State the term used for the distance between the principal focus and the optical centre of a lens.
Explain one difference between a real image and a virtual image formed by a lens.
An engineer uses a magnifying lens to inspect a trace on a circuit board.
Calculate the actual height of the trace. Use the equation:
magnification=image heightobject height \text{magnification} = \frac{\text{image height}}{\text{object height}} magnification=object heightimage height