7. Optical Instruments

A variety of everyday and scientific instruments make use of the principles of reflection and refraction discussed above, typically by combining one or more lenses and/or mirrors to form magnified, focused, or otherwise useful images.

7.1 Simple Microscope (Magnifying Glass)

The simple microscope is nothing but a single convex lens of short focal length. The object to be viewed is placed between the lens and its focus, so that the lens forms a virtual, erect, and magnified image of the object. It is commonly used by watchmakers, jewellers, and to read fine print, and is also called a magnifying glass or hand lens. Its magnifying power is higher when the focal length of the lens is shorter.

7.2 Compound Microscope

A compound microscope uses two convex lenses of short focal length arranged coaxially in a tube, to achieve much higher magnification than a simple microscope: an objective lens (facing the object, of very short focal length) and an eyepiece (ocular) lens (facing the eye, of comparatively larger focal length). The objective lens forms a real, inverted, and magnified image of the object just inside the focus of the eyepiece; this real image then acts as the 'object' for the eyepiece, which further magnifies it and produces a final image that is virtual, inverted (relative to the original object), and highly magnified. Compound microscopes are essential tools in biology and medicine, used to examine cells, bacteria, and other microscopic specimens.

7.3 Astronomical Telescope

An astronomical (refracting) telescope is used to view distant celestial objects such as planets and stars, and is also built from two convex lenses: an objective lens of large focal length and large aperture (to gather as much light as possible from a faint, distant object) and an eyepiece of small focal length. The objective forms a small, real, inverted image of the distant object at its focus, and the eyepiece then magnifies this image. Since the objects viewed are already extremely far away, the final image produced by an astronomical telescope is inverted with respect to the object — this does not matter for viewing planets and stars, but it makes the standard astronomical telescope unsuitable for viewing terrestrial (Earth-based) objects.

7.4 Terrestrial Telescope

A terrestrial telescope is used for viewing distant objects on Earth (ships, landscapes, birds) where an upright image is essential. It is built like an astronomical telescope but includes an additional erecting lens (or, in prism binoculars, a pair of right-angled prisms working by total internal reflection) placed between the objective and the eyepiece, whose sole purpose is to invert the image once more so that the final image seen by the observer is erect, matching the natural orientation of the object. Binoculars are essentially a pair of small terrestrial telescopes joined side by side, one for each eye, using prisms to both erect the image and fold the light path so the instrument can be made compact.

7.5 Camera

A camera is an optical instrument used to capture a real, permanent image of an object or scene. A basic camera has a converging (convex) lens system, an aperture (a variable opening, analogous to the pupil, that controls how much light enters), a shutter (which controls the duration for which light is allowed to fall on the sensor/film), and a light-sensitive surface (photographic film in older cameras, or an electronic image sensor in digital cameras) on which a real, inverted, and diminished image of the object is formed, analogous to the retina of the eye.

A camera's aperture setting is usually specified as an 'f-number' (such as f/2.8, f/4, f/5.6), which is the ratio of the lens's focal length to the diameter of its aperture opening. A smaller f-number means a larger aperture opening, letting in more light (useful in dim conditions but giving a shallower depth of field), while a larger f-number means a smaller aperture, letting in less light but keeping a greater range of distances in sharp focus. The shutter speed, which is the length of time the shutter stays open, works together with the aperture setting to control the total amount of light that reaches the film or sensor, and also determines whether fast-moving objects appear sharp or blurred in the resulting photograph.

7.6 Reflecting Telescope — a Brief Note

While the astronomical telescope described above is a refracting telescope (using only lenses), large modern observatories almost always use reflecting telescopes instead, in which a large concave mirror (rather than a convex lens) serves as the objective to collect and focus light. This design, pioneered by Isaac Newton, avoids several optical defects associated with large lenses (such as chromatic aberration, where different colours focus at slightly different points) and is far easier and cheaper to construct at large sizes, since a mirror needs to be precisely shaped on only one surface and does not need to be optically perfect all the way through its bulk the way a lens does. Nearly all major professional telescopes today, including space telescopes, are reflecting telescopes.

7.7 Periscope

A periscope is a simple instrument that allows an observer to view objects that are otherwise out of the direct line of sight, for example, over an obstacle or around a corner, or above the water surface from inside a submarine. It consists of a tube containing two plane mirrors (or two right-angled prisms in more advanced versions), each inclined at 45° to the vertical and parallel to each other, positioned at the top and bottom of the tube. Light from the object is reflected downward by the first mirror and then reflected again by the second mirror into the observer's eye, effectively displacing the line of sight without changing its direction. Periscopes are classically used in submarines and were also used by soldiers in trenches.