The magnifying glass and the microscope

The magnifying glass: a simple converging lens

A magnifying glass is a converging lens with a short focal length (small f', so the vergence C = 1/f' is large) used to observe an object at very close range. Placed between the object and the eye, it produces a virtual, magnified and upright image, which the eye can view without strain if this image is formed at infinity, that is, when the object is placed at the object focal point of the magnifying glass.

Magnification

The magnification G of a magnifying glass compares the angle at which the image is seen through the instrument with the angle at which the object would be seen with the naked eye, positioned at the near point (25 cm). For a magnifying glass used at infinity:

G = 25 / f' (f' expressed in cm)

Example: a magnifying glass with f' = 5 cm gives G = 25/5 = 5. The object appears 5 times larger than if it were observed with the naked eye at the minimum distance for clear vision.

The microscope: two combined lenses

The microscope combines two converging systems:

• The objective lens, which is highly converging and situated close to the object, forms a real, inverted and greatly magnified image, known as the intermediate image.
• The eyepiece, used like a magnifying glass, observes this intermediate image and produces a magnified virtual image that the eye perceives.

The total magnification is the product of the two partial magnifications, which allows magnifications ranging from several hundred to several thousand to be achieved.

Common pitfall

Do not confuse magnification (a ratio of angles, without units) with enlargement (a ratio of image to object sizes, with a sign). The microscope always produces a final virtual image, which is inverted relative to the real object being observed.