ch30_kbrl

 Chapter 30:Lenses 30.1 Converging and Diverging

A glass that has the right shape and it bends parallel light rays and these lines cross and they also form an image. The glass that is responsible for this is a lens. Converging: a lens that is thicker in the middle. This makes the rays of light start out parallel abut then the cover (come to a point)
 * Ex: projection slides

 Diverging: a lens with a thin middle and the light rays are made to diverge (spread apart)
 * Ex: viewfinder on a camera

Principal axis: a line that joins the curvature of the lens Focal point: a point of convergence of light or a point from which it diverges Focal plane: a plane that passes through either a focal point of a lens that is perpendicular to the principal axis. Focal length is the distance between the center of the lens and the focal length. The focal length doesn’t depend on what kind of lens it is.



30.2 Image Formation by a Lens

Unaided Vision: a far away object that is a seen through a small angle. When you come closer to the object the object is seen through a large angle. Magnification: an image that is observe through a wide angle. This is a simple converging lens but it differs from a regular converging lens because you can see more details with this type of lens.

Virtual Image: an image that can be made through reflection or refraction. The person can see it but it can’t be projected on a screen. The reason you can’t see it on a screen has to do with the light not being in focus. This image would be right side up.



Real Image: an image formed by a converging lens and it can be displayed on a screen. This image would be inverted (upside down)





30.3 Constructing Images Through Ray Diagrams

Ray diagrams  show principal rays that are used to figure out the size and location of the image. In order to make a ray diagram you need to know the distance from the center of the lens and the focal length of the lens The object in a ray diagram is represented with an arrow. In order to find where the image position is you have to know the paths of the two rays from a point on the object. You can really use any point but it can’t be on the principal axis.

The three useful rays that you need to make a ray diagram are: 
 * 1) A ray parallel to the principal axis that passes though the focal point after refraction by the lens
 * 2) a ray through the center of the lens that does not change direction
 * 3) a ray through the focal point in fron tof the lens tha temerges parallel to the principal aix after refraction by the lens
 * You only need two rays to locate an image.

Different Types of Ray Diagrams:











30.4 Image Formation Summarized

Converging: When the image is within one focal point then it is a virtual image that is right-side up. If the object is beyond one focal length then there would be a real image and it would be inverted. The place where the image is all depends on where it is in relation to the focal point. If an object is close to a focal point then the image would be farther away. Some examples of this would be a slide projector or a movie projector. If the object is farther from the focal point then the image is closer. An example of this would be a camera. When a real image is formed the object and the image are on opposite sides.

Diverging: When you view and image through a diverging lens the image will always be virtual, smaller and right-side up. When a virtual image is formed the object and the image are on the same side.

30.5 Some Common Optical Instruments

The Camera: In a camera there is a lens that can be moved so that you can adjust the distance between the lens and the film.

This picture shows a simple camera that has one simple lens. Most cameras use compound lens so that aberrations don’t happen as often.

To control how much light gets into a camera has to do with the shutter and the diaphragm. A shutter controls the length of time. A diaphragm controls how much light can get to the film. If you vary the opening size this also varies how much light will get to the film.

The Telescope: In a telescope it uses a lens to form real images of an object that is far away. The image that is being viewed isn’t actually being photographed but it is being projected. After it is projected then another lens magnifies it. The eyepiece is a lens of the telescope that is closest to the eye; it enlarges the real image formed by the first lens. The first lens in the telescope helps to make the image be within one focal length of the eyepiece. The eyepiece makes a virtual image of a real image. A telescope makes an image of an image.



<span style="font-family: Georgia, serif;">This shows telescope diagram of a astronomical telscope. The image that is formed from this telescope is an inverted image.

In a <span style="color: #ffcc66; font-family: Georgia, serif;">terrestrial telescope <span style="font-family: Georgia, serif;">there is a third lens and it produces a right-side up image. A pair of these telescopes die by side makes binoculars. Telescopes that use lenses are <span style="color: #ffcc66; font-family: Georgia, serif;">refracting telescopes.

The Compound Microscope: Objective Lens: In an optical device using compound lenses, the lens closet to the object observed. A compound microscope uses two converging lens and they have a short focal length. The first lens is called the objective lens ; it makes a real image of an object that is close by. The object would be enlarged because it is far from the lens. The next lens in a compound microscope is the eyepiece and it forms a virtual image of the first image.



The Projector:

This is a picture of a lens for a movie projector or a slide projector. A mirror that is concave reflects light into a pair of condenser lenses. After the condenser lens the light goes through the projection lens. The projection lens is on a tube that can slide back and forth and this helps with being able to focus images.



30.6 The Eye

The iris which is the colored part of the eye regulates how much light goes into the eye. The iris surrounds the pupil and the pupil is an opening in the eyeball which light passes through. After the pupil light goes through a transparent covering which is called the cornea. The light focuses on a layer of tissue in the back of the eye called the retina. The retina is extremely sensitive to light. There is a spot on retina and it is called the fovea. There is another part on the eye that carries information that leaves the eye and these are called blind spots. In the camera and in an eye the images are upside down. There are a few differences between the human eye and the camera has to do with how they focus things. In the eye it focuses with help from the cornea. These adjustments are changed by the thickness and the shape of the lens.



30.7 Some Defects in Vision If you have normal vision you can see objects clearly from infinity to 25 cm. Someone that is farsighted forms images behind the retina. The eyeball of a person that is farsighted has an eyeball that is too short. People who are farsighted have to hold items about 25 cm away from them to be able to see it clearly. To fix this you need to have move converging to the eye. In order to do this a person must wear glasses with converging lenses.



Someone that is nearsighted can see objects that are nearby but can’t see objects clearly. A person who is nearsighted has an eyeball that is too long. A way to fix this is to wear glasses that diverge the rays so that the images form on the retina.



Astigmatism happen when the cornea is curved more in one direction than the other. Someone that has this can’t form sharp images. To fix this problem you need to get cylindrical corrective lenses that curve in one way more than the other.

30.8 Some Defects in Lenses Aberrations are distortion in an image produced by a lens. In order to fix this you need to combine certain types of lenses. Spherical aberrations happen when light passes through the edges of a lens and focuses at a different place. You can fix a spherical aberration by covering the edges of a lens. An example of this would be a diaphragm of a camera.



Chromatic aberration happens because of different speeds of light of many different colors and the different refractions that happen to them. Red and blue light bend at different amounts so they will not come into focus at the same place. Achromatic lenses correct chromatic aberrations because they combine many different types of lenses.

<span style="font-family: Georgia, serif;">Works Cited:
 * <span style="font-family: Georgia, serif;">__[]__
 * <span style="font-family: Georgia, serif;">Conceptual Physics: Third Edition with Expanded Technology by Paul G. Hewitt
 * <span style="font-family: Georgia, serif;">[]