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    Types of Spectacle Lenses

    Information about Spectacle Lenses

    Types of Spectacle lenses and their coatings

    What type of lenses are available to fill prescriptions?

    The simplest form of spectacle or contact lens is the single vision lens, be it concave or convex.  This means that the lens is made to a single prescription to correct a particular vision deficiency, be it short or long-sightedness.

    Convex Lens

    A convex lens has either one or both of its surfaces curving outwards, that is, wider divergence from the plan at the centre. Theses lenses are used to correct long-sightedness (hypermetropia).  The more long sighted a person is, the more convex the lens has to be to correct the vision deficiency by bringing the rays of light into focus.

    A long sighted person's focus is actually some way behind the back of the eye and the convex lens pulls the rays of light together sooner than they normally would be.

    Concave Lens

    A concave lens is the opposite of a convex lens. Here one or both of the lenses surfaces are curved inwards. That is, the centre of the lens is closer to the plane than the edge. A concave lens is used to correct short-sightedness (myopia). A short sighted person's focus is focusing before the back of the eyeball. The concave lens pushes the rays of light further apart so that they arrive together in proper focus at the back of the eye.

    Photochromic Lenses

    These are lenses which react to light, so that in dull conditions they have a soft neutral tint, and in bright light react with a tint appropriate to the strength of that light. Unlike glass, the latest plastic photochromic lenses give maximum protection against harmful ultra violet light and are especially useful against harsh glare or fluorescent lights and VDU screens. These lenses can be supplied in glass or lightweight plastic to suit most prescriptions.

    Bifocals

    The bifocal lens is seen frequently, if decreasingly. It contains two optical corrections. The most common use of a bifocal is for people who have become presbyopic, as we all do with age, and need assistance with both close work and long distance vision. The upper part of the lens is used to correct the shortsightedness, whilst the lower half assists with reading or other close work.  Between the two parts, there is a distinctive line.

    Trifocals

    These can be distinguished from bifocals by the fact that they have three sections, incorporating a correction for middle distance sight. They can be seen visually by the fact that they have two lines separating the different parts of the lens. Some people find these useful once they have been able to adjust to the lines on the lens, but many find them distracting. Few of these lenses are prescribed today.

    Varifocal

    The varifocal or progressive lens is the modern day equivalent of the bi and trifocal lenses.  These lenses have no distracting lines between the different prescription areas, but have a graduated section that moves smoothly from one prescription to the other. These are the lenses of the future and are much more heavily used in many countries already. They provide the dual benefit of being better cosmetically; they do not draw attention to the ageing process, that Presbyopic is in all of us, and don't have the distracting lines across the lens for the wearer.

    Reflection-free coating

    Spectacle lenses these days can be provided with reflection-free coatings which virtually eliminate distracting reflections - from lights or windows for example - on the back or the front of the lens, helping people to see better and to be visually more satisfying. A very fashionable and expensive spectacle frame can be ruined by the glare coming back from reflections on people's lenses.

    Scratch resistant / hardening coating

    People tend not to have the time to treat their spectacles as well as they might. Beyond that there are every day accidents. Scratched lenses can at lest be an irritant to the wearer and look unsightly, and at worst become impossible to use. However, lenses can now be dispensed with a scratch resistant coating, which prolongs their lives.

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    Myopia can be corrected by using lens.

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    Question

    Myopia can be corrected by using _____ lens.

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    Updated on : 2022-09-05

    Solution Verified by Toppr

    Myopia is the human eye defect in which the eye can not see the distant objects clearly beyond few meters. It is also called short-sightedness and can be corrected by using Concave lens.

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    Short-sightedness leads to blurred distance vision, but close vision is usually normal. Short-sightedness is a common problem with numerous treatments.

    Short-sightedness

    EYE CARE Myopia

    Authored by Dr Suchita Shah, Reviewed by Dr Colin Tidy | Last edited 24 Jun 2022 | Meets Patient’s editorial guidelines

    The medical name for short-sightedness is myopia. Eyesight problems, such as myopia, are also known as refractive errors. Short-sightedness leads to blurred distance vision, whilst close vision is usually normal. Short-sightedness is a very common problem that can be corrected by glasses, contact lenses, or laser eye surgery.

    What is a refractive error?

    Before we can understand short-sightedness, we need to understand what a refractive error is.

    A refractive error is an eyesight problem. Refractive errors are the most common reason worldwide for reduced level of eyesight (visual acuity).

    Eye cross-section

    Refraction refers to the bending of light, in this case by the eye, in order to focus it. A refractive error means that the eye cannot focus light on to the retina properly. This usually occurs either due to abnormalities in the shape of the eyeball, or because age has affected the workings of the parts of the eye that have the job of focusing light.

    There are four types of refractive error:

    Short-sightedness (myopia).

    Long-sightedness (hypermetropia).

    Age-related long-sightedness (presbyopia).

    Astigmatism (a refractive error due to an unevenly curved cornea).

    In order to understand refractive errors fully, it is useful to know how we see.

    Anatomy of the eye

    When we look at an object, light rays from the object pass through the eye to reach the retina. This causes nerve messages to be sent from the cells of the retina down the optic nerve to the vision centres in the brain. The brain processes the information it receives, resulting in an image that we can see.

    Focusing the eye

    Light rays come off an object in all directions, as they result from the light around us (for example, from the sun, moon and artificial light) bouncing back off the object. The part of this bounced light that comes into the eye from an object needs to be focused on a small area of the retina. If this doesn't happen, what we look at will be blurred.

    The cornea and lens have the job of focusing light. The cornea does most of the work, as it bends (refracts) the light rays which then go through the lens, which finely adjusts the focusing. The lens does this by changing its thickness. This is called accommodation. The lens is elastic and can become flatter or more rounded. The more rounded (convex) the lens, the more the light rays can be bent inwards.

    The shape of the lens is varied by small muscles in the ciliary body. Tiny string-like structures called the suspensory ligaments are attached at one end to the lens and at the other to the ciliary body. This is a bit like a trampoline with the central bouncy bit being the lens, the suspensory ligaments being the springs, and the ciliary muscles being the rim around the edge.

    When the ciliary muscles in the ciliary body tighten, the suspensory ligaments slacken, causing the lens to become fatter. This happens for near objects. For looking at far objects, the ciliary muscle relaxes, making the suspensory ligaments tighten, and the lens thins out.

    More bending (refraction) of the light rays is needed to focus on nearby objects, such as when reading. Less bending of light is needed to focus on objects far away.

    What is short-sightedness (myopia)?

    Short-sightedness

    Short-sightedness (also known as myopia or near-sightedness) occurs when light coming from distant objects is 'overfocused', so that the point of focus is in front of the retina. It occurs because either the eyeball is too long or, less commonly, because the cornea is too curved. Despite maximum flattening of the lens, the eye is not able to focus the light rays further back and on to the retina.

    Light coming from near objects requires a stronger focusing activity anyway, so in myopia light from near objects is more likely to be focused in the right place.

    People with short-sightedness are not able to see distant objects clearly. The terms short-sightedness and near-sightedness mean exactly what they suggest. When you are short-sighted,you can see objects that are at short (near) distance. Close objects (for example, when reading a book) can often be seen well. This is because when looking at near objects, the light rays come into the eye going slightly outwards. These will focus further back in the eye than light rays that come in straight from distant objects.

    The diagram above shows the differences in focusing between a normal and a short-sighted (myopic) eye.

    What is long-sightedness?

    The opposite of short-sightedness is long-sightedness, which means you can see things clearly that are far away but not up close.

    What causes short-sightedness (myopia)?

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