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3: Optical Instruments

The title of this chapter is to some extent false advertising, because the instruments described are the instruments of first-year optics courses, not optical instruments of the real world of optical technology. Thus a telescope consists of a long focal length lens called the object glass and a short focal length lens called the eyepiece, and the magnification is equal to the ratio of the focal lengths. Someone whose experience with telescopes is limited to this concept of a telescope would scarcely recognize a real telescope. A real telescope would consist of an overwhelming mass of structural engineering intertwined with a bewildering array of electronics, wires and flashing lights. There would be no long focal length lens. Instead there would be a huge mirror probably with a hole in the middle of it. There would be no eyepiece, nor anyone to look through it. The observer would be sitting in front of a computer terminal, quite possibly in another continent thousands of miles away.

Thus the intent of the chapter is mainly to give a little bit of help to beginning students who are struggling to answer examination question of the type “A microscope consists of two lenses of such-and-such focal lengths. What is the magnification?” None of this means, however, that the simple and fundamental principles described in this chapter do not apply to real instruments. They most certainly do apply. This is just a beginning.

• 3.2: The Driving Mirror
The Driving Mirror that is outside on the passenger’s side is usually a convex mirror with some words inscribed on it that say something like “OBJECTS IN MIRROR ARE CLOSER THAN THEY APPEAR”. The image formed by the convex mirror is actually an erect, diminished, virtual image, and it “appears” just a few inches behind the surface of the mirror. The object is much further away that it “appears” to be!
• 3.3: The Magnifying Glass
Two points about a magnifying glass to begin with. First, apparently rather few people understand how to use this complicated scientific instrument. The correct way to use it is to hold it as close to your eye as possible. The second point it that it doesn’t magnify at all. The angular size of the image is exactly the same as the angular size of the object.
• 3.4: Spectacle Lenses
When parallel light enters the relaxed eye, it may be brought to a focus before the retina. In effect the lens, or the cornea, of the eye is too strong, or perhaps the eyeball is too deep. It is easy to see objects that are close up, but light from more distant objects is brought to a focus too soon. The eye is said to be myopic or shortsighted or near-sighted.
• 3.5: The Camera
The camera is a box with a lens in one side of it and a photographic film or a CCD on the opposite side. The distance between camera lens and film can be changed so as to focus on objects at various distances. The aperture can also be changed. In dim light you need to open the aperture up to let a lot of light in; but this makes the image less sharp, and you have a smaller depth of field.
• 3.6: The Telescope
A telescope is an optical instrument that makes distant objects appear magnified by using an arrangement of lenses or curved mirrors and lenses, or various devices used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. The first known practical telescopes were refracting telescopes invented in the Netherlands at the beginning of the 17th century, by using glass lenses.
• 3.7: The Microscope
Recall the way a microscope works. First, the objective produces a magnified real image of the object. Then you look at this primary image with an eyepiece. The overall magnification, then, is the product of the linear magnification produced by the objective and the angular magnification produced by the eyepiece.

Thumbnail: The photographer can see the subject before taking an image by the mirror. When taking an image the mirror will swing up and light will go to the sensor instead. Camera lens Reflex mirror Focal-plane shutter Image sensor Matte focusing screen Condenser lens Pentaprism/pentamirror Viewfinder eyepiece. Image used with permission (CC BY-SA 3.0; Cburnett).