Optical Instruments, Uses and Functions: Complete Notes

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The “Optical instruments list” is part of the chapter on ray optics. It is an important topic in general science. In this article, we provide all the major optical instruments with the previous year’s questions asked in SSC. Also, we include frequently asked questions and answers that are very important for various competitive exams like SSC, Railway, RRB NTPC, UPSC, CDS, and all other state PSC exams.

What are optical instruments?

  • An optical instrument is a device that processes light waves either to enhance an image for viewing or to analyze and determine its characteristic properties.
  • Common examples include periscopes, microscopes, telescopes, and cameras.

The magnifying power of optical instruments

  • The Magnifying power of an optical instrument is defined by the ratio of the angle subtended by the image at the eye (visual angle) to the angle subtended by the object seen directly when both lie at the least distance of distinct vision or nearest point. It is also called angular magnification and is denoted by M.
Magnification of Optical Instrument
Magnification of Optical Instrument

What is Visual Angle?

  • Visual angle is the angle subtended by the object in the human eye.  A larger Visual angle means a larger Object, and similarly, a Small visual angle means a smaller object.
  • The optical instrument enhances the visual angle of the object, causing us to see it as larger than its actual size.
  • In other words, the optical instrument creates a larger image of the object, and this final image creates a larger visual angle for our eyes.

α1 – Small visual angle = O1 – Smaller Object

α2 – Bigger visual angle = O2 – Larger Object

Visual Angle
Visual Angle

Types of Optical Instruments

Here is the major optical instruments list, which are commonly in use:

1. Camera
2. Simple Microscope or Magnifying Glass
3. Compound Microscope
4. Telescopes
5. Periscopes
6. Binoculars
7. Kaleidoscope

✅ Read Also: Image formed by Plane Mirror: Detaild Notes

Photographic Camera

  • A camera is an optical instrument that captures a visual image. At a basic level, cameras’ bodies are sealed lightproof boxes with a small hole known as an aperture that allows light through to capture an image on a light-sensitive surface, usually photographic film or a digital sensor.
  • Cameras have various mechanisms to control how the light falls onto the light-sensitive film.
  • The amount of time the photosensitive surface is exposed to light is controlled by the shutter mechanism.
  • Lenses focus the light entering the camera, and the aperture size can be widened or narrowed. The f-number represents the size of the aperture.

✔️ F-Number = Focal length of the lens / Diameter of lens

✔️ Generally, 2, 2.8, 8, 11, 22, and 32 are f-numbers.

  • The amount of light entering the camera is directly proportional to the area of the aperture.

✔️ L A d2

✔️ The Brightness of an image d2/f2

✔️ Here, d=diameter of the lens and f=focal length of the lens

  • Exposure time is when light is incident on photographic film.
  • A real, inverted image of the object is formed on the film by the lens system.


What are Microscopes?

  • A Microscope is an optical instrument that forms a bigger or magnified image of a small nearby object by increasing the visual angle subtended by the image at the eye so that the object is seen to be bigger and distinct.
  • The optical microscope, often referred to as the light microscope, uses visible light and a system of lenses to magnify images of small subjects.

Magnifying Power of Microscope

  • The magnification in a microscope is defined by the ratio of the visual angle formed by the final image (β) to the visual angle formed by the object kept at potion D (α).
  • So, Magnification (M) = β / α
  • Here D is the least distance of distinct vision.

What is meant by the least distance of distinct vision?

  • It is the distance up to which the human eye can see the object clearly without any strain on it. For a normal human eye, this distance is generally taken to be 25 cm.

Types of Microscopes

There are two types of microscopes:

1. Simple microscopes
2. Compound microscopes.

Simple microscope

Simple Microscope Definition

  • A simple microscope is a magnifying glass of a single convex lens with a short focal length. It magnifies the object through angular magnification, thus producing an erect virtual image of the object near the lens.

Magnifying Power

  • Since a simple microscope uses only one objective lens, its magnification capability is greatly limited. Most simple microscopes have a 10x magnification power.
  • The formula for calculating the magnifying power of a simple microscope is:

M = 1 + D/F

Where D is the least distance of distinct vision, and F is the focal length of the convex lens. The shorter the focal length of the lens, the higher the magnifying power of the microscope.

Magnifying Power of Simple Microscope
Magnifying Power of Simple Microscope

Compound Microscope

Compound Microscope Definition

  • A compound microscope consists of two convex lenses. A lens of short aperture and short focal length faces the object and is called the objective. The eyepiece is another lens of short focal length but a large aperture facing the eye. The objective and eyepiece are placed coaxially at the two ends of a tube. The eyepiece lens is nothing but a simple microscope.

