Concave Lenses

The main characteristic of a concave lens is thinner than the middle edges . Concave lens or a concave lens is often called a negative lens . The types of concave lens based curved surface form are as follows :

1. Bikonkaf lens is a lens that has two concave surfaces .
2. Plan - concave lens is a lens that has a concave surface and a flat surface .
3. Concave - convex lens is a lens with one convex and one concave surface . In this case , the concave surface is more dominant than the convex surface .

Light is refracted by a concave lens is much fainter than the light coming. This is because the rays that come in parallel concave lens refracts the pervasive that concave lens called a diverging lens.

Parallel rays that come on the concave lens refracts the directions as if coming from a point in front of the lens. Point is called the focal point of a concave lens . Because the focal point formed by the intersection of the extension of the rays bias , the focal point of a concave lens is virtual or real. Beam comes on concave lenses could also come from the front or back of the lens. That is why the focus of a concave lens as well be two. Focus in front of the lens is called the focus (F1) and focus behind the lens called the back focus (F2).

                                                       Figure II.20 Illustration concave lens

Like a mirror , the concave lens also siniar that there are three preferentially refracted by the lens . Special rays and refraction properties of the concave lens is as follows :

• a. Beam is parallel to the major axis comes refracted as if coming from the focal point .

                              Figure II.21 Refraction rays that come in parallel to the main axis
                              concave lens

• b . Rays that seemed to come to the rear focal point refracts parallel to the main axis .

                Figure II.22 Refraction rays coming through the focal point of a concave lens

• c . The rays coming through the optical center passed without refracted .

               Figure II.23 Refraction rays coming through the optical center of the concave lens

Equation is the same with the concave lens convex lens equation , namely :
1 1 1
 + = ................................. ( 2.15)
So Si f
- The hi
 M == .............................. ( 2.16)
 so ho

Description :
So = the distance from the object to the optical center of the lens .
Si = the distance from the shadow to the optical center of the lens .
ho = height of objects .
hi = high shadows .
M = magnification of objects .

Concave lens focal distance is negative because it is virtual . Real object located in front of the lens and the calculation is positive . Virtual object is located behind the lens and in the calculation is negative. Real image located behind the lens and in the calculation is positive . Virtual image is located in front of the lens and the calculations are negative .

Concave lenses can be used for patients with nearsightedness ( myopia ) . People who suffer from myopia can not see distant objects clearly because the shadow of the object falls on the retina . With the help of a concave lens , the image is spread so happened to fall on the retina .

When discussing the benefits of a convex lens , there is little mention of binoculars that use a convex lens and a concave lens . Concave lenses on these binoculars serves to reverse the image produced by a convex lens. Thus, the image produced by the telescope to be upright. Binoculars are using a concave lens as inverting the shadow is called the Galileo telescope or binoculars stage.

                              Example II.24 shadow image formation in concave lens