So draw on the refraction at each surface, emphasising the bending towards the normal as the light goes from a fast to a slow medium. Also emphasise the bending away from the normal as the light changes from a slow to a fast medium. The rest of lens design is about arranging the angles between the two normals, so that you get the right amount of bending. Here we show only convex lenses. Use the animation to show how these are made out of slices of prisms of glass of increasing steepness as you get towards the centre of the lens.
Step 8: The block is removed from the lens, and the lens is washed and inspected. Sometimes special coatings may be applied to the lens. At this point the lens blank has had additional curves ground in the back of the lens and it has been polished. However, the large diameter blank still has to be sized and shaped to fit into the frame selected by the patient.
Several methods are used, depending on the equipment, but they are all based on the following description. Step 9: The lens blank is shaped on a linear lathe called an edger using either a ceramic or diamond grinding wheel or stainless steel blades.
The lens must again be prepared to accept a chuck, but since only the edge is being cut, a much gentler system is used. A small chuck receiver is placed where the geometrical center of the finished lens will be, and the lens is then oriented on the axis.
Usually, only an adhesive pad is needed to hold the receiver on the lens. The lens is chucked in the edger and held in place by a pressure pad that presses on the opposite side of the lens like holding a very large coin between your thumb and forefinger at its center.
Step A pattern in the shape of the frame is inserted in the edger. Patterns are commonly plastic and may be supplied by the frame manufacturer or made in the lab. Newer edgers do not use patterns; instead, the shape is determined by a probe that measures the frame and stores the information in a computer, which in turn controls the edging operation. As it operates, the slowly turning lens is brought into the fast turning cutting surface, which is either a grinding wheel or steel blades, until a guide contacts the pattern, which is rotating to match the lens.
If the frame has a complete rim surrounding the lens, a bevel, or ridge, is cut along the edge of the lens that will fit into a groove in the frame; otherwise, the edge is left flat.
Step The lens is inserted into the frame. Fit and orientation is double checked, any worn screws or hinges are replaced as needed, and the frame is made square. The finished eyeglasses are then thoroughly cleaned and packaged for delivery to the patient.
Glass lenses are ground and polished much the same way as plastic except that diamond cutting surfaces are used, and some details may vary. The blanks are made of relatively soft glass and must be tempered, either by chemicals or heat, to strengthen them before inserting into the frame. Advances in automation are rapidly changing how lenses are made. For example, the vast majority of labs now use computers to determine curve parameters and lens choice, and equipment is available that will combine several steps or even do the entire operation automatically.
For more information about corrective lenses and related topics, check out the links on the next page. Bob Broten is an American Board of Opticianry-certified optician and certified laboratory technician at Lenscrafters Inc.
He holds a bachelor's degree in biology and did extensive research in fish vision while pursuing his degree. Author's note: I am indebted to Erik Schopp, A. O-certified optician and general manager of Lenscrafters , and Dr. Dawne R. Griffith, O. Robert D. Optics and optometry are complex subjects beyond the scope of this article. In presenting the basic principles of these two disciplines, I've oversimplified somewhat for the sake of brevity. For this I apologize. Any errors in fact or theory are entirely mine.
I encourage interested readers to seek professional advice, as this article is a brief overview and not intended as a guide to diagnoses. Also, I am grateful to Lenscrafters store in Portland and to Joshua Boyd, lens technician, for help in taking the photos used with this article. Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close. Mobile Newsletter chat dots. Mobile Newsletter chat avatar.
Mobile Newsletter chat subscribe. Everyday Innovations. How Corrective Lenses Work. Test your eyes. As mentioned above, light rays incident towards either face of the lens and traveling parallel to the principal axis will either converge or diverge. If the light rays converge as in a converging lens , then they will converge to a point.
This point is known as the focal point of the converging lens. If the light rays diverge as in a diverging lens , then the diverging rays can be traced backwards until they intersect at a point. This intersection point is known as the focal point of a diverging lens. The focal point is denoted by the letter F on the diagrams below. Note that each lens has two focal points - one on each side of the lens.
Unlike mirrors, lenses can allow light to pass through either face, depending on where the incident rays are coming from. Subsequently, every lens has two possible focal points. The distance from the mirror to the focal point is known as the focal length abbreviated by f.
Technically, a lens does not have a center of curvature at least not one that has any importance to our discussion. However a lens does have an imaginary point that we refer to as the 2F point. This is the point on the principal axis that is twice as far from the vertical axis as the focal point is. These three rays lead to our three rules of refraction for converging and diverging lenses.
These three rules are summarized below. These three rules of refraction for converging and diverging lenses will be applied through the remainder of this lesson. The rules merely describe the behavior of three specific incident rays. While there is a multitude of light rays being captured and refracted by a lens, only two rays are needed in order to determine the image location. So as we proceed with this lesson, pick your favorite two rules usually, the ones that are easiest to remember and apply them to the construction of ray diagrams and the determination of the image location and characteristics.
Physics Tutorial. My Cart Subscription Selection. Student Extras. Why just read about it and when you could be interacting with it? Interact - that's exactly what you do when you use one of The Physics Classroom's Interactives. We would like to suggest that you combine the reading of this page with the use of our Optics Bench Interactive. You can find this in the Physics Interactives section of our website.
The Optics Bench Interactive provides the learner an interactive enivronment for exploring the formation of images by lenses and mirrors. Its like having a complete optics toolkit on your screen. Visit: Optics Bench Interactive. Refraction Rule for a Converging Lens Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens.
Refraction Rules for a Converging Lens Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens. Any incident ray traveling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.
Refraction Rule for a Diverging Lens Any incident ray traveling parallel to the principal axis of a diverging lens will refract through the lens and travel in line with the focal point i. Refraction Rules for a Diverging Lens Any incident ray traveling parallel to the principal axis of a diverging lens will refract through the lens and travel in line with the focal point i. Any incident ray traveling towards the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.
An incident ray that passes through the center of the lens will in effect continue in the same direction that it had when it entered the lens.
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