Different Optical Instruments

Dhristi JEE 2022-24

What are Optical Instruments?

The study of the light that is visible to the naked eye is known as optics. In optics, we also study the ways by which optics can be used in order to extend human vision and perform other tasks. Optical instruments are instruments that reflect and refract light in order to form images by using mirrors and lenses. For the invention of optical instruments, knowledge of light was required. Some of the optical instruments are telescopes, microscopes, cameras, etc.


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


Optical Instruments Example

Let’s look at the optical instruments list:

  • Eyes

  • Lenses

  • Magnifying glass

  • Telescope

  • Microscope

Let’s discuss each example one by one.


Eyes


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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 centre 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.


Lenses

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 diverge 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 is 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.


Types of Optical instruments

Light Microscope

The instrument that forms enlarged images, of very tiny objects that are not visible to the human eye, by using lenses is known as a light microscope. 


There are four types of microscopes; these are:

  • Compound microscope

  • Stereomicroscope

  • Light microscope

  • Digital microscopes, and

  • Pocket or handheld microscope


Compound Microscope

The most widely used microscope is the compound microscope. Compound microscopes have a minimum of two convex lenses, one or more eyepiece lenses, and one or more objective lenses. The lenses that are close to the object are known as objective lenses while the ones closer to the human eye are known as eyepiece lenses. The objective lenses form an enlarged image inside the microscope of the object being viewed and the eyepiece lenses form an enlarged image of the first image. 


Telescope

The instruments that form enlarged images of far-away objects that appear very small because of the distance such as distant stars are known as telescopes. The type of lenses that are used in the telescope in order to make enlarged images are convex lenses. Telescopes are basically of two types- Reflecting and Refracting telescopes. The telescopes in which a concave mirror is used to collect and focus light are known as reflecting telescopes and the telescopes in which a convex lens is used in order to collect and focus light are known as refracting telescopes. In both the telescopes convex lenses are used in the eyepiece for the enlargement of the image


Periscope

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.


Camera

The optical instrument which helps in the formation of an image and also recording it is known as a camera. The image may be detected by electronic sensors which help in storing the image digitally or it may be recorded on a film. In a camera, the light is passed and entered into a camera through an opening known as the aperture. When the light is passing through the lens, it leads to a formation of a reduced real image. It is possible that in order to bring the image into focus, the lens of the camera may move back and forward. The amount of light striking the sensors is controlled by the shutter and it stays open for a longer time in case of dim lights to let more light in.


Lasers

The lasers are devices that form a focused beam of visible light having one wavelength and one single colour. The waves in the case of laser light are synchronous which lines up the crests and troughs of the waves.


Optical Fibres

Laser lights encode sounds or pictures in their pulses which are sent through optical fibres. There is no chance that the light can escape the fibre since all of the light is reflected off of inside the fibre. Therefore, even in long distances, the signals of the light remain strong. It is not necessary that only one signal can travel through an optical fibre at the same time. Some of the examples in real life where fibres are used are cable TV, Telephone, Internet Signals, and so on.


Conclusion

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.

FAQs on Different Optical Instruments

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

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 virtual image)/(angle subtended by unaided eye) 

M = ፀ’/ፀ

M = 8/2

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

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

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

M = (angle between the centre 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

3. What are the Advantages of a light Microscope?

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.

4. List the Three Types of Lenses.

The types of lenses are:

  • Normal lenses

  • Zoom lenses

  • Telephoto lenses

5. What is meant by the persistence of vision?

The human eye continues to see the image for some time when the image of the object is formed at the retina of the eye even when the object is removed. This ability is known as the persistence of the vision of the human eye. The image of an object persists on the retina of the human eye for at least 1/16th part of a second. The most common application of the principle of persistence of vision is in cinematography where the movies are projected on a screen which is taken by a movie camera. The picture is projected at the rate of 24 pictures per second.

6. What are some of the differences between a telescope and a microscope?

The most primary difference between a telescope and a microscope is that the telescope is used to see extremely large objects which are at a far-away distance while the microscope is used to see minute objects which cannot be seen by the human eye. In the case of microscopes, the aperture of the objective is smaller while in the case of telescopes the aperture of the objective is larger. The focal length of the eyepiece is greater than that of the objective lens in the case of a microscope while the focal length of the objective is greater in the case of the telescope. 

7. What are some of the differences between a convex and a concave lens?

The lenses which are thicker around their edges than at their centres are known as concave lenses while the ones thicker at their centres are known as convex lenses. The focal point, in the case of the concave lens, lies in the front of the lens and is the point from which the spread light beam appears to originate through the lens. In the case of a convex lens, the focal point lies on the other side of the lens where the light beam is focused. 

8. What is meant by short-sightedness?

An eye condition when objects at a distance start to appear blurred while the objects which are closer are seen clearly is known as short-sightedness or sometimes myopia. It is a very common eye condition. Mild shortsightedness may not be required to be treated. In order to prevent short-sightedness, one should not sit in front of the television or any other screen for too long. Short-sightedness can be treated with the help of laser treatments, corrective lenses, or lens implants.

9. What are some of the similarities between lenses and mirrors?

Convex lens and concave mirror can both have a positive focal length and form images with similar characteristics and both of them can converge parallel rays to a focal point. Convex mirror and concave lenses can both have a negative focal length, formation of small and virtual images only and both of them can diver the parallel rays away from a focal point.

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