Optical Instruments and Human Eye

Ever wondered how is the Navy able to detect any threat while they are underwater? What is the device that helps them detect the threat? Or what’s makes a Kaleidoscope so amusing? And what’re these devices? Well, all these devices are ‘Optical Instruments’. They act as an extra pair of eyes for us. And how do these devices work alongside working the eye? Scroll ahead to know more.

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Optical Instruments – Periscope

Objects that are not in direct line of sight can be viewed by optical instruments named ‘Periscopes’. Periscopes help a lot for navigation under water and are of great help to submariners.

Simple periscopes require only mirrors, but periscopes that are little more complex use prisms instead of mirrors. Submarines use the complex periscopes that require tubes up to 10 meters in length for a greater range of sight.

Recently in news, development in science and technology have led to the use of fiber optics in periscopes.

Construction of a Periscope

A pair of mirrors or prisms alongside a rectangular hollow tube having a window for viewing at the top and bottom are used for constructing the optical instrument ‘Periscopes’. At both ends of the rectangular hollow, mirrors are fixed at an angle of 45o.

Working Principle of a Periscope

Almost all optical instruments including periscopes work on the ‘Laws of Reflection’.Rays of light from the object fall on one of the mirrors and reflect back. These reflected rays then fall on the other mirror and reflect back again until they reach our eyes.

Applications of Periscopes

Periscopes are synonymous with their use in submarines for navigation and to detect threats that are nearby.

Another use of periscopes is in medicine. In comparison to a surgery, periscopes are a less abrasive method for looking into the human body for detection of illness and disease.

Optical Instruments – Kaleidoscopes

Kaleidoscope comes from Greek words meaning ‘beautiful form to see’. Kaleidoscope is a device that consists of a tube containing pieces of mirrors and colored glass or paper, whose reflections produce changing patterns when the tube is rotated.

Working of Kaleidoscopes


When two or more mirrors or reflective surfaces are positioned at an angle to each other, usually that form a V-shape or a triangle; Kaleidoscopes are constructed. The case that we see when we buying or making kaleidoscopes is actually the body surrounding it. Inside this case collection of objects are positioned at one end of the mirrors, and at the other end, there’s an eyehole.

When we look through the eyehole we can see the beautiful reflection of the collection of things. The tube that holds this collection of things is usually as large as the kaleidoscope tube. The reflection of only those things can be seen that fall within the space of the triangle inside the object holder.

Construction of Kaleidoscopes

The optical instruments kaleidoscope consists of a hollow tube with three mirrors placed in form of a triangular tube and with their reflecting surfaces facing each other. The transparent sheet is used to cover one end of the tube and an opaque sheet is used to cover the other end. This opaque sheet has an eyehole in it.

Small pieces of colorful papers and glass are put inside the tube. Hence when we rotate a kaleidoscope, we get to see various patterns in it. These patterns occur because of multiple reflections in it.

The Human Eye

‘Human Eye’ is the organ of vision of the human body that enables us to see.  The human eye(s) are located in the specialized sockets carved out in the human skull. The normal human eye sizes for approximately 2.5 cm in diameter. The main parts of a human eye are:


The white outer region of our eye which protects the internal parts of our eyes is known as ‘Sclera’. Its made up of fibrous tissues. It’s continuous with the cornea.


The transparent portion of our eye that allows the light to enter our eye is known as the ‘cornea’ and is made up of transparent tissue. The cornea covers the pupil, anterior chamber, and the iris. Along with the anterior chamber and lens, cornea refracts light and accounts for two-thirds of the eye’s total power. Usually, the refractive power of the cornea is approximately 43 dioptres.


A circular, thin structure made up of contracting and relaxing muscles in the eye that controls the size of the pupil and the light reaching the retina are known as the ‘Iris’. Iris defines a person’s eye color. If we define the human eye as a camera then the iris becomes aperture of the eye.


The part of the eye located in the center of the iris allowing light to reach the retina. The pupil appears black in color since the eye tissues absorb or diffusely reflect the light entering the pupil. Iris controls the pupil.


The lens is a biconvex, transparent structure present in the eye behind the pupil. The lens along with the cornea refracts the light, so as to focus it on the retina. By changing its shape, the lens is capable of changing the focal distance of the eye. 


The retina is a light-sensitive tissue in the inner coat of the eye that sends electrical signals after converting them from light to the brain for processing. Rods and cones are the two light-sensitive types of cells present in the retina. Rods help us for night-time vision and cones help us see colors. 

Optic Nerve:

The optic nerve sends electrical impulses from the retina, at the back of the eyes to the brain.

Solved Example For You

Q. What is the maximum capacity of a normal human eye to see small objects?

a. 10,000 micrometer

b. 10 micrometer

c. 100 micrometer

d. 1000 micrometer

Sol: c. 100 micrometer

The maximum capacity of a normal human eye to see small objects is 100 micrometers.

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