Subject knowledge and
 challenging concepts

• The formation of images is a product of the refraction of light rays. Rays are redirected so as to converge at a particular place; this process is referred to as focusing and happens as a result of curved mirrors or lenses.
  
  
• There are two types of image � real and virtual. A real image is one that can be projected onto a screen, such as one produced by a data projector. It is an inversion of the object. A virtual image is one that can be seen by the viewer looking into an optical device, such as with a microscope. It cannot be projected onto a screen but is the right way up.
  
  
• A convex lens causes the paths of light rays to converge. If this is sufficient to make the rays meet (diverging rays may simply be made less diverging or parallel) an image will be formed. A concave lens causes rays of light to diverge.
  
  
• Figure 7 shows image formation by convex and concave lens.
  
  

Figure 7. Convex and concave lenses refracting parallel rays of light

• The focal length is the distance beyond the lens that parallel rays going into the lens are brought to meet. The nature of the image produced by a convex lens depends upon the distance between the object and the lens. (A concave lens also has a focal length: it is the distance from the lens to the point from which the diverging rays appear to be emanating.) Examples include:
  • a magnifying glass making rays converge to produce a virtual image that can be seen by looking into the lens
  • a camera lens causing an image to form on the film or the CCD by making the rays converge on it
  • a projector causing rays to converge on a screen.
    
    
• Focusing involves altering the distance between the object and the lens. This alters the point at which the rays meet, so that they can be made to meet on the back of the camera or the screen.
  
  
• Image formation is usually shown in three ways:
  • By using lenses or mirrors set up to model an arrangement or device. For example, a pair of convex lenses can be used to model a telescope.
    
    
  • By using a ray box lamp with a multi-slot plate (to produce several rays) and cylindrical lenses and mirrors. In a dimmed room the rays are then reflected and refracted to show their paths. Students can then see how they are made to converge.
    
    
  • By using ray diagrams. There are various conventions used in the production of these diagrams; the diagrams show the ray paths.
    
    
• Optical diagrams are tricky to produce; if the angles are slightly wrong the diagram may well be completely wrong. One way forward is for students to complete incomplete ray diagrams; another is to ask students to answer questions, either individually or in small groups, about the diagrams. The important thing is for students to interact with the diagram.
  
  
• The relative size of the image and the object is the magnification. It can be calculated by measuring and dividing thus: magnification = image size/object size.