What is a Simple Microscope?

A simple microscope is used to see the magnified image of an object. Antonie Van Leeuwenhoek, a Dutch, invented the first simple microscope, consisting of a small single high powered converging lens to inspect the small micro-organisms of freshwater. It is chiefly designed from the light microscope. The main property of the convex lens is to produce a virtual, erect and enlarged image when the object is placed within the focal length. A convex lens is used to construct a simple microscope. Convex lens is most widely and popularly used as a reading glass or magnifying glass. Now, to obtain higher magnification, combinations of two or more convex lenses are used to form a compound microscope.

What are the Parts of the Simple Microscope?

A simple microscope consists of various optical parts and other supporting (or mechanical) components as discussed in this section. Figure-1 shows the schematic of a basic modern simple microscope with its various parts labelled numerically. In figure 1, Eyepiece (8) is connected with an objective lens (5) through a tube (7). The eyepiece is the lens through which the image of the object can be seen. Tube length is variable by rotating the knob (6) so that the clear image can be obtained by changing the focus. Objective lens (5) enlarges the magnification as the magnification power of the objective lens is higher.

Figure 1: The Schematic of a Simple Microscope showing different parts of it.

The specimen stage (4), which is made of a metallic plate, consists of metallic clips that hold the sample, which is put on the glass slide, under observation. A mirror (3) focuses the light on the sample. All components are placed on the base (1). Base (1) is a mechanical part that provides support to hold the other parts of the microscope. Arm (2) of the microscope is connected to the optical elements and the base.

Magnification of Simple Microscope

Principally, the magnification is the ratio of the size of the object and size of the image. The equation of the magnification for a lens is given as below:

\[{\text{Magnification (M)}} = [\frac{H_{image}}{H_{object}}] = [\frac{D_{image}}{D_{object}}]\]

However, in the case of a simple microscope, which is a convex lens of shorter focal length used to magnify the image of an object, the angle subtended on the eye due to the object and the image.

Figure 2: Working of a Biconvex lens as a Magnifying glass.

The magnification in terms of the angle subtended by the image and the object is given as below:

\[{\text{Angle magnification (M)}} = \frac{\text{Angle subtended by the image}}{\text{Angle subtended by the object}}\].

From Figure 2, we can write in terms of angle

\[{\text{Angle magnification (M)}} = \frac{\alpha}{\beta} = \frac{tan\alpha}{tan\beta}\]

\[tan\beta = \frac{A'B'}{AB} = \frac{AB}{BO}\]. . . .(1)

\[tan\alpha = \frac{AB}{B'O}\]....(2)

Divide the equation (2) by equation (1)

\[M = \frac{\frac{AB}{BO}}{\frac{AB}{B'O}} = \frac{B'O}{BO} = \frac{D}{U}\] ....(3)

U is the distance between the object and the centre of curvature lens. Using the lens formula which is given as \[\frac{1}{F} = \frac{1}{(-U)} - \frac{1}{(-D)}\], equation 3 can be written as \[M = 1 + \frac{D}{F}\].

Therefore, the magnification of a simple microscope is determined by an equation written below:

\[M = 1 + \frac{D}{F}\]

Here, M defines the magnification of the simple microscope, D is the least distance of distinct vision and F is the focal length of the convex lens. As ‘F’ is in the denominator in the equation of magnification, hence smaller focal length will result in higher magnification.

Working of Simple Microscope

A simple microscope consists of a convex lens of short focal length. Figure 3 shows the ray diagram which subsequently forms the image of an object (or we can say a source of light).

Figure 3: Working of a Biconvex lens as a Magnifying glass.

F is the focal length of the lens. An object is placed between the focal length and the centre of the curvature. A ray of light emanating from the object (source) passing through the centre of curvature the lens (O). Another ray of light passes through the focus of the lens which lies on the other side of the lens on the principal axis. Both the rays of light enter the eye and the image is formed by tracing the rays in the backward direction as shown in Figure. The resultant image will be formed at the point of intersection of the rays. The final image is upright, enlarged and virtual. Therefore, a convex lens functions as a Simple microscope.

Simple Microscope Experiment

Here, we perform a simple experiment to calculate the magnification of a biconvex lens with a focal length F which works as a magnifying glass.

Apparatus Required:

A biconvex lens of shorter focal length ‘F’ with a holder.

A newspaper article which has a small font size.

Procedure:

Put the reading material under the biconvex lens and your eyes near to the lens.

Move the lens slowly to the article and also move your head with the lens to observe.

As the lens is brought closer to the article, you would notice the alphabets will be small and blurred to read.

As you keep moving the lens you would see a clear and enlarged image of the alphabets printed on articles. Consider the position as ‘A’. At this position, let say the distance between the lens and the article is ‘D’.

If you go beyond this position, the image will be magnified but blurred and difficult to read.

Using the formula,\[M = 1 + \frac{D}{F}\] magnification can be calculated.

Conclusion:

A Simple experiment can be done to find out the magnification of a lens. Magnification basically tells how big an image of an object will be formed by a particular lens.