# Lens Form and Theory

## The Electromagnetic Spectrum

Posted by on May 5, 2010 in Lens Form and Theory | 0 comments

Produced by the nuclear cauldrons of stars and all matter in the cosmos, energy in the form of electromagnetic radiation permeates our entire universe. Every second of every day we are bombarded with and surrounded by electromagnetic radiation; some bounces off of our bodies, some passes through us, and some we absorb, but most goes undetected and unperceived. Electromagnetic energy travels at the speed of light (2.9x10^8 m/s or 180,000 miles/sec) in the form of a wave. In fact, we classify electromagnetic energy according to its wavelength. Wavelength is defined as the distance between...

## Lens Power

Posted by on May 5, 2010 in Lens Form and Theory | 1 comment

As light rays pass through a lens with power, the rays are bent or refracted. In a lens with a plus power, the light rays converge or are refracted toward one another. The point at which the light rays converge is called the focal point and in a plus lens, is behind the lens surface. In a lens with a minus power, the light rays diverge or are refracted away from one another. If these rays are extrapolated or traced back toward the light source, the lines will converge and form a focal point in front of the lens surface. The lens power is relative to the focal distance or the distance...

## Lens Form: Sphere, Cylinder, and Axis

Posted by on May 5, 2010 in Lens Form and Theory | 7 comments

It can be helpful to think of very basic lens forms in terms of prisms. Recall, as light passes through a prism it is refracted toward the prism base. Minus lenses therefore resemble two prisms apex to apex spreading light rays outward as they pass through the lens, while plus lenses resemble two prisms base to base converging light rays as they pass through the lens. Of course, most lenses are not comprised of angular prismatic surfaces but consist of curved surfaces. The most basic of these curves is a sphere. The curve on the surface of a spherical lens, if extrapolated in all...

## Prism

Posted by on May 5, 2010 in Lens Form and Theory | 1 comment

Prism can be used to correct vision for an individual whose eyes are not perfectly aligned as with, for example, a patient with strabismus. When the eyes are not aligned, the right and left eye see different images resulting in blurred or double vision. Sometimes the brain can even "shut off" one eye, in an attempt to remedy the vision, resulting in monocular vision and loss of depth perception. Prism can sometimes be used to align the images seen by both eyes, so the eyes can fuse or see the same image, restoring visual clarity and depth perception. Prism, like lens power, is...

## Prism by Decentration

Posted by on May 5, 2010 in Lens Form and Theory | 3 comments

Normally, eyeglasses are fitted with the optical center of the lens directly in front of the eye. If the lens is fit off-center, image displacement can occur due to induced prism. The higher the power or the further the lens is fit off-center, the higher is the induced prismatic effect. Induced Prism To better understand why there is induced prism, the cross-section of a plus lens can be likened to two prisms base-to-base, as the lens is thicker in the middle and thinner at the edges. Likewise, a minus lens can be likened to two prisms apex-to-apex, thinner in the middle and thicker at...