Instructions

Measure the lens diameter (Figure 2-1). If you wish to plot the relationship between lens diameter and power, use a micrometer to measure the size of the glass sphere (a $1 plastic one works just fine for this) before the microscope is assembled.

Project laser beam though TEM grid and lens – Basic principles. To measure the power of the lens, set up the components in this order: the laser source, then the EM grid, then the lens, then a surface, with the EM grid and the lens situated very close to one another. The EM grid is a small, usually copper disk with a fine metal mesh, the size of which is known exactly. For the 100-mesh grid suggested above, the hole width is 204µm and the bar width is 50µm. When the laser beam is passed though the grid and lens, it projects a greatly enlarged image of the grid pattern on to the surface. If you then measure the distance between two grid bars on projected image and compare it to the known size of the grid (i.e. 204µm), taking into account the distance it has been projected, you can calculate the power of the lens using the following two equations.

First,

(X (mm) / d (mm)) x 250mm = D (mm)

Then,

(D (mm) / A (mm)) = P

Where,
d = Distance between lens and projected image
X = Measured size of grid hole in projected image
D = Size of projected grid hole at 250 mm
A = Actual size of grid hole (0.204 mm)
P = power of lens

There are two equations because the power of a lens is dependent on how far you are from it, so there is a convention in optics to record the power as being from 250 mm. So we first measure your projected size at whatever distance you chose, then convert that size to what it would be if the distance between the lens and the projection were 250 mm. In the second equation you take that projected size and compare it to the known size to determine the power of the lens.

Figure 2-3. Align the laser so that the beam passes through the centre of the EM grid.

Project laser beam though TEM grid and lens:

Method 1 – using a stand (Figures 2-2, 2-3, 2-4 and 2-5). I built a wooden stand for this . This is convenient if you are measuring the power of several lenses, but it is not really necessary, so an alternate method is presented below. A stand holds the laser steady and provides a platform to attach the EM grid (I stuck it to over a hole in a sheet of tin foil using film tape, Figure 2-2). The laser beam is first aimed at the centre of the grid by placing paper shims under the pointer to align the beam, with the switch held on by the elastic that holds the pointer in place. When aligned (Figure 2-3), the microscope is mounted using bulldog clips so that the lens is against the grid (Figure 2-4). Now turn on the laser and point it at a surface to measure the projected image size and projected distance (Figure 2-5).

Method 2 – no stand. Since the only requirements of this procedure are the alignment of the laser beam with the grid and then lens, there is another way that does not require a stand, but more patience. The EM grid can simply be taped on to the cardstock side of the microscope over the lens (with the tape to the side of the grid, not over the light path). Hold the microscope steady and carefully point the laser beam into the grid, while another person measures the projected image size.

 

Figure 2-5. Turn on the laser, and the beam will pass though the grid, then the lens, projecting a magnified image of the grid onto a surface. Measure the distance between two points on this projected image (where the actual size on the grid is known), and measure the distance from the lens to the surface. For example, measure between the positions indicated by two red dots, which are 0.204mm apart on the actual grid. From these values using the equations given, the power of the lens can be calculated.

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