PHYSICS || COMPOUND MICROSCOPE

Compound Microscope

The Structure of a Compound Microscope

Describe the structure of a compound microscope

A compound microscope is an optical instrument used to produce much greater magnification than that produced by simple microscope. The main features of a compound microscope includes two short-focus convex lenses, the objective lens, and the eyepiece.

Demonstration

The Mode of Action of a Compound Microscope

Describe the mode of action of a compound microscope

The most commonly used microscope for general purposes is the standard compound microscope. It magnifies the size of the object by a complex system of lens arrangement.

It has a series of two lenses; (i) the objective lens close to the object to be observed and (ii) the ocular lens or eyepiece, through which the image is viewed by eye. Light from a light source (mirror or electric lamp) passes through a thin transparent object.

The objective lens produces a magnified ‘real image’ (first image of the object). This image is again magnified by the ocular lens (eyepiece) to obtain a magnified ‘virtual image’ (final image), which can be seen by eye through the eyepiece. As light passes directly from the source to the eye through the two lenses, the field of vision is brightly illuminated. That is why it is a bright-field microscope.

The Magnification of a Compound Microscope

Determine the magnification of a compound microscope

The object lens forms a real and inverted image I_Iof the object O ( the image is slightly magnified). The eyepiece lens acts as a magnifying glass for the first image II and produces a magnifical virtual image.

The object is placed just beyond the principal (fo) of the objective lens so that that the real image I, is formed inside the principal focus (F) of the eye piece. The eyepiece treats the real image I, as an object and then forms its magnified virtual image I2.

Magnification of a compound microscope: This isthe ratio of the image distance produced by a compound microscope to the object distance. The magnification produced by objective lens is v/u.

Where

V is the image distance

U is the object distance

The magnification given by the eyepiece is given by;

Me = 25/fe + 1

If the final image is formed at the least distance of distinct vision (V = 25cm).

Mc = M_om_e

Combine eqn (i) and (ii)

Then

Mc = (v/u) (25/fe+1)

The above formula shows that the final virtual image is formed at the least distance of distinct vision.

Uses of a Compound Microscope

Mention uses of a compound microscope

The uses of a compound microscope includes the following:

• Used to magnify microorganism such as bacteria which cannot be seen by naked eyes.
• Used in hospitals widely to detect microorganisms in specimens provided by patients. A specimen is a small amount that is taken for testing. Blood is an example of specimens. In hospitals microscopes can detect parasites such as plasmodium ssp (a causative agent for malaria) in blood specimen.

Example 2

A certain microscope consists of two converging lenses of focal length 10cm and 4cm for the objective and eyepiece, respectively. The two lenses are separated by a distance of 30cm. The instrument is focused so that the final image is at infinity. Calculate the position of the object and the magnification of the objective lens.

For the objective lens

I/U + I/V = I/Fo

Where

Fo = 10cm

The objective lens forms a real image of the object at the principal focus of the eyepiece.

Thus

V = (30 – 4)

= 26cm

Thus I/U + I/V = I/10

I/U + 1/26 = 1/10

1/U = (1/10 – I/26)

(I/U) -1 = (4/65)

(1/U) -1 = (4/65)-1

U = (65/4)

The distance of object, U= 16.25cm

The magnification given by the objective lense is given by:

Whereas:

V = 26cm

U= 16.25cm

Mo = (26cm/16.25cm)

The magnificent given by objective lens, Mo = 1.6.