List of optical instruments

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List of optical instruments
List of optical instruments


List of optical instruments and their uses of the chapter ray optics  is an important topic under general science. In this article we provide all the major optical instrument with previous year questions asked in SSC. Also, we include frequently asked questions and answer which is very important for various competitive exams like SSC, RaiLway, RRB NTPC, UPSC,  CDS and all other state PSC’s exams.

Optical instruments introduction

What are optical instruments? or Optical Instrument Definition 

• An optical instrument is a device that processes light waves either to enhance an image for viewing or to analyse and determine their characteristic properties.

• Common examples include periscopes, microscopes, telescopes, and cameras.

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 at human eye.  Larger Visual angle means larger Object and similarly Small visual angle means smaller object. The optical instrument enhances the visual angle of the object so that we can see the object larger than its actual size. Or in other words, we can say that the optical instrument creates a larger image of the object, and this final image creates a larger visual angle to 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

Photographic Camera

• A camera is an optical instrument that captures a visual image. At a basic level, the bodies of cameras are a sealed lightproof box with a small hole known as aperture that allows light through to capture an image on a light-sensitive surface usually photographic film or a digital sensor.

Also see Total Internal Reflection Mirage formation

• 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 shutter mechanism.

• Lenses focus the light entering the camera, and the size of the aperture 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, 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 the time for which 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 which 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, is a type of microscope that 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 a least distance of distinct vision?

• It is the distance upto 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.

1. Simple microscope

 Simple Microscope Definition

• A simple microscope is nothing but a magnifying glass which is made up 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 only makes use of one objective lens, its magnification capability is greatly limited. Most simple microscopes only 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. Another lens of short focal length, but large aperture facing the eye is called the eyepiece. 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 unaided eye or naked eye.

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

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

When, v0=distance of 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)

Image formation by a compound microscope
Image formation by a compound microscope


• Telescopes are used to see distant objects which are 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 object on earth surface.

• In naked or unaided eye the distant objects are not seen properly due to the small visual angle which is 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 objective lens and eyepiece, separated by a distance, It is used for observing distinct images of heavenly bodies like stars, planet etc.

• The objective  lens has a large aperture and large focal lengths (fo) and it  faces towards the object

• The eye lens, which is towards the eye, is called the eye lens or eyepiece. It has a small aperture and 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 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 large magnifying power of a telescope fo 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 Periscopes is a simple optical instrument consists an outer case with two plan mirrors which 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 known as “polemoscope” which was developed by Johannes Hevelius.

• First naval periscope invented by Hippolyte Marié-Davy.



Also see Image formed by Plane Mirror


• A Binoculars or field glasses are nothing but two refracting telescopes mounted side-by-side and it is aligned to a point in the same direction, which allowing 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) with 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.

Porro prism binoculars design
Porro prism binoculars design (Image source  Antilived, CC BY-SA 3.0, via Wikimedia Commons)


• A kaleidoscope is an optical instrument which 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, colored 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 which is fitted in the chest of the patient to regularise the irregular heart beats
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 that has two sets of lenses : an occular lens and an eyepiece.
C. A microscope whose lenses are concave.
D. A microscope whose lenses are convex.

Answer: A microscope that has two sets of lenses: an occular 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 compaired 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 has been 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 of 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.

Optical instruments questions and answers

Question 1: Who invented the microscope in 1666?
Answer: Dutch tradesman Antoni Van Leeuwenhoek

Question 2: Who invented the first microscope?
Answer: In 1590, Two Dutch spectacle-makers and father-and-son team, Hans and Zacharias Janssen, create the first microscope.

Question 3: Who invented simple microscope in 1665?
Answer: scientist Robert Hooke

Question 4: Who invented the compound microscope?
Answer: Two Dutch spectacle-makers Hans and Zacharias Janssen [because they use two lens system]

Question 5: What optical instrument produces a virtual image?
Answer: Microscope

Question 6: What optical instrument uses concave lenses?
Answer: Telescopes, cameras, lasers, glasses, binoculars (most of the advanced optical instrument uses the combination of concave and convex lenses)

Question 7: What optical instruments use convex lenses?
Answer: Magnifying glasses, Eyeglasses, Cameras, Microscopes

Question 8: What optical instruments contain plane mirrors?
Answer: kaleidoscope and Periscopes

Question 9: Which optical instruments that works like a human eye ?
Answer: Cameras work very similarly to how the human eye works.

Question 10: Which optical instrument uses 2 convex lenses?
Answer: The Compound Microscope

Question 11: 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)

Question 12: Which optical instrument works on the principle of laws of reflection?
Answer: A periscope is based on the Laws of Reflection

Question 13: Who invented the first prototype of electron microscope for the first time?
Answer: Ernst Ruska

Question 14: Who invented the electron microscope?
Answer: Ernst Ruska and Max Knoll


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