Virtual Images (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award)): Revision Note

Converging lens - virtual image

Extended tier only

• Images produced by lenses can be one of two types:

  • A real image

  • A virtual image

• Images can be described compared to their object as:

  • Enlarged/same size/diminished

  • Upright/inverted

  • Real/virtual

Real images

• A real image is defined as:

  An image that is formed when the light rays from an object converge and meet each other and can be projected onto a screen

• A real image is one produced by the convergence of light towards a focus

• Real images are always inverted

• Real images can be projected onto pieces of paper or screens

  • An example of a real image is the image formed on a cinema screen

A real image

A real image can be projected onto a screen

Virtual images

• A virtual image is defined as:

  An image that is formed when the light rays from an object do not meet but appear to meet behind the lens and cannot be projected onto a screen

• A virtual image is formed by the divergence of light away from a point

• Virtual images are always upright

• Virtual images cannot be projected onto a piece of paper or a screen

  • An example of a virtual image is a person's reflection in a mirror

A virtual image

A reflection in a mirror is an example of a virtual image

Constructing converging ray diagrams of virtual images

• A single lens placed at a distance less than the focal length of an object can be used as a magnifying glass

A converging lens ray diagram for an object placed less than f

Ray diagram showing light converging through a magnifying glass to form a magnified virtual image

• The image that forms will have the following properties:

• A converging lens will produce a real image of an object which is placed at a distance greater than the focal length from the lens

• If the object is placed closer to the lens than the focal length f then a virtual image will be formed and the converging lens ray diagram will be drawn in the following way:

A virtual image is formed when the object is placed closer than the focal length

1. Start by drawing a ray going from the top of the object through the centre of the lens. This ray will continue to travel in a straight line

2. Draw a dashed line continuing this ray upwards

3. Next draw a ray going from the top of the object, travelling parallel to the axis to the lens. When this ray emerges from the lens it will travel directly through the principal focus f

4. Also, draw a dashed line continuing this ray upwards

5. The image is the line drawn from the axis to the point where the two dashed lines meet

• In this case, the image is:

  • Virtual: the light rays appear to meet when produced backwards

  • Magnified: the image is larger than the object

  • Upright: the image is formed on the same side of the principal axis