Refraction and Lenses Refraction is the when a ray changes mediums. Examples of mediums: Lenses are optical devices which take advantage of the refraction of light to 1. produces images real and 2. change the size of the image. Lenses have to be carefully constructed to produce clear images. (avoid any ) Over the history of time, many different shapes with many different materials have been created but the most basic shapes are listed below. Match the following shape with the correct definition for lenses. note these terms are adjectives for the shape of the lens. Concave: hollowed or rounded inward like the inside of a bowl Convex: curved or rounded outward like the exterior of a sphere or circle Concave convex: a lens that has a concave section and a section that is convex. Part 1 Convex and Concave Lenses Which is converging and which is diverging? 1. Using the 3 slit cover in the ray box, aim the rays at both a concave and a convex lens such that they are perpendicular to each lens as indicated in the sketch below. 2. Sketch, not on your diagram sheet but on the pictures below,what happens to the 3 rays to determine which lens is converging and which is diverging. centre line centre line Convex lens is a lens Concave lens is a lens 1
Part 2 the Optical Centre "O" 1. On a blank piece of paper, trace the shape of a concave and convex lens, with each lens in the middle of the paper. Label each lens as concave or convex. 2. Using the single slit in the ray box, aim the ray at the lens and move the ray box until the light rays travel straight through the lens without bending. Mark dots (or x's) on the ray to be drawn in later. Find two other rays which also travel straight through the lens and mark these lines. Remove the ray box and with a straight edge, draw the 3 rays. (note the optical centre should be at the intersection of the centre line and the principal axis but lenses are much more difficult to manufacture. See application note below.) 3. Add a centre line to each diagram. The centre line is a line that divides the lens in two as indicated in the diagram from part 1. 4. Add the principal axis to your diagram. The principal axis should meet the centre line at a right angle and it should contain the optical centre. Part 3 Finding the focal point of each lens 1. On the back of your sheet trace the each lens like you did in Part 2. Including in your drawing a principal axis and a centre line. Use techniques from Part 2 to help you find the P. and the centre line. 2. Change the plastic cover on the ray box so that the light creates 3 parallel rays. Aim the middle ray along the principal axis and indicate the path of the other 2 rays with 2 small "x" marks. Your middle ray should remain on the P. after it passes through the lens. Indicate the path of the refracted rays with small dots. Once the light is removed, draw in both the incident and reflected rays. 3. Convex lenses have "real" focal points. Label this point on your convex diagram. Concave lenses have "virtual" focal points. Extend the refracted rays back from your concave diagram using dotted lines to find this "virtual" focal point. 4. Measure the length from the Focal Point to the Optical Centre for each lens. Record this value below the diagram using lines and arrows as indicated below. This length is called the focal length. http://amazing space.stsci.edu/resources/explorations/groundup/ Application Telescopes It is difficult to make a quality lens. Lenses were used in the first telescopes. Galileo adapted an optical instrument in 1609 and pointed it up into the night time sky. Galileo was amazed at what he saw. Galileo used the information he collected to argue in support of Copernicus's Sun centered model of our Solar System. As telescopes developed, the difficulty with developing lenses became too great a challenge. In 1680, Newton changed one of the lenses to a concave mirror a much easier device. The result of this change was to have greater magnification and improved image quality. Today, the major telescopes of the world use mirrors which are easier to construct. In 1993, using a combination of smaller mirrors, a 10 m diameter telescope was built in Hawaii. 2
Ray Diagrams Convex Lens A convex lens is a converging lens because parallel rays will. For any object, millions and millions of rays are emitted in all directions. Some of these rays hit the lens and are. Because these refracted rays converge, multiple rays that leave an object happen to reunite at one location. This location is where the image is formed. Like mirrors, 3 special incident rays can be used to find the image. 3 Special Incident Rays 1. An incident ray which is traveling parallel to the principal axis will 2. An incident ray which travels through the secondary focal point (F') will 3. An incident ray that travels through the optical centre will. Note 1 only 2 rays need to be drawn to find the location of the image. When drawing 3 incident rays, if the rays aren't carefully drawn, or if the lens is not perfect (spherical aberration), then the 3 rays will not meet at exactly the same spot. Note 2 properly label each ray diagram before you draw any rays. Important labels include the Principal Axis (PA), Optical Centre ( ), Principal Focus ( ) and its symmetrical partner F' Note 3 light refracts (bends) at each interface but for symmetry reasons, we draw the bending at the centre line. 3
For each of the following, find the image and state the characteristics of the image. How is a convex lens similar to a concave mirror? 4
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