# The magnifying glass: quantitative

##### Class practical

Finding the position and size of the image in a magnifying glass, and hence the magnification.

#### Apparatus and materials

For each student or group of students

Plano-convex lens, + 14D

Graph paper (2 mm), sheet

Wooden strip or tongue depressor

Retort stand and bosses (2 if metre rule is used)

Bright lamp to illuminate the object

#### Health & Safety and Technical notes

This experiment is more easily performed using a commercial mount. If a metre rule is used it should be firmly fixed horizontally using the retort stands and bosses.

The object '0' and the image-catcher 'I' are made by attaching pieces of graph paper to the wooden strips. These in turn are fixed in the holders in a vertical position. The top of 'I' should be higher than the lens. The top of '0' should be level with or just below the top of the lens. The object must be very well illuminated by a lamp close to it.

#### Procedure

a Fix the +14D lens in a holder on the mount, to act as the single magnifying glass, with its less convex side towards the observer.

Position the image catcher 'I' 25 cm from the lens (the average near-point of comfortable vision). Place the eye close to the lens 'L' and move the object '0' until its image seems to sit clearly on the catcher. To make this adjustment, keep both eyes open, looking at the catcher with one (naked) eye, and looking through the lens at the virtual image of the object with the other eye. Concentrate attention on the naked eye, continuing to keep the catcher in focus. Meanwhile move the object nearer and farther away, until the eye looking at the image sees that image also sharply in focus, 'sitting on the catcher'.

b The magnification may be estimated. Or you can simply observe that the image is back at the image-catcher, and obviously considerably bigger than the object.

#### Teaching notes

1 An alternative method, which is neither so easy nor such good teaching of the meaning of images, is the 'method of no-parallax'. Keep both eyes open and (concentrating attention on the naked eye) move the head to and fro laterally, moving the object until image and catcher seem to stay together.

2 There is a third, poorer method, only to be recommended where a student has uneven eyes or can use only one eye well. The head is moved up and down rapidly, looking through the lens at the virtual image, then over the lens at the catcher, then through the lens at the image, and so on. During this rapid alternation of glimpses of image and catcher, the object is also moved until both image and catcher seem, successively, equally in focus.

3 The sizes of the object and the image on the graph paper may be compared and hence the magnification calculated.

This experiment was safety-checked in January 2007