Convex & Concave Ray Diagrams (OCR Gateway GCSE Physics)

• 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:

Convex Lens Ray Diagrams

• If an object is placed further from the lens than the focal length f then a real image will be formed, and the converging lens ray diagram will be drawn in the following way:

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 through the principal focus f
3. The image is the line drawn from the axis to the point where the above two rays meet

• When describing an image, consider if it is:
  • Real or virtual 
  • Magnified (larger) or diminished (smaller)
  • Upright or inverted

• For example, in the above diagram, the image is:
  • Real: the light rays meet each other after refraction
  • Magnified: the image is larger than the object
  • Inverted: the image is formed on the opposite side of the principal axis

• A convex 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 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

Concave Lens Ray Diagrams

• Concave (diverging) lenses can also be used to form images, although the images are always virtual in this case
• If an object is placed further from the lens than the focal length f then a concave lens ray diagram will be drawn in the following way:

Concave lenses only produce virtual images

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 upwards away from the axis
3. Draw a dashed line continuing this ray downwards to the focal point, f
4. The image is the line drawn from the axis to the point where the above two rays meet

• In this case, the image is:
  • Virtual: the light rays appear to meet when produced backwards
  • Diminished: the image is smaller than the object
  • Upright: the image is formed on the same side of the principal axis

Comparing Convex & Concave Lenses

• The image produced by a convex lens can be either real or virtual
  • This means the image can be inverted (real) or upright (virtual)

• The image produced by a concave lens is always virtual
  • This means the image will always be upright

Step 1: Draw a line from the top of the object through the middle of the lens

• • The top of the image lies somewhere along this line

Step 2: Draw a line from the focal point through the top of the lens

• • The dashed line shows the continuation of the upwards arrow
  • The top of the image is where the two lines cross