4. Optical Instruments

Optical instruments use lenses, mirrors, and prisms to form images, magnify objects, or analyse light, based on the laws of reflection and refraction.

A convex (converging) lens is the workhorse of nearly every magnifying optical instrument, since it can bend parallel rays of light to converge at a real focus and, when an object is placed within its focal length, form an enlarged virtual image — the basis of the simple microscope, the eyepiece of the compound microscope, and the eyepiece of the refracting telescope. Concave mirrors, by contrast, are preferred as the main light-gathering (objective) component in large telescopes because a mirror's reflecting surface does not disperse different colours of light differently the way a lens does, avoiding the coloured-fringe distortion (chromatic aberration) that troubles large lenses.

4.1 Microscope (Simple and Compound)

Simple Microscope. An optical instrument consisting of a single convex lens of short focal length, used to obtain a magnified image of small, nearby objects by placing the object within the focal length of the lens so that a virtual, erect, and magnified image is formed. It is used, for example, by watchmakers and jewellers as a magnifying glass.

Compound Microscope. An optical instrument using two convex lens systems — an objective lens (of very short focal length, close to the object) and an eyepiece (ocular) lens — arranged coaxially in a tube to achieve very high magnification of extremely small objects such as cells, bacteria, and tissues, far greater than a simple microscope can provide. The objective lens forms a real, magnified, inverted image, which is further magnified by the eyepiece to give a final virtual image. It is a basic and essential tool in biology and medical laboratories.

4.2 Telescope

Telescope. An optical instrument used to view distant objects, such as celestial bodies, by collecting more light than the naked eye and forming a magnified image. The refracting telescope (invented in the Netherlands in 1608 and famously improved and used astronomically by Galileo Galilei in 1609) uses a large-diameter convex objective lens to collect light and form an image, magnified by an eyepiece. Reflecting telescopes, developed by Isaac Newton, use a large concave mirror instead of a lens as the objective, which avoids certain optical distortions (chromatic aberration) and allows much larger apertures, and are the basis of most modern large observatory telescopes.

Beyond visible-light telescopes, modern astronomy also uses radio telescopes (large dish antennas that collect radio waves emitted by celestial objects rather than visible light) and space-based telescopes placed in orbit above Earth's atmosphere, which avoids the blurring and absorption effects of the atmosphere and allows observation of wavelengths, such as X-rays and ultraviolet light, that do not reach the ground at all.

4.3 Periscope

Periscope. An optical instrument that allows an observer to see objects that are otherwise out of the direct line of sight — for example, viewing over an obstacle or above the water surface from inside a submerged submarine. It typically consists of a tube with a pair of plane mirrors (or right-angled prisms) fixed parallel to each other at 45°, each of which reflects light through 90°, so that light entering the top of the tube is redirected down to the observer's eye at the bottom.

Apart from submarines, simple periscopes are also used in tanks and armoured vehicles (allowing crew to see outside while remaining protected), in some school science demonstrations to teach reflection, and historically by soldiers in trench warfare to observe enemy positions without exposing themselves to fire.

4.4 Binoculars

Binoculars. A hand-held optical instrument consisting of two small refracting telescopes, one for each eye, mounted side by side and aligned to point in the same direction, used to view distant objects with depth (binocular/stereoscopic vision) unlike a single telescope. Most modern binoculars use a system of prisms (such as Porro prisms) to fold the light path, which both corrects the image orientation and allows a shorter, more compact body.

4.5 Spectrometer

Spectrometer (Spectroscope). An instrument used to produce and analyse the spectrum of light emitted, absorbed, or reflected by a substance, typically by splitting light into its constituent wavelengths (colours) using a prism or diffraction grating and measuring the intensity at each wavelength or the angles of deviation. It is used extensively to study the composition of stars and gases, and forms the basis of spectroscopy, a technique fundamental to modern chemistry and astrophysics.

Each chemical element produces a unique pattern of spectral lines (rather like a fingerprint) when it emits or absorbs light, which is how astronomers determine the chemical composition of stars and distant galaxies without ever physically sampling them — indeed, the element helium was first discovered in the spectrum of the Sun, before it was found on Earth, which is why it was named after the Greek word 'helios', meaning sun.

4.6 Photometer

Photometer. An instrument used to measure the intensity (brightness) of light, or to compare the luminous intensities of two different light sources. Simple photometers compare the illumination produced by two sources on a common screen (adjusting distances until the illumination appears equal, using the inverse square law of light), while modern photometers use photoelectric sensors (photocells) that convert light intensity directly into a measurable electric current.

The light meter used by photographers to judge correct camera exposure settings, and the automatic light sensor that switches on streetlights at dusk, are both everyday practical applications of the photometer principle — converting incident light intensity into a usable, measurable signal.

4.7 Refractometer

Refractometer. An instrument used to measure the refractive index of a substance (how much it bends light passing through it), which in turn allows quick determination of the concentration or purity of a solution — for example, the sugar content of fruit juice, the salinity of seawater, or the concentration of a chemical solution. It works by precisely measuring the critical angle at which light passing from the sample into a prism of known refractive index undergoes total internal reflection, since this angle depends directly on the sample's refractive index.

Quick Revision Table — Optical Instruments