SOME OPTICAL INSTRUMENTS

Note: The periscope and prismatic binoculars are discussed in another Grade 10 topic.

1. The refracting telescope

The telescope is a device for producing a magnified image of distant objects. It is likely that the first telescope was invented in Holland in about 1608. In 1610, Galileo built such an instrument, shown here on the right, and used it to make important astronomical discoveries, such as the moons of Jupiter and the rings of Saturn.

One of the tubes, still to be found on smaller modern astronomical telescopes, is called the "seeker" (or "finder") , and is used to aim the telescope in the right direction

Very large refracting telescopes have been built, such as the 40-inch Yerkes telescope.

For many years, the South African Astronomical Observatory used the Victoria telescope (shown here on the left) as its main instrument.

It has been superseded by SALT, the South African Large Telescope, which is a reflecting telescope.


The principle of the refracting telescope is quite simple, and in it most basic design, such instruments are made up of two converging lenses, a long focal length OBJECTIVE and a short-focal length EYEPIECE.

Parallel rays (shown in red in the above diagram) enter the objective lens (why are they parallel?), and form a real, inverted, and diminished image (red arrow) in the focal plane of that lens, at a distance F1 from that lens. The eyepiece is at a distance such that its principal focal plane (at a distance F2 from the front of that lens) overlaps the focal distance F1 of the objective. Note that the focal length of the objective, F1, is longer than the focal length of the eyepiece, F2.

The rays (shown in blue in the above diagram) are now refracted by the eyepiece, and form a magnified, inverted virtual image (blue arrow), which is seen by the observer.

Normally, the eyepiece is adjusted in such a way that the principal focal planes of the two lenses coincide (F1 + F2 = the length of the telescope). The image is then formed at infinity.

What exactly is SALT?

(Click here for a discussion)

2. The microscope

          

A microscope is an instrument designed to produce magnified imaged of small objects. The simplest is simply a single converging lens with a short focal length, which generates a virtual, magnified image of an object placed within its principal focus. Here, however, we are concerned with the so-called COMPOUND MICROSCOPE, which consists of a tube, in which two converging lenses (or two assemblies of lenses, each acting as a converging lens), the EYEPIECE and the OBJECTIVE are mounted. The specimen to be examined is placed on a glass slide, fixed to a STAGE, which is very close to the objective, which has a short focal length. Light from a lamp is reflected onto a mirror, passes through a CONDENSER (which forms a beam of parallel rays, and hence through the stage and the specimen.

Most of the better microscopes have several objective lens assemblies, mounted on a turret, thus enabling the viewer to use objectives of different focal lengths, as the situation demands it. Such a microscope is shown above, on the left.

The viewer looks through the eyepiece, which also has a short focal length.

The object is located at a distance between F1 (the focal length of the objective) and 2F1. It forms a real image between the focal plane (at F2) of the objective and the objective itself.


This real image is magnified by the eyepiece, and is viewed as a virtual image. Magnification of 1000 times or more the normal size of the object are readily obtained with better class instruments.

3. The camera

A diagram of a (very!) simple camera is shown on the right, showing the principle of its operation.

Light from a distant object is refracted through the objective. The diaphragm adjusts the amount of light which is allowed into the camera (the so-called "f-stops"), and an adjustable shutter isolates the inside of the camera from light until the picture is taken. When the picture is taken, the shutter opens for a predetermined length of time (for example, 1/250th of a second), allowing the image to form on the film, which is then said to be EXPOSED.

Digital cameras do not use film; instead, the image is formed onto an image sensor that turns the light into electric signals. Note that the principle of image formation by a the camera is basically the same as that of image formation in the human eye

4. The slide or film projector

A slide or film projector is a device which sends a beam of light onto a slide or a film, forming a greatly magnified, real, inverted image onto a screen. The basic principle of its construction is shown above. A lamp acts as a powerful source of light. In order to increase its efficiency, a concave mirror is placed on one side, reflecting some of the light which would otherwise be lost. The light passes through a condenser lens, whose purpose is to form a uniform beam of light.

The light then passes through a slide, which acts as object. Rays coming from the slide then pass through a projection lens (normally a combination of lenses), and then onto the screen. Focussing the image is achieved by moving the lens forward or backwards. The lamp gives off a lot of heat, and projectors are fitted with a fan to cool the lamp/mirror assembly.

5. Additional questions


Refractors and reflectors

Most modern telescopes are of the so-called REFLECTOR type, where a parabolic (that is to say, in the shape of a parabola) mirror is used as objective.

Newton built such an instrument, so the principle of their construction, which is beyond the scope of this discussion, has been around for quite some time.


Camera f-stops

A camera diaphragm adjusts the area or APERTURE through which light will enter the lens. The f-stops are values which determine this. Successive f-stops have half the area of the previous value. For example, a value of 5.6 has a diaphragm opening which is half the area provided by an f-stop value of 2.8. So-called FAST LENSES have small f-stop values (that is, large apertures), allowing a relatively large amount of light to enter the lens in any given time interval.


Why are the rays from a distant object drawn as parallel rays?

We draw rays from a distant object as parallel rays because the further the object O is from the lens, the smaller the angle θ. If the object is very distant, it is at infinity as far as the objective lens is concerned, and θ is effectively zero. In other words, the two beams as drawn on the right are parallel.