Most materials have a refractive index greater than one, which means that as light enters the material from air, the angle of the ray in the material will be more nearly "normal" perpendicular to the surface than it was before it entered. Give it a shot: shine a flashlight through a jar of water. Does the beam look straight?
Put a pencil in a dish of water; it looks like it bends where the water and air meet. The diagram to the right shows the refraction of light as it passes from one medium to another. The red line is normal perpendicular to the surface of the material. As the light passes through the second material, the light bends towards the normal because density of the second material is greater than the first.
The angle between the incoming ray and the normal is the angle of incidence. The angle between the refracted ray and the normal is the angle of refraction. So why is learning about refraction important? Lenses are all about refraction. If you want to create an image you need to be able to focus all of the incoming rays into a single point.
If we can direct each beam of light by bending it slightly — a little right for the light in the left side of the beam, a little left for the light in the right side of the beam — then the light will focus into a single point.
This is exactly what a lens does. When a lens is created, there are two main factors in its design. The refractive index of the material, which is how much the material it's made out of slows down the beam, and the angle of incidence.
The greater the angle of incidence, the more bending occurs. This is why wide-angle lenses, which need to bend the light a long way, have such a bulging appearance.
A nearsighted person sees near objects clearly, while objects in the distance are blurred. Farsightedness is the result of the visual image being focused behind the retina rather than directly on it. It may be caused by the eyeball being too small or the focusing power being too weak. Farsightedness is often present from birth, but children can often tolerate moderate amounts without difficulty and most outgrow the condition.
The cornea and the crystalline lens are both important for the eye to focus light. The distance from the magnifying lens to the piece of paper is the focal length. This bending is possible because of the curve of the cornea as well as the change in refractive index as light moves from air into the cornea and then into the aqueous fluid between the cornea and the iris.
Air has a refractive index of 1. This becomes noticeable if you try to look at something when you are under water. Things appear out of focus because the cornea is designed to work with light passing into it from air rather than from water. Wearing swimming goggles under water allows the layer of air to be present. Behind the aqueous fluid is the second lens system. It consists of a convex lens that is soft and pliable.
The ciliary muscle is a circular ring of muscle that attaches all the way around the lens. This ciliary muscle can change the shape of the crystalline lens by stretching it at the edges. It is attached to the lens by zonules ligament fibres that can be tight or loose. When you are looking at a near object, the lens needs to become more rounded at the central surface in order to focus the light rays.
This ability to change focus for close-up objects is called accommodation. The Schachar mechanism can be demonstrated using a Mylar balloon a shiny silver flat balloon that is often used with helium.
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