray diagram for a magnifying glass.

The image is:

  • Virtual
  • Magnified
  • Upright

Example of a Concave Lens

Concave lenses always produce images that are:

  • Virtual
  • Diminished
  • Upright

Below is an example of a ray diagram for a concave lens.