Determination of Focal Length and Principal Points of Telescope Objective and Eyepiece, Lab Reports of Physics

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Physics 262

Lab #5: Geometric Optics

John Yamrick

Abstract

The purpose of this experiment was to use methodologies based upon the principles of geometric optics in order to characterize a pair of lenses. Following a technique involving auto- collimation and the rotation of the lens, the focal length of a telescope objective was determined to be 36.9 cm ± 0.1 cm and the separation of the nodal points to be 6.7 mm ± 0.2 mm. Using a technique involving the magnification of an image, the focal length of a telescope eyepiece was determined to be 5.36 cm ± 0.05 cm. The telescope was then assembled and its magnification tested at various distances.

Introduction

Geometric Optics is the study of reflection and refraction of light at various surfaces based upon its geometric incidence. It is, at its core, an application of Snell’s Law. Given the radii of curvatures and the index of materials comprising any lens system, it is theoretically possible to determine the final trajectory of any light beam entering the system. However, due to the difficulty of accurately obtaining such complete information about a lens, and due to the cumbersomeness of such calculations, methods have developed to concisely characterize the effects of a lens or lens system in such a way that the image produced by an incident ray can be much more easily determined. This experiment derives a few of the equations which govern such interactions, and then uses practiced methods for determining the relevant quantities from observable data.

Theoretical Background

Snell’s Law explains the conditions under which light passing from one medium to another will change its trajectory. A plane tangent to the point of incidence can be imagined, and a normal axis drawn. The angle of incidence is taken to be the angle from this normal to the incident ray, and the angle of refraction is taken to be the angle to the exiting ray on the refracted side. The equation:

describes this phenomenon, where n 1 and n 2 are the refractive indices of the two mediums.

Light traveling from a medium with a smaller index of refraction (for instance, air) to one with a larger index of refraction (glass) will bend towards the normal axis. This is what causes light hitting the outer rounded surface of a glass lens to converge, and light hitting the inner crescent surface to diverge.

Often in geometric optics, light is considered to travel as a sum of individual rays, and particular instances of these rays are examined to help visualize a problem. In the case of lenses, rays travelling parallel to the axis of the lens are often considered because they rather closely approximate light emanating in all directions from a distance object. When these parallel rays hit the curved surface of the converging lens, they bend inwards. When they hit the crescent made by the second interface, they

Experimental Procedure

Apparatus

Telescope objective, telescope eyepiece, length-delineated mounting brace, plane mirrors, glass disc inscribed with 5 mm ruled scale (used as object in magnification experiment), white light sources, rotating lens holder with adjustable x-axis positioner, observation lens with y-axis positioner.

Procedure

Telescope objective:

The first task of the experiment was to characterize the telescope objective. The focal length was determined via a 3-step process. The first step was to use auto-collimation to place the lens one focal length away from its first principle point. Auto-collimation is a process where a mirror is set up behind the lens and the distance between the lens and an object is changed until an inverted image appears in the same plane as the object. This occurs because of the definition of the focal point, which says that any rays leaving the focal point will exit the lens parallel to the geometric axis, and any ray entering the lens parallel to the axis will be focused upon the opposite focal point. Because the mirror redirects any beam leaving the lens back towards the lens at a similar angle off the axis, an image will form in the plane of the source of the same size and with bilateral inversion. Provided the lens and mirror are properly aligned, the clarity of this image indicates that the lens has been positioned at the focal point of the lens.

Even though the lens is now known to be at the focal point, simply measuring this distance is insufficient to determine what the focal length is. The second step is to remove the collimating mirror and slide the lens forward a distance x so that it focuses an image of the source on a second source across the lab table. Since the lens was previously one focal length away from the original source, x + f is now the object distance. It creates an image at a distance of x’ + f from the second principle point in image space.

The third step is to move the lens still further and record the position at which the second source can be auto-collimated. Now the lens is one focal length away from the plane where the image was formed in step 2. The distance between this lens new position and the previous one is x’ (previously the lens was x’ + f away from the second source, and now it is f).

With the object and image distances obtained from the image created in step 2, Newtonian laws of optics can be invoked and the focal length of the lens determined. Since the medium is air on both sides of the lens, the focal lengths will be the same on both sides.

