Refraction

Refraction is bending of light rays that occur when passing from one medium to the other. With law of physics light has a constant speed when traveling through some transparent medium, but light travels with different speeds through mediums like vacuum, air, glass, water, etc. The fastest speed of light is through vacuum and somewhat slower through other mediums.

Because of this difference of speed, light will either “slow down”, or “speed up” when passing from one medium to the other. If the light enters different mediums from angle of 90 degrees, it won’t change its direction – only speed. But if light enters different mediums from some other angle, the effect of “speeding up” or “slowing down” will make it change its original direction.

If light enters from less dense material to more dense material, it will refract toward normal. If light enters from more dense material to less dense material, it will refract away from normal. The intensity of refraction will depend on the optical density of medium and angle of light.

Lens is an object designed to either converge or diverge light. It is created from optically denser material than air (usually glass) and has at least one surface curved. This curvature is essential and will determine whether the lens will diverge or converge light. Convex lenses converge light, while concave lenses diverge light.

Imagine many parallel light rays coming from the air to the curvature of the lens (denser material). Each of these light rays will enter lens from different angle and will refract differently. Only the central ray will not refract since its angle is 90 degrees. All other rays will enter lens at different angle and will refract toward normal of the lens curvature at that point. As I mentioned, if the lens is convex, angles will cause the light to converge and if it is concave, angles will cause it to diverge.

Refraction of an eye is no different from every other form of refraction. Eye has some optically transparent mediums: cornea, humor aqueous, lens and vitreous body. Cornea is optically thicker than air, while humor aqueous is less dense. Lens of an eye is denser than humor aqueous and vitreous body.

From the anatomy of eye and laws of refraction we can see that refraction only happens when light enters and exits cornea and enters and exits the lens. Most important factor in refraction is curvature where light enters the medium, so we can simplify and say that the two parts of an eye responsible for refraction are frontal curvature of cornea and frontal curvature of lens. Since they are both convex, we can easily deduct that both cornea and lens converge the light.

Only convex lenses have real focus. Focus is a point where all light rays that pass through the lens meet. Concave lenses don’t have real focus, since they diverge light. Their focus is imaginary and can be projected in front of the lens.

The power of lens is the capability of refraction. Lenses that are more curved and have thicker optical density possess high capability of refraction and are more powerful.

The power of lens is measured in diopters. Convex lens that focus parallel light rays one meter behind lens has optical power of 1 diopter. Lens with 3 diopters focus light 1/3 of meter behind lens. Since convex lenses have real focus, diopters for convex lenses usually have plus prefix. For example: +1.00, +2.00, +3.00, etc.

Concave lens diverge light and they have no real focus, but imaginary focus that can be projected in front of the lens. Concave lens with the power of 1 diopter has imaginary focus 1 meter in front of it and lens with power of 3 diopters has imaginary focus 1/3 meter in front of it. Because of imaginary focus diopters for concave lenses have minus prefix. For example: -1.00, -2.00, -3.00, etc.

Human eye when focused for distance has refractive power of around +60.00 diopters. With accommodation this power can increase by around +12.00 diopters.