Magnification of compound microscope

  • The Magnifying power of a compound microscope is defined as the ratio of the angle subtended by the final image at the eye to the angle subtended by the object at the unaided or naked eye.
  • When the final image is formed at least a distance of distinct vision (D), then

M = v0 /u0 (1 + D/fe)

When, v0=distance of the image, formed by the objective lens

u0=distance of objects from the objective lens.

  • When the final image is formed at infinity, then

M=v0 /u0 (D/fe)

Optical Instruments
An image formed by a compound microscope


  • Telescopes are used to see distant objects not visible to the naked eye, such as celestial bodies or heavenly bodies like the sun, moon, planets, and stars, and terrestrial bodies such as distant objects on the earth’s surface.
  • In the naked or unaided eye, the distant objects are not seen properly due to the small visual angle subtended by the distant objects at the eye.
  • The use of a telescope increases the visual angle and brings the image nearer to the eye.

Types of Telescope

Mainly two types of telescopes are in common use:

1. Refracting telescope and
2. Reflecting telescope.

The refracting telescopes are also of two types:

1. Astronomical telescopes
2. Terrestrial telescopes

Astronomical Telescope

  • It is also a combination of two convex lenses, called an objective lens and an eyepiece, separated by a distance. It is used for observing distinct images of heavenly bodies like stars and planets.
  • The objective lens has a large aperture and large focal lengths (fo), and it faces towards the object. 
  • The eyepiece is the lens facing the eye. It has a small aperture and a short focal length (fe).
  • The objective and eyepiece are mounted coaxially in two metal tubes.
  • An astronomical telescope produces a virtual and erect image.

Magnifying Power

  • When the final image is formed at least a distance of distinct vision (D), then

M = – fo/fe (1+fe/D)

  • Where, fo and fe focal lengths of objective and eyepiece, respectively.
  • Length of the Telescope (L) = (fo +fe)
  • When the final image is formed at infinity, then  M = – fo/fe
  • Length of the Telescope (L) = (fo +fe)
  • For telescope’s magnifying power should be large, and fe should be small.
  • For large magnifying power of a microscope; fo < fe but fe should be small
Ray diagram of Astronomical Telescope
Ray diagram of Astronomical Telescope


  • A periscope is an instrument for observation over, around, or through an object, obstacle, or condition that prevents direct line-of-sight observation from an observer’s current position.
  • A Periscope is a simple optical instrument that consists of an outer case with two plan mirrors that are set parallel to each other at a 45° angle.
  • This form of periscope, with the addition of two simple lenses, served for observation purposes in the trenches during World War I.
  • The earliest periscope was known as the “polemoscope” which was developed by Johannes Hevelius.
  • Hippolyte Marié-Davy invented the first naval periscope.


  • Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to a point in the same direction. They allow the viewer to use both eyes (binocular vision) when viewing distant objects.
  • Unlike a (monocular) telescope, binoculars give users a three-dimensional image.
  • This optical system of a Binocular consisted of an objective lens and the ocular lens (eyepiece), which had two facing, right-angle prisms arranged to invert and correct the orientation of the image.
  • The most common configuration is that invented in 1849 by Carl Kellner.


  • A kaleidoscope is an optical instrument that creates beautiful patterns.
  • It consists of two or more reflecting surfaces (or mirrors) tilted to each other at an angle so that one or more (parts of) objects on one end of the mirrors are seen as a regular symmetrical pattern when viewed from the other end due to repeated reflection.
  • The reflectors are usually enclosed in a tube, often containing on one end a cell with loose, coloured pieces of glass or other transparent (and/or opaque) materials to be reflected into the viewed pattern.
  • Rotation of the cell causes motion of the materials, resulting in an ever-changing view being presented.
Kaleidoscope, patterns

Note: There are many Kaleidoscope photo apps on play store you can download and try them out. In the above image, Kaleidoscope  patterns applied on two detergent bubbles through mobile app.

Optical instruments previous year MCQ’s

Question 1: What is an endoscope?
A. It is an optical instrument used to see inside the alimentary canal
B. It is a device that is fitted into the chest of the patient to regularise the irregular heartbeats
C. It is an instrument used for examining ear disorders
D. It is an instrument for recording electrical signals produced by human muscles
Answer: A. It is an optical instrument used to see inside the alimentary

Question 2: A periscope works on the principle of _
(SSC Combined Matric Level (PRE) Exam. 30.03.2008 (Ist Sitting)
A. refraction
B. total internal reflection
C. diffraction
D. reflection

Answer: D. reflection

Question 3. A microscope used in pathological laboratories forms _
(SSC Combined Matric Level (PRE) Exam. 13.05.2001 (Ist Sitting)
A. magnified, virtual, erect image
B. diminished, real and erect image
C. magnified, virtual and inverted image
D. diminished, virtual and erect image

Answer: C. magnified, virtual and inverted image

Question 4: The final image in a simple microscope is
(SSC Combined Matric Level (PRE) Exam. 16.06.2002 (Re-Exam)
A. real, diminished and inverted
B. real, magnified and erect
C. virtual, magnified and erect
D. virtual, diminished and erect

Answer: C. virtual, magnified and erect

Question 5: What is a compound microscope?
(SSC Constable (GD) Exam. 12.05.2013)
A. A microscope that has one lens.
B. A microscope has two sets of lenses: an ocular lens and an eyepiece.
C. A microscope whose lenses are concave.
D. A microscope whose lenses are convex.

