Types of Optical Instruments and Their Applications

Optical instruments are those devices that use properties of light for observing, measuring and manipulating light waves. These instruments can be categorised into imaging, spectroscopy, measurement, vision correction, light control, medical and industrial instruments. We use optical instruments for seeing fine details of objects and viewing them more clearly. For making things look bigger, we will use a converging lens or a diverging lens. Magnifying glass, human eye, microscope and telescopes are some of the optical instruments.

We will now be taking a look at the different types of optical instruments that are used in our day-to-day lives: 

1. Microscope

A simple microscope is a convex lens that magnifies small objects about twenty times their original size. For large magnification, compound microscope is used which is a combination of two lenses. 

1.1 Simple Microscope

A simple microscope consists of a convex lens with short focal length to see a magnified image of small objects. The lesser the focal length of a convex lens, greater will be the value of magnifying power of simple microscope. An object is placed between optical center and focus of this lens to form an image that is virtual, erect and magnified. The image formed is on the same side as the object. Such a lens is held close to the eye and distance of object is adjusted till a clear image is formed at least the distance of distinct vision.

1.2 Compound Microscope

The compound microscope has two convex lenses. One convex lens is called as the objective which has a short aperture and short focal length that faces the object. The other lens has a short focal length but a larger aperture that faces the eye and is known as an eyepiece. The objective and eyepiece are placed coaxially at two ends of a tube. 

When the object is placed between F and 2F of the objective, it forms a real, inverted and magnified image beyond 2F on the other side of the objective. This image acts like an object for eye lens, which acts as a simple microscope. The eye lens is adjusted in a way that image lies between its focus and optical center to form a magnified image at least the distance of distinct vision from the eye lens.

2. Telescopes

These are the optical instruments that are used for viewing distant objects like celestial and terrestrial bodies that are otherwise not visible to the unaided human eye. Visual angle subtended by distant objects at the eye is so small that the object cannot be perceived. Telescopes increase this visual angle and bring the image nearer to the eye. There are mainly two types of telescopes: refracting telescopes and reflecting telescopes. Let us discuss these in further detail.

2.1 Refracting Telescopes

Also known as refractors, these are one of the types of optical telescopes that use lenses for gathering and focusing the light from distant objects like stars, planets and galaxies. This is the first type of telescope invented. In a refracting telescope, a large convex lens is placed at the front of the telescope that collects and bends incoming light. It then focuses the refracted light to a point inside the telescope tube.

• Astronomical Telescope: An astronomical telescope is designed to use two convex lenses: an objective lens (large f0 ) and an eyepiece lens (small fe ) and is used to observe stars, planets and other celestial bodies in the outer space. The objective lens first forms a real, inverted and diminished image magnified for distant objects. The eyepiece then uses the image created by the objective lens as its object and produces a virtual, erect and diminished magnified image of the distant object. Magnification of astronomical telescope is given by the mathematical formula: $m=\frac{f_0}{f_e}$ where $f_0$ is the focal length of objective lens and $f_e$ is the focal length of eyepiece lens.
• Terrestrial telescopes: Terrestrial telescopes are optical devices that are designed specifically for viewing objects on Earth. These telescopes are useful for daytime use and most commonly for activities like birdwatching, sports events, nature observation and surveillance.

2.2 Reflecting Telescopes

These are used for seeing distant stars and possess large light-gathering power to obtain bright image of the faintest of stars in the space. The objective in this type of optical instrument is made of a concave mirror which has a large aperture and large focal length. This concave mirror is parabolic in shape and, it is free from spherical aberration. Before reflected rays of light meet to form real, inverted and diminished image of a distant star at focal plane of concave mirror, they are intercepted and reflected by plane mirror inclined at an angle of 45 to principal axis of concave mirror. The function of eyepiece is to form a magnified and virtual image of the star that is distinctly visible to the eye.

Most of the optical instruments work on four main principles that have been mentioned below:

• Reflection: Mirrors obey a simple rule: the in-angle equals the out-angle. That is why periscopes and reflecting telescopes can steer light so precisely. A curved mirror gathers faint starlight and redirects it toward a focus and then the eyepiece.
• Refraction: When light crosses from air to glass, it changes speed and bends. This is described by Snell’s law. Lenses use this to focus images in glasses, cameras, microscopes, and refracting telescopes.
• Diffraction: Narrow slits or fine gratings make light fan out and form patterns. Spectrometers exploit this: a diffraction grating spreads colours by wavelength so you can read a material’s “barcode.”
• Interference: Overlapping light waves can boost brightness (constructive) or dim the brightness (destructive). Interferometers measure tiny distance changes from those patterns; coatings use destructive interference to cut reflections.
• Absorption & Polarisation: Atoms/molecules grab specific wavelengths; spectrophotometers measure how much gets eaten to infer concentration. Polarisation filters pass only one vibration direction of light which is useful for sunglasses and lab optics.
• Focusing and magnification: Lenses/mirrors bring rays to a focus; an objective forms the image, an eyepiece enlarges it. In a compound microscope, total magnification is objective × eyepiece. Reflecting telescopes do the same job with mirrors instead of big glass lenses.

Let us take a look at the applications of optical instruments:

• Endoscopes put a fiber-optic camera into the body for minimally invasive checks and minor procedures (polyps, biopsies). It is a live video from inside the body rather than surgery by guess.
• Optical Coherence Tomography (OCT) gives cross-section view of the eye including retina, macula, optic nerve in seconds through an “optical ultrasound” for tracking glaucoma or macular disease. 
• Telescopes gather faint light with big mirrors/lenses to view distant objects because of their bigger aperture. Spectrographs split the gathered light to read chemistry, temperature, and motion of celestial bodies to understand their composition and atmosphere.
• Spectroscopy/spectrophotometry are used for water/air testing, pharma quality control, food testing, and materials science since different molecules absorb and emit light differently.
• Optical tweezers trap and move viruses, bacteria, and organelles through focused laser “fingers” which is a standard tool in the biophysics labs. 

Class 12th NCERT solutions for Physics chapters have been shared below for the convinience of students:

This lesson on Optical Instrument has been reviewed by Raghvendra Kumar, who is a subject matter expert with years of experience in teaching. He has qualified for the AIEEE and GATE exams, which helps him in preparing students for exams like JEE Main exam and NEET exam as well as for Class 11 and 12 subjects. At Shiksha, he ensures that the content being published has clarity, accuracy and exam readiness. As an author, I ensure the quality of the content by referring to the study material provided by NIOS, NCERT and other standard Physics books.