The short answer
An intraocular lens, usually shortened to IOL, is a small artificial lens that a surgeon places inside your eye to replace your eye's own natural lens. Your eye is an optical system built from two lenses working together: the cornea at the front, and the crystalline lens sitting just behind your iris. The crystalline lens is the one that becomes cloudy with a cataract, and it is the one an IOL is designed to replace. Removing the natural lens takes away an important part of the eye’s focusing power. An IOL replaces that contribution and works together with the cornea.
Why the eye needs a replacement at all
A cataract is not a film or coating that can be wiped away or dissolved with drops. It is the natural lens itself losing transparency as its tissue changes. During surgery, the surgeon removes that clouded lens through a very small incision. The eye still needs optical power in that position to focus an image, so an IOL replaces the removed lens.
What it physically is
Today's IOLs are a world apart from the very first artificial lenses used decades ago, which were made of rigid plastic, measured around six millimeters across, and required a large incision with stitches to implant. Many modern IOLs use foldable acrylic; some use silicone. Acrylic materials differ in how much water they contain and how they behave over time. A separate postoperative issue is posterior capsule opacification (PCO): the thin capsule behind the IOL becomes cloudy, rather than the cataract growing back. The cause of any clouding must be checked before choosing treatment. Because the lens is foldable, it can be rolled up, passed into the eye through an incision often smaller than three millimeters, and then gently unfolds into place once inside — which is a major part of why modern cataract surgery is so much less invasive than it used to be.
Filters built into the lens
Modern IOLs include ultraviolet filtering. Some models also filter part of the violet or blue-light spectrum. That extra filter is a design characteristic, not proof that one lens will give you better vision or better general health. Any comparative claim needs evidence for the exact models and outcome being discussed.
Where it sits inside your eye
During surgery, your surgeon works to preserve the thin, transparent capsule that used to hold your natural lens, emptying it of the cloudy lens material but leaving the capsule itself in place. The IOL is then positioned inside that same capsule, which holds it centered and stable behind your iris and pupil — essentially the same location your natural lens occupied for your entire life before the cataract developed. This is one reason the eye tolerates the implant so well: the new lens sits where the body already expects a lens to be, supported by tissue that is naturally part of the eye rather than something foreign to it.
A permanent fixture, not an adjustable accessory
This is the point that catches many patients by surprise: unlike glasses or contact lenses, an IOL is not something you swap out as your preferences or prescription change over time. It is implanted once and is generally meant to stay in your eye for the rest of your life. Removing or exchanging an IOL after the fact is a real surgical procedure in its own right, more involved and more delicate than the original implantation, and it is not something to plan around casually. In practice, you don't get to "try before you buy" the way you might test a pair of glasses for a week. That is exactly why the power and type of lens are calculated carefully beforehand, using measurements of your eye's own optics, and why it is worth taking the decision seriously rather than treating it as an afterthought to the surgery itself.
Why there's more than one type
A young natural lens changes shape and moves focus automatically; this is called accommodation. A fixed IOL does not fully restore it. Some IOLs create one main focus, while others extend or divide the useful range to reduce glasses use. The result for contrast, night symptoms and near vision is model-specific. A wider range does not automatically mean that something else gets worse, and a simple design does not guarantee perfect quality in an individual eye.
Where to go from here
Start with four questions: what is medically suitable, which distances matter, when glasses are acceptable, and what the proposed model actually showed for contrast and night symptoms. This site helps you prepare those questions. The surgeon who examined your eyes must confirm the final model, power and treatment plan with you.