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 cannot 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. Note: 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 represented by a vertical line with outward-facing arrows on either end to resemble its shape.

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 from the top of the object through the focal point before it reaches the lens. Once the line reaches the lens, refract the ray so it is 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 object is placed closer to the lens than the focal length.

The Human Eye and Cameras

Both cameras and human eyes use convex lenses. The ray diagram below illustrates a distant object positioned at more than twice the focal length from the lens.

The image is:

• Real

• Diminished

• Inverted

Magnifying glasses

A magnifying glass uses a convex lens to produce an enlarged image when the object is placed at a distance less than its focal length. The ray diagram below illustrates this scenario.

The image is:

• Virtual

• Magnified

• Upright

Examples with a Concave Lens

A concave lens always produces an image that is:

• Virtual

• Diminished

• Upright

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