5.18 Using Ray Diagrams

Describing images formed by lenses

• Images are described using three concepts
• They are either:
  • Real or virtual
  • Bigger than, the same size as or smaller than the object
  • Inverted or the same way up as the object
• A real image is one formed by the convergence of rays of light (the rays meet)
  • A real image can be projected onto a screen
• A virtual image is seen but not formed on a screen
  • The rays of light have not met, they have been perceived by the eye
  • An image viewed through a magnifying glass is a virtual image
• 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:

Diagram showing the formation of a real image by a lens

Drawing a Ray Diagram

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 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:

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:

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

Magnifying glasses

• If the object is placed closer to the lens than the focal length, the emerging rays diverge and a virtual image is formed
• When viewed from the right-hand side of the lens, the emerging rays appear to come from a point on the left. This point can be found by extending the rays backwards (creating virtual rays)
• A virtual image will be seen at the point where these virtual rays cross

A virtual image is formed by the divergence of rays from a point

• In this case the image is:
  • Virtual
  • Enlarged
  • Upright

• Using a lens in this way allows it to be used as a magnifying glass
• When using a magnifying glass, the lens should always be held close to the object