What are Eyeglasses?
Eyeglasses are more commonly known as glasses are the ones that are used to correct vision. Eyeglasses are simple concave or convex lens that works by adjusting the beam of light on the retina to form a clear image. This use of eyeglasses dates back to the 13th century. The first use and development of eyeglasses or glasses are surrounded by conflict as both eastern countries and western countries claim to be the first to develop eyewear. In this article, we will focus on the brief anatomy and functioning of the eye, defects in the vision, types of lenses used in glasses, and their mode of action.
Eyes
The eyes are one of the most important sensory organs, they are involved in receiving the sensory stimulus, according to which image is formed on the surface of the retina. This image is now interpreted by the brain, the optic nerves are the major nerves that carry the stimulus from the eye to the central nervous system and the motor neurons brings back the information from the brain. It is notable that a simple act of seeing things has quite a complicated and intricate mechanism associated with it. Any problem in vision, precisely the improper image formation on the retina can be corrected using an eyeglass. They work by adjusting the light rays to form the image. To understand the functioning of an eye it is important to understand the basic anatomy and physiology of the eye.
The Structure
The structure of the eyes include the following main parts,
The cornea is the frontal covering of the eye. It plays role in refracting and bending of light that enters the eye. The cornea does not have any blood vessels associated with it is because blood vessels can form a cloudy layer that hinders the ability of the cornea to bend light. Since there is no blood vessel associated with it the cornea is supplied with nutrients via tears and the aqueous humor.
The iris is the ring-shaped structure around the pupil. It is responsible for giving distinct colours to every person. Iris has an opening called a pupil that can adjust according to the light. The pupil and iris combined to decide the amount of light that enters the eye.
The pupil, these are the adjustable opening for the iris. Pupils are the little black dot on the centre of the eye, they can dilate or contract in response to the light.
The lens, these are the organs that focus on the light that is they change their shape to focus on the incoming light, the ciliary muscles help in contraction or relaxation of the lenses. Their anatomical position is right behind the iris.
The retina is the surface on which an image is formed, it has photoreceptors and blood vessels, photoreceptors are the light sensing cells. The region in the retina called the macula contains a large number of photoreceptor cells known as the cones. These are the cells that help in creating the visual details of a sensory stimulus.
Structure of an Eye
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Defects in Vision
The defects in vision can be due to various reasons, in this article we will mainly focus on the defects that are caused due to improper image formation on the retinal plane. The bending of light is known as refraction, the improper refraction of light on the retinal surface leads to the formation of a blurry or unclear image. It is important to note that such problems can be categorized into two groups called nearsightedness and farsightedness. Both of these can be managed by using corrective glasses that help incorrect refraction patterns of the light to focus light on the retainal surface to create a clear image.
Nearsightedness - It is the condition where a person can not focus light to form a clear image of the retina. The nearsightedness is also known as myopia, in this case, corrective glasses that concave lens are used as part of treatment. In myopic condition, light rays refract in such a way that images are formed in front of the retina rather than on the retina.
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Farsightedness - It is the condition where the iris does not contract properly, the refraction of the light forms an image that is focused behind the retinal surface, in this case, a person can not form a proper image of the things that are placed at a relatively large distance. Another term used for it is hyperopia.
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Astigmatism - It is also a very common refractive disease, it the disease where the cornea is in uneven shape since the cornea is the organ responsible for the refraction, any defects in it will lead to an uneven refraction pattern which will lead to blurry vision.
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Eyeglasses
There are various types of eyewear available, some of them are corrective whereas others can be categorized into bio optics, mixed double frames, or sunglasses. It is important to note that only corrective glasses are used to treat the problems associated with defective refraction.
Corrective Eyeglasses
These are the lens that is used to treat defective refraction patterns. There are two types of lenses namely concave and convex, these lenses are used to focus images on the retina. Convex lenses are used for hyperopia, in this case, the image is formed behind the retinal membrane, since convex light focuses the ray over a short distance, these lenses are used to treat hyperopia. When a person with farsightedness uses a convex lens the light rays get converged and the image gets formed on the retinal surface creating a clean visual.
In case of myopia or shortsightedness, corrective lenses that contain concave lenses are used. Concave lenses generally diverge the light rays. In myopic conditions the person forms an image in front of the retinal surface when the concave lens is used as corrective eyewear by the patient, refracted light rays get diverged and form the image on the retinal surface creating a clean vision.
Blurry vision usually causes headaches and pain in the frontal lobe of the brain, it can be due to stress on the optic nerves of a patient, the use of corrective lenses can help to reduce the pain.
