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. What is an optical instrument and what fundamental principle do they use?
An optical instrument is a device that processes light waves to enhance an image for viewing or analysis. The fundamental principle behind most optical instruments, like microscopes and telescopes, is the use of lenses and/or mirrors to cause refraction and reflection of light, thereby forming magnified, diminished, or inverted images of an object.
2. What are some common examples of optical instruments and their primary functions?
Common optical instruments and their functions include:
- The Human Eye: A natural optical instrument that focuses light onto the retina to form an image.
- Microscope: Used to view highly magnified images of tiny, nearby objects.
- Telescope: Used to view magnified images of very large, distant objects like stars and planets.
- Camera: Captures and records an image on a digital sensor or photographic film.
- Spectacles: Used to correct vision defects like myopia and hypermetropia by altering the focal point of light entering the eye.
3. What is the main difference between a simple microscope and a compound microscope?
The main difference lies in their lens system and magnification capability. A simple microscope uses a single convex lens to produce a magnified virtual image and has a limited magnifying power. In contrast, a compound microscope uses a system of at least two lenses (an objective lens and an eyepiece) to achieve much higher magnification. The objective lens forms a real, inverted, and magnified image, which is then further magnified by the eyepiece acting as a simple magnifier.
4. How do the objective and eyepiece lenses in a telescope differ from those in a microscope?
The primary difference is in their respective focal lengths and apertures, which are designed for their specific purposes:
- In a telescope, the objective lens has a long focal length and a large aperture to gather maximum light from a distant object. The eyepiece has a short focal length.
- In a microscope, the objective lens has a very short focal length to produce high magnification of a tiny, nearby object. The eyepiece's focal length is larger than the objective's but still relatively short.
5. Why is the objective of an astronomical telescope designed with a large aperture and a long focal length?
A large aperture and long focal length are crucial for a telescope's performance. A large aperture enhances its light-gathering power, making faint, distant objects appear brighter, and improves its resolving power, allowing for clearer distinction between two close objects. A long focal length of the objective, combined with a short focal length of the eyepiece, contributes to a higher angular magnification, making the distant object appear larger.
6. What is the importance of setting a microscope for 'normal adjustment'?
Normal adjustment in a microscope or telescope refers to configuring the instrument so that the final image is formed at infinity. The primary importance of this setup is to allow for comfortable and prolonged observation. When the image is at infinity, the observer's eye can remain in its most relaxed state, significantly reducing eye strain and fatigue.
7. Why are reflecting telescopes often preferred over refracting telescopes for astronomical use?
Reflecting telescopes, which use a large mirror as the objective, are generally preferred over refracting telescopes for several key reasons:
- No Chromatic Aberration: Mirrors do not disperse light, so they are completely free from the chromatic aberration (colour fringing) that affects lenses.
- Easier Construction: It is mechanically easier to manufacture, polish, and support a large mirror from behind than it is to create a large, heavy, and optically perfect lens supported only at its edges.
- Reduced Spherical Aberration: By using a parabolic mirror instead of a spherical one, spherical aberration (blurriness due to different focal points) can be eliminated.
8. How do corrective lenses in eyeglasses function as simple optical instruments?
Corrective lenses are used to fix vision defects by ensuring light correctly focuses on the retina. They act as simple, fixed-focus optical instruments:
- For myopia (nearsightedness), where the eye focuses light in front of the retina, a concave lens is used. It diverges the incoming light rays just enough so that the eye's natural lens can then focus them properly onto the retina.
- For hypermetropia (farsightedness), where the eye focuses light behind the retina, a convex lens is used. It pre-converges the light rays, assisting the eye's lens to achieve a sharp focus on the retina.
