Real & Virtual Images (Cambridge O Level Physics)

• Images produced by lenses can be one of two types:
  • A real image
  • A virtual image

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

Projecting a Real Image

A real image can be projected onto a screen

• Real images are where two solid lines cross in ray diagrams

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

Virtual Image in a Reflection

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

• Virtual images are where two dashed lines, or one dashed and one solid line crosses in ray diagrams

Real Images with Converging Lenses

• Lenses can be used to form images of objects placed in front of them
• The location (and nature) of the image can be found by drawing a ray diagram:

Object Between f and 2f

Diagram showing the formation of a real image by a lens

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. 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 towards the principal focus
3. The image is found at the point where the above two rays meet

• The above diagram shows the image that is formed when the object is placed at a distance between one focal length (f) and two focal lengths (2f) from the lens
• In this case, the image is:
  • Real
  • Enlarged
  • Inverted

• The following diagram shows what happens when the object is more distanced – further than twice the focal length (2f) from the lens:

Object over 2f from Lens

Diagram showing the formation of a real image by a lens with the object at distance

• In this case the image is:
  • Real
  • Diminished (smaller)
  • Inverted

• If the object is placed at exactly twice the focal length (2f) from the lens:

Object Exactly 2f from Lens

Diagram showing the formation of a real image with the object at 2f

• In this case the image is:
  • Real
  • Same size as the object
  • Inverted

Virtual Images with Converging Lenses

• 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:

Object less than f from the Lens

A virtual image 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
• Real images are formed by converging rays
• Virtual images are formed by diverging rays