Optical Instruments

Optical instruments are the devices which process light wave to enhance an image for more clear view.

Use of optical instruments, such as a magnifying lens or any complicated device like microscope or telescope usually makes things bigger and helps us to see in a more detailed manner. The use of converging lenses makes things appear larger and on the other hand, diverging lenses always gets you smaller images. While using a converging lens, it’s important to remember that, if an object is at a larger distance then the image is diminished and will be very nearer to the focal point. While the object keeps on moving in the direction of the lens, the image moves beyond the focal point and enlarges. When object is placed at 2F, which is two times the focal distance from the lens, the image and object becomes of the same size. When the object moves from 2F towards the focal point (F), it’s image keeps moving out of the lens and enlarges till it goes to infinity when the object reaches the focal point, F. As the object moves closer to the lens, the image moves in the direction of the lens from negative infinity and gets smaller when the object gets closer to lens.

Image enhancement

The first optical instruments were telescopes used for magnification of distant images, and microscopes used for magnifying very tiny images. Since the days of Galileo and Van Leeuwenhoek, these instruments have been greatly improved and extended into other portions of the electromagnetic spectrum. he binocular device is a generally compact instrument for both eyes designed for mobile use. A camera could be considered a type of optical instrument, with the pinhole camera and camera obscura being very simple examples of such devices.

Another class of Optical Instrument is used to analyze the properties of light or optical materials. They include:

  • Interferometer for measuring the interference properties of light waves.
  • Photometer for measuring light intensity.
  • Polarimeter for measuring dispersion or rotation of polarized light.
  • Reflectometer for measuring the reflectivity of a surface or object.
  • Refractometer for measuring refractive index of various materials, invented by Ernst Abbe.
  • Spectrometer or monochromator for generating or measuring a portion of the optical spectrum, for the purpose of chemical or material analysis.
  • Autocollimator which is used to measure angular deflections.
  • Vertometer which is used to determine refractive power of lenses such as glasses, contact lenses and magnifier lens.
  • DNA sequencers can be considered optical instruments as they analyse the color and intensity of the light emitted by a fluorochrome attached to a specific nucleotide of a DNA strand.

Applications of Optical Instruments

Multiple Lenses – There are many devices like telescopes and microscopes which uses multiple lenses to create images. Analyzing any system having multiple lenses shows that it works in stages where each lens forms an image of the object. The original object will work as the object for the first lens and creates an image. This new image will be the object for the second lens and so on..


A microscope has two converging lens. This is because it is easier to get higher magnification with two lenses rather than just one. Use of one lens can magnify 5 times more and using a second will magnify 7 times, and you will get an overall magnification of 35 which is not possible in one lens. It’s an easy procedure than to get magnification by a factor of 35 with a single lens. A ray diagram of microscope arrangement is given below. Here you can see the image is the object for the second lens and the image formed by the second lens is the image that you would see when you looked through the microscope.

Types of Microscopes

Simple Microscope

A simple microscope is an optical instrument, we use for the magnification of small objects to get a clear image or vision. It is a convex lens having a short focal length. This microscope is at a small distance from the object for the magnification and hence this forms a virtual image. The simple microscope enables us to view very small letters and figures

Compound Microscope

With a compound microscope, we get very large values of magnification. We use this microscope to see microscopic objects like microorganisms. It comprises of two convex lenses and magnification occurs in both of these lenses. the components of a compound microscope are eyepiece, objective lens, fine and rough adjustment screw.


As we use telescope to view a object that is at very distant place, therefore, a telescope needs at least two lenses. The first lens forms a diminished image which is nearer to its focal point. This device is designed in a way so that real and inverted image formed by the first lens is just nearer to the second lens than its focal length. With the help of a magnifying glass, we gets a enlarged image which is virtual. The final image then inverted with respect to the object. This doesn’t really matter with the astronomical telescope. While observing an object which is on earth, we usually prefer a straight image which is obtained using a third lens.

Types of Telescopes

Refracting Telescopes

All refracting telescopes use the same principles. The combination of an objective lens 1 and some type of eyepiece 2 is used to gather more light than the human eye is able to collect on its own, focus it 5, and present the viewer with a brighter, clearer, and magnified virtual image 6. The figure above is a diagram of a refracting telescope. The objective lens (at point 1) and the eyepiece (point 2) gather more light than a human eye can collect by itself. The image is focused at point 5, and the observer is shown a brighter, magnified virtual image at point 6. The objective lens refracts, or bends, light. This causes the parallel rays to converge at a focal point, and those that are not parallel converge on a focal plane.

Reflecting Telescopes

Reflecting telescopes, such as the one shown in, use either one or a combination of curved mirrors that reflect light to form an image. They allow an observer to view objects that have very large diameters and are the primary type of telescope used in astronomy. The object being observed is reflected by a curved primary mirror onto the focal plane. (The distance from the mirror to the focal plane is called the focal length. ) A sensor could be located here to record the image, or a secondary mirror could be added to redirect the light to an eyepiece.

