Different Optical Instruments

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What are Optical Instruments?

Optics and optical instruments are the devices that process photons to enhance images for viewing/analyzing their characteristics. Eyes is a natural optical instrument viz: a camera that allows us to do the same.

Whether or not we're a photographer, every day we deal with lenses, images, and keeping objects in focus. However, these visual marvels are possible because of the laws of Physics.

Like in a camera when light passes through the lens and hits the digital sensor in the back of a camera, we get a recorded photograph. Similarly, there are different optical instruments about which we will learn in this article.

Optical Instruments Example

Let’s look at the optical instruments list:

  • Eyes

  • Lenses

  • Magnifying glass

  • Telescope

  • Microscope


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If we look at the structure of eyes when light falls on the eyes, the iris controls how much light enters the eye, it expands when it’s dark all around and contracts in the presence of bright light.

The lens is controlled by the muscles called the ciliary muscles that change the focal length to focus on varying objects at varying distances. The incoming rays then enter the cornea and reach at the back of the eye viz: retina.

The retina (acts as a sensor) captures the image and sends it to the brain in the form of electric signals. And, there is a small area, which is 1/4 mm in size lies in the center of the retina called the fovea. The fovea is a source of sharp central vision that helps us in reading a book or focusing on a single point.


On average, the near vision of an eye is 25 cm. If a person’s near point is less than 25 cm, then the person can see the nearby objects easily, however, he faces difficulty in observing the far-off objects. In this case, concave lenses are employed to focus the far-off diverged light rays to the retina.

However, when the near point is farther than 25 cm, this person can see the far-off objects and faces difficulty observing the nearby objects. Such a person is said to have an optical defect called Hyperopia or farsightedness.

In such a case, if an image is brought very close to the eye, the eyes can’t converge the light rays at the retina instead they converge outside the retina and the image of an object seems blurred.

For this, convex lenses are employed to converge and focus the light rays on the retina.

Magnifying Glass

A magnifying glass a converging lens that produces a virtual image of an object and enlarges it.

Now, if we wish to see a small object viz: tree, and place it before the focal point of the lens (far from the near point), the lens diverges these rays and forms a virtual image of the tree, and enlarges it.

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Every object takes some amount of light vision by how much angle an object is subtended. However, if you bring the tree closer to the eye, the image gets blurred.

So, if we desire to look very far away objects like stars, planets, and so on, for this, we need an astronomical telescope.

Astronomical Telescope

A telescope contains an objective lens on the end of a closet and an eyepiece at the other end. This eyepiece converges the image formed by an objective lens.

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The parallel light rays coming from the far-off objects fall on the objective lens, this, in turn, converges these rays to form a real, flipped image inside the telescope. Since due to minute size, the eyepiece works as a magnifier and enlarges the virtual image.


Microscopy is the science of observing objects at a microscopic level. A microscope is an optical instrument that helps observe objects that are not visible by the naked eye.

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There are four types of microscopes; these are:

  • Compound microscope

  • Stereomicroscope

  • Light microscope

  • Digital microscopes, and

  • Pocket or handheld microscope

Out of these compounds and stereo microscopes are commonly used.

A compound microscope comprises two systems of lenses for greater magnification; these are:

  • The first lens or the objective lens is nearest to the object.

  • The ocular or eyepiece lens magnifies the image formed at the objective lens for viewing it with a naked eye.

If the microscope comprises a mirror, the mirror is used to reflect light from an external source of light up through the bottom of the area.


A periscope lets us explore the world by sitting at one corner of our room through a video streaming from our mobile or laptop. It is an optical instrument that uses a group of devices viz: prisms, lenses/mirrors to reflect images via a tube, and that tube is called the Periscope tube.

The light coming from a distance source hits the top mirror and is then reflected at an angle of 90° down the tube.

FAQ (Frequently Asked Questions)

Question 1: How do you Calculate the Magnifying Power of a Magnifying Glass?

Answer: The magnifying power is determined by how much angle an object subtends. So, if an unaided eye subtends an angle of 2, and if the object subtends some angle, then the virtual image subtends an angle of 8 magnifying power can be calculated as:

M = (angle subtended by the virual image)/(angle subtended by unaided eye) = ፀ’/ፀ

= 8/2

So, the magnifying power of a magnifying glass is M  =  4.

Question 2: How do you Calculate the Magnifying Power of a Telescope?

Answer: The magnifying power of a telescope can be calculated by using the following formula:

M = (angle between the center of the objective lens and the height of the real image)/(angle subtended by an unaided eye) = - ፀ’/ፀ

The negative sign indicates that the image is flipped. Here, angle ' is the amount that the object subtends when viewed from the eyepiece.

In terms of focal length:

M = (focal length of the objective lens)/(focal length of the eyepiece) = - fo/fe

Question 3: What are the Advantages of a light Microscope?

Answer: The advantages of a light microscope are as follows:

  • Easy to use, small, and lightweight.

  • High observational quality, and remains unaffected by electromagnetic fields.

  • Don’t require radiation to function.

Question 4: List the Three Types of Lenses.

Answer: The types of lenses are:

  1. Normal lenses

  2. Zoom lenses

  3. Telephoto lenses