ch30_amas

LENS A lens is a piece of glass or other transparent material that is specially shaped to change the path of the light passing through it. [] []
 * 30.1 Converging and Diverging Lenses**
 * Converging lens**- cause parallel light rays to all come together to a single point(called the focal point)
 * Diverging lens**-cause parallel light rays to spread out as if they came form a focal point on the other side of the lens.

The figure below illustrate some important terms for a lens. The **principal axis** of a lens is the line joining the centers. The **focal point** is the point at which a beam of a parallel light, parallel to the principal axis, converges. The **focal length** of a lens is the distance between the center of the lens and its focal point. []

If the rays of light coming from an object actually come together to form an image it will be able to be projected on a screen so it is a **real image**. Real image formed by a single lense are always **upright.** []
 * 30.2 Image Formation by a lens**

If the ray can't be projected because the rays do not actually converge at the image then it is a **virtual image**. Virtual image will form an **inverted image**. Ray diagram can also be drawn to locate and describe the image that will be formed by a lens. The usual rays to be drawn are: 1) A ray that comes in parallel to the principal axis and goes out through the focal points. 2) A ray that comes in through the focal points and goes out parallel to the principal axis. 3) A ray that goes straight through the center of the lens. The relationship between the focal length (//f//), object distance (p), and image distance (q) can also be found from the thin lens equation: 1/p + 1/q = 1///f// [] Diverging lens, virtual image (ray diagram) []
 * 30.3 Constructing Images Through Ray Diagram.**

Any image formed by a diverging lens will be upright and virtual. An image formed by a converging lens will be inverted and real if the object is farther than the focal length from the lens, will be upright and virtual if the object is between the lens and the focal point, and no image will be formed if the image is at the focal point.
 * 30.4 Image Formation Summarized**

The Camera** A camera consists of a lens and sensitive film mounted in a lighttight box. In many cameras, the lens is mounted so that it can be moves back and forth to adjust the distance between the lens and film.The lens forms a real, inverted image on the film. The image below show a camera with a single simple lens. In practice, most cameras make use of compound lenses to minimize distortion called aberrations. [|**http://www.physast.uga.edu/~rls/1020/ch7/fig7-04.jpg**]
 * 30.5 Some Common Optical Instruments

A simple telescope uses a lens to form a real image of a distance object. Telescope that use lenses are refracting telescope. Larger astronomical telescope use mirror instead of lenses. A pair of reflecting prism is used in the terrestrial telescope, which produce an image that is right-side up. A pair of these telescope side by side, each with a pair of prisms to provide four reflecting surfaces to turn images right-side up, makes up a pair of binoculars(picture below) [|**http://www.odec.ca/projects/2005/dong5a0/public_html/PPrBinoc.png**]
 * The Telescope**

**[|**http://physics.2lv.in/blog/sites/default/files/microscope1.GIF**]
 * The Compound Microscope

A compound microscope uses two converging lenses of short focal length, shown above. Since the image is farther from the lens than the object, it is enlarged.The instrument is called a microscope because it enlarges an already enlarged image.

A projector is arrange by a concave mirror that reflects light from an intense source back onto a pair of condenser lenses.The condenser lenses direct the light through the silde or movie frame to a projection lens. [] The human eye is similar to the the camera. The amount of light that enters is regulated by the iris, the colored part of the eye that surround the opening calle dthe pupil. Light enters through the transparent covering called the cornea, passes through the pupil and lens is focuseed on a layer of tissue at the back of the eye; the retina.
 * The Projector**
 * 30.6 The Eye**

[]
 * 30.7 Some Defects in Vision**
 * Farsighted** causes image to form behind the retina and **nearsighted** causes image to form in front of it, instead of fromoing images on the retina. These can be corrected by placing corrective lenses in front of the eyes A converging lens is needed to correct farsightedness while a diverging lens is needed to correct nearsightedness.
 * Astigmatism** of the eye is a defect that result when the cornea is curved more in one direction than other, somewhat like the side of a barrel. because of this defect, the eye does not form sharp images. The cure for this is cylindrical corrective lenses that have more curvature in one direction than in another.

[] The above picture shows aberration; defects of lens. The most common of all defects is **spherical aberration**. It is light passing through the edges of a speherical lens will focus at a different point than ight passing close to the center. While **chromatic aberration** is light of different colorswill focus at different points. Correcting spherical aberration requires that lenses be made parabolic in cross section rather than as a section a sphere. To correct chromatic aberration requires that carefully chosen pairs of lenses be used so that their chromatic aberratioon will cancel each other out.
 * 30.8 Some Defects of Lenses**

Work Cited

Hewitt, Paul G. __Conceptual Physics__. Third Edition ed. Menlo Park, California: Scott Foresman Addison Wesley, 1999