ch30_jcms


 * //Chapter 30: Lenses//

//30.1 Background and Snell’s Law// ** When light waves hit a lens they slow down and bend. This happens because one part of the wave hits the lens first and causes it to slow that part of the wave down which makes it bend and pulls the rest of the wave with it and bends the entire wave. Snells’s law is used to describe the relationship between the angles of incidence and the angles of refraction when referring to light and other waves passing through two different media. Sin theta 1/sin theta 2=v1/v2=n2/n1 or n1 sin 1=n2 sin 2. There are many differences between converging and diverging lenses. A lens is a piece of glass that is formed in specific way so that when light hits the lens it bends the rays and forms an image. There are different types of lens converging and diverging. A converging lens is thicker in the middle and thinner on the outside. A converging lens bends light inward towards a focal point on the other side of the lens. A diverging lens is thinner in the middle and thicker on the outside and causes light rays to bend outwards like they came from a focal point before the lens. A converging lens is thicker in the middle (convex) that at the edges and will cause parallel rays of light to converge to a single point. A diverging lens is thinner in the middle that at the edge and will cause parallel rays of light to diverge. The focal length of a converging lens is the distance between the center of the lens and its focal point. If it is a thin lens the focal lengths on both sides will be equal even if the curvature of the lens on either side is not. You can find the focal length of a converging lens using household items like a meter stick, magnifying glass, an index card and sunlight. To find the focal length first attach the magnifying glass to the end of the meter stick at the 0 cm mark. Then hold an index card at the other end, while pointing the meter stick at an object that is far away and fairly large. Use the sunlight coming through the lens to look for an image on the index card; you should see an image as long as you are holding the index card at the right distance from the lens. Once the image is focused, this can be changed by moving the index card forward or backwards, mark on the meter stick how far away the index card is from the lens and that will be your focal length. Ray diagrams show the principle rays that can be used to show the size and location of an image. In order to make a ray diagram the size and location of the object, its distance from the center of the lens and the focal length of the lens must be known. An arrow is used to show the object. To locate the position of an image you only have to know the path of two lines from a point on the object. The path of one reflected line is known, a ray drawn parallel to the principle axis will be refracted by the lens to the focal point. Another path that is known is through the center of the lens where the faces are parallel to each other. A ray of light will pass straight through the center of the lens with no change in the direction of the ray. A ray from the tip of the object goes in a straight line through the center of the lens. The third path known is a ray of light passing through the focal point in front of the lens emerges from the lens and goes parallel to the principle axis. The image is located where the three rays intersect, any two of the three rays can be used to determine the size and location of the image. Thin film interference happens when light reflects off of two surfaces that are very close to each other. Reflected light comes to the eye in two different paths. The light that hits the lower surface has a farther distance to go before it reaches your eye. This extra distance results in the waves from the upper surface and the lower surface being a half wavelength out of phase. This causes destructive interference to occur and a dark band of light to appear. At the same time destructive interference will not occur and a band of light will be seen. A real image can be projected onto a screen, a virtual image can only be seen through a lens. Virtual- rays that can reach your eye behave as they come from the image position. Real- formed by a single diverging lens upside down (inverted) Virtual- when object is far enough away to be beyond the focal point of a converging lens, light from object does converge. **//Camera- lens and sensitive film mounted in a light tight camera box//** **//Telescope//** **//Compound Microscope//** There are three defects in human vision that our lenses can correct, farsightedness, nearsightedness and astigmatism. The eye works much like a camera, the amount of light that enters the eye is regulated by the iris. The iris is the colored part of the eye that surrounds the black opening called the pupil. Light enters the eye through a transparent covering called the cornea. After the light enters the eye it passes through all of the layers and focuses on a layer of tissue at the back of the eye called the retina which is extremely sensitive to light. Since the retina is not the same in all directions there is a small region in the middle of our field of view where we have the most distinct vision, this area is called the fovea. In the fovea things can be seen with much greater detail than at the side parts of the eye. There is also a blind spot in the retina. This is because all the nerves that carry the information leave the eye in a narrow bundle. The image created by the eye is upside down but our brain automatically flips them over. Even though the eye is similar to a camera the way they focus is much different. In a camera the focusing is done by changing the distance between the lens and the film. In a human eye the focusing is accomplished by the cornea. The cornea changes its thickness and shape of the lens to regulate the focal length of the image. Farsightedness is one of the defects that our lenses can correct. A farsighted person has an eyeball that is too short. Therefore the images are formed behind the retina. This means that these people have to hold things farther than 25 cm away in order to focus them. The way to fix farsightedness is to increase the convergence of the eye. This can be done by wearing glasses or contacts that have converging lenses. These lenses will converge the rays of the image that enter the eye on the retina instead of behind it. Nearsightedness is another defect that our lenses can fix. A nearsighted person can see close objects clearly but has trouble see objects that are far away. This means that distant objects are formed in front of the retina and the eyeball is too long. This can be remedied by wearing divergent lens that focus the image on the retina instead of in front of it. Astigmatism is a defect where the cornea is curved more in one direction than the other. Because of this the eye does not form sharp images. This can be fixed by wearing cylindrical lenses that are curved more in one direction than the other. Distortions of images are called aberrations. There are two types of aberrations, spherical and chromatic. Aberrations can be minimized by combining lenses in certain ways. This is the reason why most instruments use compound lenses instead of a single lens. Spherical aberration happens when light focuses at a different point when it enters through the side of a lens than it does when it enters through the center of a lens. Chromatic aberration is a result of different speeds of light consisting of various colors undergoing different refractions. In a simple lens red and blue light bend in different amounts therefore they do not focus in the same place. Achromatic lenses combine multiple simple lenses to correct the defect. People see better in bright light because the pupils are smaller. This means that the light can only pass through the center of the eye where spherical and chromatic aberrations are minimal. Light also bends the least in the center of the lens therefore there is low amounts of focusing required.
 * //30.2 Converging and Diverging Lenses// **
 * //30.3 Focal Length of Converging Lens// **
 * //30.4 How to Draw a Ray Diagram// **
 * //30.5 Thin Film Interference// **
 * //30.6 Real and Virtual Images// **
 * //30.7 Cameras, Telescopes and Microscopes// **
 * Lens is mounted so that it can be moved back and fourth to adjust the distance between the lens and the film
 * Lens forms, real inverted image
 * Aberrations-compared lenses to minimize distortions
 * Shutter-length of time film is exposed to light
 * Diaphragm- opening that light passes through to reach film
 * Size of opening controls the amount of light let through
 * Uses a lens to form a real image of a distant object
 * Real image is projected into space and examined by another lens used as a magnifying glass.
 * Second lens is an eyepiece and is positioned so that the image produced by the first lens is within one focal length of the eyepiece. The eyepiece forms an enlarged virtual image of the real image therefore when you look through a telescope you are seeing an image of an image. Because of this astronomical maps are printed upside down.
 * Terrestrial telescopes are built differently and produce images that are right side up. Binoculars are a type of terrestrial telescope and have two telescopes side by side with a pair of prisms in each telescope to provide four reflecting surfaces so that the image is turned right side up.
 * A compound microscope uses two converging lenses of short focal length.
 * The first lens is the objective lens produces a real image of a close object.
 * The object is enlarged because the image is farther from the lens than the object.
 * A second lens, the eyepiece, creates a virtual image of the original image and enlarges it further.
 * It is called a compound microscope because it enlarges an already enlarged image.
 * //30.8 Defects in Human Vision// **
 * 30.9 Lens Defects **
 * Ray Diagrams **












 * //Microscopes://**




 * //Farsighted:

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