Patients that suffer from Astigmatism, the defect in the corneal shape are prescribed the corrective glasses according to the need of the patients. In this case, a type of soft contact lenses also known as toric lenses is used to correct the refraction pattern. Toric lenses can bend light more in one direction compared to the other.
The eyeglass frame of a corrective lens constitute of the following parts,
Eye wires or rims that surround and holds the lenses in place
A bridge connecting the two eye wires
Nose pads which allow the eye wires on the nose
Pad arms connect the nose pads to the eye wires
Earpieces
FAQs on Eyeglasses
1. What are eyeglasses and how do they function to correct vision?
Eyeglasses are optical devices consisting of a frame that holds a pair of lenses. They function by correcting refractive errors in the eye. The lenses in the eyeglasses precisely bend (refract) the light rays before they enter the eye, adjusting the focal point so that light focuses directly onto the retina. This process compensates for the eye's inability to do so on its own, resulting in clear and sharp vision.
2. What is the main difference between eyeglasses for nearsightedness (myopia) and farsightedness (hypermetropia)?
The primary difference lies in the type of lens used to correct the specific vision defect.
- For Myopia (nearsightedness): Eyeglasses use concave lenses. These lenses diverge light rays to move the eye's natural focal point backward onto the retina.
- For Hypermetropia (farsightedness): Eyeglasses use convex lenses. These lenses converge light rays to move the eye's natural focal point forward onto the retina.
3. What type of lens is used in eyeglasses to correct myopia, and what is its function?
To correct myopia, eyeglasses use a concave lens. In a myopic eye, images of distant objects are focused in front of the retina, causing blurriness. The function of the concave lens is to diverge the incoming light rays before they reach the eye's lens. This effectively increases the focal length, pushing the final image back so that it forms precisely on the retina, thus providing clear distance vision.
4. What type of lens do eyeglasses use to correct hypermetropia, and why?
Eyeglasses use a convex lens to correct hypermetropia. This is necessary because a hypermetropic eye has a focal point that falls behind the retina, making it difficult to see nearby objects clearly. A convex lens is a converging lens; it increases the overall refractive power and bends light rays more, pulling the focal point forward so that it lands directly on the retina for a sharp, focused image.
5. What does the 'power' of an eyeglass lens, measured in dioptres, actually represent?
The 'power' of an eyeglass lens, measured in dioptres (D), represents its ability to bend light. It is mathematically defined as the reciprocal of the focal length of the lens measured in metres (Power = 1 / focal length). A lens with a higher dioptre value (e.g., +4.0 D or -4.0 D) is stronger and bends light more significantly than a lens with a lower dioptre value (e.g., +1.0 D or -1.0 D).
6. Why is the power of a lens for correcting myopia always negative, while for hypermetropia it is positive?
The sign of the power is determined by the type of lens, based on standard optical sign conventions.
- A concave lens, which is used for myopia, diverges light and has a virtual focal point, giving it a negative focal length. Therefore, its power is always negative (e.g., -2.5 D).
- A convex lens, used for hypermetropia, converges light to a real focal point, giving it a positive focal length. Consequently, its power is always positive (e.g., +2.0 D).
7. How do bifocal eyeglasses work to help a person see both distant and nearby objects clearly?
Bifocal eyeglasses have a single lens with two distinct optical powers, designed to correct vision at two different distances. The lens is typically divided into two sections:
- The upper section has the prescription for correcting distance vision (e.g., for myopia).
- The lower section contains a segment with added converging power (a more convex prescription) for correcting near vision (e.g., for reading).
8. What is presbyopia, and how are the corrective eyeglasses for this condition designed?
Presbyopia is an age-related vision condition where the eye's natural lens stiffens and loses its ability to change shape to focus on close objects. Eyeglasses designed to correct presbyopia use a convex (converging) lens to provide the additional focusing power needed for reading and other near tasks. These can be simple 'reading glasses' with a single prescription, or if the person also has a distance vision issue, they would use bifocal or progressive lenses to incorporate both prescriptions.
9. Can eyeglasses also correct a condition called astigmatism? If so, how?
Yes, eyeglasses are highly effective at correcting astigmatism. This condition arises from an irregularly shaped cornea or lens, causing light to focus at multiple points instead of one. To correct this, eyeglasses use a special cylindrical lens. Unlike a spherical lens that has the same curve all over, a cylindrical lens has curvature in only one direction. This precisely compensates for the specific irregularity of the eye, ensuring all light rays are brought to a single, clear focal point on the retina.