Answer: A microscope with two sets of lenses: an ocular lens and an eyepiece.

Question 6: An electron microscope gives higher magnifications than an optical microscope because :
(SSC CAPFs SI, CISF ASI & Delhi Police SI Exam, 21.06.2015 (Ist Sitting)
A. The velocity of electrons is smaller than that of light
B. The wavelength of electrons is smaller as compared to the wavelength of visible light
C. The electrons have more energy than the light particulars
D. The electron microscope uses more powerful lenses

Answer: The wavelength of electrons is smaller as compared to the wavelength of visible light

Question 7: Which country is in the process of building the largest single Aperture Radio Telescope – FAST
(SSC CGL Tier-I (CBE) Exam. 31.08.2016 (Ist Sitting))
A. Japan
B. China
D. Russia

Answer: B. China

• The world’s largest radio telescope, the Five-hundred-metre Aperture Spherical Telescope or FAST, was installed in Pingtang in China’s Guizhou province.

Question 8: The instrument used to see the distant objects on the Earth is _
(SSC Tax Assistant (Income Tax &Central Excise) Exam. 29.03.2009
A. Terrestrial telescope
B. Astronomical telescope
C. Compound microscope
D. Simple microscope

Answer: A. Terrestrial telescope

Question 9: One can distinguish a telescope from a microscope by observing _
(SSC CPO(SI, ASI & Intelligence Officer) Exam. 28.08.2011 (Paper-1)
A. length
B. colour
C. size of the lens
D. length and size of the lens
Answer: D. Length and size of the lens

Question 10: The magnifying power of an astronomical telescope can be decreased by _
(SSC Combined Matric Level (PRE) Exam. 27.05.2001 (IInd Sitting (East Zone)
A. decreasing the focal length of the eyepiece
B. increasing the focal length of the eyepiece
C. increasing the focal length of the objective
D. None of these
Answer: B. Increasing the focal length of the eyepiece

Question 11: Where is the largest reflecting telescope in Asia?
(SSC Tax Assistant (Income Tax &
Central Excise) Exam. 25.11.2007)
A. Kodaikanal
B. Ooty
C. Kavalur
D. Nainital

Answer: D. Naini Tal

• A reflecting telescope is able to collect light from outer space using a highly reflective surface or a mirror. A reflecting or reflector telescope uses a single mirror or a series of curved mirrors to collect and focus light in order to make an image.

• The largest reflecting telescope in Asia is located in the Devasthal  Observatory at Nainital, Uttarakhand, India. The 3.6m Devasthal Observatory Telescope (DOT) is currently the largest reflecting telescope in Asia.

FAQs on List of Optical Instruments

Q1: Who invented the microscope in 1666?

Answer: Dutch tradesman Antoni Van Leeuwenhoek.

Q2: Who invented the first microscope?

Answer: In 1590, Two Dutch spectacle-makers and a father-and-son team, Hans and Zacharias Janssen created the first microscope.

Q3: Who invented the simple microscope in 1665?

Answer: scientist Robert Hooke

Q4: Who invented the compound microscope?

Answer: Two Dutch spectacle-makers Hans and Zacharias Janssen [because they use a two-lens system]

Q5: What optical instrument produces a virtual image?

Answer: Microscope

Q6: What optical instrument uses concave lenses?

Answer: Telescopes, cameras, lasers, glasses, binoculars (most of the advanced optical instrument uses a combination of concave and convex lenses)

Q7: What optical instruments use convex lenses?

Answer: Magnifying glasses, Eyeglasses, Cameras, Microscopes

Q8: What optical instruments contain plane mirrors?

Answer: kaleidoscope and Periscopes

Q9: Which optical instruments work like a human eye?

Answer: Cameras work very similarly to how the human eye works.

Q10: Which optical instrument uses 2 convex lenses?

Answer: The Compound Microscope

Q11: Optical instruments based on the principle of interference are known as?

Answer: Optical Flat(It is used in conjunction with a monochromatic light to measure the difference between two surfaces)

Q12: Which optical instrument works on the principle of laws of reflection?

Answer: A periscope is based on the Laws of Reflection

Q13: Who invented the prototype of the electron microscope for the first time?

Answer: Ernst Ruska

Q14: Who invented the electron microscope?

Answer: Ernst Ruska and Max Knoll

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