Catadioptric Telescopes

Catadioptric telescopes, such as the one shown in, combine mirrors and lenses to form an image. This system has a greater degree of error correction than other types of telescopes. The combination of reflective and refractive elements allows for each element to correct the errors made by the other.

X-Ray Diffraction

X-ray diffraction was discovered by Max von Laue, who won the Nobel Prize in physics in 1914 for his mathematical evaluation of observed x-ray diffraction patterns. Diffraction is the irregularities caused when waves encounter an object. You have most likely observed the effects of diffraction when looking at the bottom of a CD or DVD. The rainbow pattern that appears is a result of the light being interfered by the pits and lands on the disc that hold the data. Shows this effect. Diffraction can happen to any type of wave, not just visible light waves

X-ray Imaging

X-ray radio-graphs are produced by projecting a beam of X-rays toward an object, in medical cases, a part of the human body. Depending on the physical properties of the object (density and composition), some of the X-rays can be partially absorbed.
The portion of the rays that are not absorbed then pass through the object and are recorded by either film or a detector, like in a camera. This provides the observer with a 2 dimensional representation of all the components of that object superimposed on each other.


  • Tomography refers to imaging by sections, or sectioning. demonstrates this concept.
  • The three-dimensional image is broken down into sections.
  • (S1) shows a section from the left and (S2) shows a section from the right.

CT Scans

  • CT scans, or computed tomography scans use a combination of X-ray radiography and tomography to produce slices of areas of the human body.
  • Doctors can analyze the area, and based on the ability of the material to block the X-ray beam, understand more about the material. shows a CT Scan of a human brain.
  • Doctors can cross reference the images with known properties of the same material and determine if there are any inconsistencies or problems.
  • Although generally these scans are shown as in, the information recorded can be used to create a 3 dimensional image of the area. shows a three dimensional image of a brain that was made by compiling CT Scans.


  • A laser is a device that produces a very focused beam of visible light of just one wavelength and color.
  • Waves of laser light are synchronized so the crests and troughs of the waves line up.
  • Electrons in a material such as a ruby crystal are stimulated to radiate photons of light of one wavelength.
  • At each end of the tube is a concave mirror. The photons of light reflect back and forth in the tube off these mirrors.
  • This focuses the light. The mirror at one end of the tube is partly transparent.
  • A constant stream of photons passes through the transparent part, forming the laser beam.

Optical fibres

  • One use is carrying communication signals in optical fibres.
  • Sounds or pictures are encoded in pulses of laser light, which are then sent through an optical fiber.
  • All of the light reflects off the inside of the fiber, so none of it escapes. As a result, the signal remains strong even over long distances.
  • More than one signal can travel through an optical fiber at the same time, Optical fibers are used to carry telephone, cable TV, and Internet signals.


A camera is an optical instrument that forms and records an image of an object. The image may be recorded on film or it may be detected by an electronic sensor that stores the image digitally. Regardless of how the image is recorded, all cameras form images in the same basic way.

  • Light passes through the lens at the front of the camera and enters the camera through an opening called the aperture.
  • As light passes through the lens, it forms a reduced real image. The image focuses on film (or a sensor) at the back of the camera. The lens may be moved back and forth to bring the image into focus.
  • The shutter controls the amount of light that actually strikes the film (or sensor). It stays open longer in dim light to let more light in.

Human EyeAn Optical Instrument

  • The human eye is an optical instrument that enables us to view all the objects around us is a very complex organ.
  • The white protective membrane seen when looked into the eye directly is a Sclera. It is tuff, opaque and fibrous outer layer of the eyeball.
  • The circular part is the Iris. The color of the eye is determined by the color of the Iris. The iris works like the shutter of the camera. It absorbs most of the light falling on it and allows it to pass through the pupil.
  • The center transparent area of the iris is the Pupil. The amount of light that enters the inner part of the eye depends on the size of the pupil.
  • In bright light, the iris contracts the pupil to restrict the light, whereas in low light it widens the pupil to emit more light into the eye. The eyeball is spherical in shape.
  • The retina of the eye is able to detect the light and its color because of the presence of senses known as rods and cones.
  • Light entering the human eye is first refracted by the cornea. The refracted light is then incident on an iris. The lens is just behind the iris and light after refracted through the pupil falls on it and forms a sharp image. Image formation exactly on the retina enables us to see the object clearly.

Defects Of Human Eye

Myopia – A condition in which close objects appear clearly, but far ones don’t.

Hypermetropia – A vision condition in which nearby objects are blurry.

Presbyopia – It is the gradual loss of your eyes’ ability to focus on nearby objects. It’s a natural, often annoying part of aging. Presbyopia usually becomes noticeable in your early to mid-40s and continues to worsen until around age 65.

Astigmatism – A common imperfection in the eye’s curvature. With astigmatism, the front surface of the eye or the lens inside the eye is curved differently in one direction than the other. A common symptom is blurry vision.


Based on the property of reflection and refraction many optical instruments have been designed to understand the behaviour of light more better. Based on the total internal reflection, the phenomenon like Mirage, Transmission through optical fibre, and property of Diamonds could be well understood. Likewise other optical instruments like microscope, telescope etc has been designed to help in the advancement of technology and research studies.