Because the objective is a thick lens, the focal length alone is not sufficient to characterize it. Also important is to find the separation of the lens’ principal points. Again, because the medium on either side of the lens is air, the principle points are the same as the nodal points of the lens (which are easier to measure).

The key attribute of the nodal points which allow them to be found is that they produce a nodal plane such that parallel rays incident upon it will be focused to an image in the focal plane. This will happen regardless of the angle of the lens, but the focal plane itself rotates to remain parallel to the lens. However, the plane always intersects the point on the axis that is exactly one focal length away from the second nodal point. If the lens is rotated about this nodal point, an image produced by a distant source will remain stationary and focused upon the focal point. If, however, the lens is rotated about another point, then the second nodal point will move as the angle is changed and the position of the image will change as well.

By adjusting the arm position of the rotating lens mount, the axis of rotation can be moved forward or backward relative to a fixed point along the axis of the lens. A source can be set up to shine through the lens and focus an image onto a point approximately one focal length away from the lens (put at the correct general vicinity using auto-collimation again). Once the position of the nodal point is determined, an image can be created by shining light through the lens in the opposite direction and the process repeated. This will yield the distance between the two nodal (and in this case principal) points.

Telescope eyepiece:

To determine the focal length of the eyepiece (which is expected to be much smaller than that of the objective) a different technique is necessary. Light will be shown through a small glass disc inscribed with ruled markings. This forms an optical object which, if projected through the lens, will create a larger sized image as long as the object distance is made to be greater than the focal length of the lens. Observing the size of the image created will allow calculation of the lens’ magnification. Image size can be measured by viewing the image through the observation lens and recording the distance shown on the y-positioner as focus is shifted from one side of the image to the other. Finding two data points worth of magnification will allow sufficient information to determine the focal length of the lens.

Assembled telescope:

In the final part of this experiment, the eyepiece and objective are combined into a working telescope. To do this, the two lens are arranged along the axis in such a way that their inside focal lengths coincide. A magnified image of a distance object can then be viewed through the lens with the shorter focal length (the eyepiece). In this experiment, magnification of the distant object was determined by comparing the size of a full meter stick seen without the telescope to the portion of the scale readable when viewed through the telescope. Results for magnification where observed at intermediate (a few focal lengths) and long (many focal lengths) distances.

d 1 = 80.1 cm – 54.2 cm = 25.9 cm ± 0.2 cm

M 1 = (1.498 cm ± 0.001 cm) / (0.500 cm) = 2.996 ± 0.

d 2 = 81.0 cm – 42.0 cm = 39.0 cm ± 0.2 cm

M 2 = (2.719 cm ± 0.001 cm) / (0.500 cm) = 5.438 ± 0.

From these the focal length can be determined to be:

With error

f = 5.36 cm ± 0.05 cm

Assembled telescope:

In the final portion of the experiment the eyepiece and objective were combined to produce a telescope. Measurements were made comparing the image size to the apparent size of the real object at that distance.

For full hallway (d (^) object = approximately 14 m or 40 focal lengths):

1 real meter = 14 cm ± 0.5 cm observed

For half hallway (d (^) object = approximately 7 m or 20 focal lengths):

1 real meter = 12.5 cm ± 0.1 cm observed

For 2.20 m away:

66 real cm = 5.8 cm ± 0.1 observed

For 143 cm away:

66 real cm =4.7 ± 0.1 observed

Summary

The application of geometric optics allowed the problems of this experiment to be handled with far greater simplicity than would be possible if relying purely on the laws of reflection and refraction. Characterization of the lenses in terms of such optical parameters as focal length and separation of principle points is systematic and reliable. As far as numerical results, the focal length of the telescope objective was determined to be 36.9 cm ± 0.1 cm and the separation of the nodal points to be 6.7 mm ± 0.2 mm. The focal length of the telescope eyepiece was determined to be 5.36 cm ± 0.05 cm. The magnification of the assembled telescope was shown to decrease with distance from the object. This more significantly corresponds to a decreased angle of vision for light coming from the object.