508.

The index of refraction of glass may be found by tracing a of light through a triangular glass prism. Light passing such a prism is bent away from the vertex of the prism a entering and again on emerging from the glass. The angle ween the incident and emergent rays is called the angle of ation. This angle may have different values, but when the leaves the glass with an angle of emergence equal to the e of incidence at the other face, then the angle of deviation minimum. Call the angle of the prism A and the angle of imum deviation D; then it may be shown that the index of action is given by the formula:

n

sin (AD)

sin A

he angles of incidence and refraction at each face of the m can be measured, the index can be found by the method he preceding experiment.

(a) Place a prism on a sheet of paper and trace its triular outline, carefully and accurately. Set up two pins at nd B. Viewing these pins through the prism, set up two e, C, and D, apparently in line with the first two.

Now remove the prism and trace out the path of the ray, longing lines as shown in the diagram in order to facilitate measuring of the angles.

Measure, to the nearest tenth of a degree, the angles of incidence and refraction at the face where the ray enters the prism. Make the corresponding measurements for the second face of the prism, where the ray leaves the glass.

Measure also, the angle of deviation.

Perform the experiment three times with different angles of incidence, and record in a table the angles measured, with the sines of the angles of incidence and refraction. Calculate for

each trial, the index of refraction of the glass, and record the mean value.

(b) Trace the path of a ray which passes symmetrically through the prism, i. e., which enters the glass at the same angle Iwith which it leaves. In securing this condition, C and B should be placed against the prism faces at equal distances from 0.

Measure the angle of the prism and this angle of minimum deviation and then compute the index of refraction by the formula given above.

EXPERIMENT NO. 506

INDEX OF REFRACTION BY MICROSCOPE.

References: Stewart, Physics, Sect. 591; Kimball, College Physics, Sect. 836-837; Duff, College Physics, Sect. 436; Spinney, Text-Book of Physics, Sect. 487.

It can be shown that if a point is situated in a medium of refractive index, n, at a distance, t, from the surface (supposed plane) an image is formed when the light is refracted out into the air, and the distance of this image from the surface is not t

but d, where d =—. This is why an object immersed in water

appears to be nearer to the surface than it really is.

We make use of this fact to determine refractive indices as follows: Make a mark with pencil or ink on the stand of a microscope, and then focus the microscope on this mark so that there is no parallax between the mark and the cross-hairs. By

means of a pair of vernier calipers find the distance between some point fixed on the microscope and a point fixed on the stand. Record four such measurements, each time refocusing the microscope. Now place the block of glass over the mark. The latter

Fig. 18.

will no longer be in focus, but may be brought into focus by raising the microscope. Again take four measurements between the same points as were used before. It is easy to see that the difference between the means of the two sets of measurements gives the value of t-d. Moreover, t is simply the thickness of the glass itself, which should be determined by four measurements with the calipers.

In finding the index of the liquid, we cannot measure the thickness accurately with a caliper. Consequently we proceed as follows: First obtain four settings on a scratch in the bottom of the empty dish, then pour in the liquid and make four settings on the scratch through the liquid, and finally scatter some chalk dust on the surface and take four settings on this. The difference between the means of the first and third sets gives t. The difference between the second and third gives d.

Find the index for a block of glass and for water and alcohol.

EXPERIMENT NO. 507

LENSES.

References: Stewart, Physics, Sect. 601-610; Kimball, College Physics, Sect. 852-857; Duff, College Physics, Sect. 442447; Spinney, Text-Book of Physics, Sect. 490-494.

The focal lengths of simple converging lenses are quite easily found by determining conjugate focal distances and substituting the values in the lens formula:

With certain modifications, described below, the focal length of a diverging lens can also be found by this method.

An optical bench is used. This consists of a metal bar on which can be clamped a number of holders for lenses and screens. At one end is placed an opaque screen with a small opening through which light may shine. This opening may be a small cross, or a circle with cross-wires or a clock-hand fastened across it. Back of this opening must be a lamp of some kind, preferably an electric lamp with frosted bulb. Some distance. out on the bench is the lens holder and beyond it a ground-glass screen with the ground side toward the lens. Look through the screen and move the lens or screen or both until a clear image is formed on the screen. Such a real image cannot be formed unless both the object and the ground glass are well outside the principal foci of the lens. For this reason, in the case of a lens of very long focus, it may be necessary to use an extra stand some distance away to hold the ground glass. A diverging lens will not give any real image at all unless another lens is used in connection with it.

When the image is obtained measure the distance from the lens to the object and from the lens to the image, to the nearest millimeter. If the lens is plano-convex, measure to the tip of the convex surface, if double-convex measure to the middle of the lens. Call the respective distances p and q, and let ƒ be the focal length. Then substitute in the lens formula and solve for f.

Take three such sets of measurements for each of the converging lenses, and calculate ƒ for each set. Record the number of each lens.

For the diverging lens the following method must be used: First set up the shortest-focus converging lens so as to form a clear image on the ground glass at a point such as P. Then insert the diverging lens between P and the converging lens.

The image will no longer be formed at P, but at a point Q farther out. We have now only to measure the distances from

P

Fig. 19.

Q and P to the diverging lens. Let these be called p and q. Then

For, imagine the path of the light to be reversed, so that Q would be the object. Then the light leaving the lens would diverge as if it came from P. Therefore P would be the virtual image, and since it lies on the same side of the lens as the object its distance from the lens is to be used with the negative sign.

Take three sets of observations for the diverging lens, and calculate its focal length. Record the number of the lens.

The focal length of a converging lens may be checked by focusing on a white screen the image of a distant object and measuring the distance from lens to screen. Do this for each of the three lenses.

EXPERIMENT NO. 508

OPTICAL INSTRUMENTS.

References: Stewart, Physics, Sect. 622-625; Kimball, College Physics, Sect. 855, 889-892; Duff, College Physics, Sect. 492-494; Spinney, Text-Book of Physics, Sect. 495-499.

While the actual telescopes and microscopes are made up of multiple lenses carefully figured and made of different kinds of glass, so as to be as free as possible from refractive errors, the principles of these instruments can be shown by the use of simple lenses. The purpose of this experiment is to construct rudimentary models of the compound microscope, the Galilean

S