ab #4: Telescope Optics (continued) Page 5 of 6 ke a refracting telescope using the lenses in the white envelopes on the lab table as follows: arefully insert the thin lens in one of the lens holders. ternatively, you may hold it securely between your fingers shown in the figure at right. Insert the thick lens into ens holder or hold it securely. lign the lenses so that you can look through both in the direction of the paper on the wall eled "NORTH". While keeping the thick lens close to your eye, move the thin lens slowly vard or away from the thick lens. At some point, the word "NORTH" should come into focus. is the image you see upside down & reversed? Is the size of the image larger than what you see with the naked eye? Do you notice anything peculiar around the edges of the letters and paper? mplete the chart on the next page. Your instructor will distribute concave mirrors binoculars. A Newtonian reflecting telescope is located in the northwest corner the lab room. THICK LENS To measure the diameter, place the lens or concave mirror on the ruler at right. Use a meter stick to measure the diameter of the Newtonian telescope's tube. To compute the radius, divide the diameter by two. To find the area, square the radius and multiply it by ♬ (~3.14). To find the Light Gathering Power (LGP) as eye area equivalents, divide the area. of the lens or mirror by the area of the lens of the human eye. To find the Resolving Power (RP), use the following relationship: Resolving power of lens or mirror in degrees Diameter of eye's lens in cm x 0.02⁰ Diameter of lens or mirror in cm o measure the focal lengths of the lenses, mount them in the optical bench as own in the diagram below and adjust the screen until a focused image appears on screen. The distance between the screen and lens is the focal length. Note: Keep light source as far from the lens as possible. You can also use light from a source side such as reflected light from a distant building or from a streetlight. For the cave mirror, reflect the light back to its source and find the point where the light brought to a focus. Use a meter stick to measure the length of the Newtonian escope's tube. screen lens in holder optical bench light source THIN LENS o find the Magnifying Power (MP), divide the focal length of the objective lens or rror by the focal length of the eyepiece. O 6

Part 3 - Keplerian Telescope A telescope magnifies far-off images. Rays hitting the objective are almost parallel and are focused to form an inverted, real image at the focal point of the eyepiece. The eyepiece refracts the light so the rays leave the eyepiece again almost parallel. The object remains inverted, and so the angular magnification is defined as M= -B/a, where the angles are shown in Figure 2. objective eyepiece f. f. f. F e F. h inverted image at infinity Figure 2: A Keplerian telescope, invented by Johannes Kepler in 1611, uses two converging lenses as shown. An objective with focal length fo and an eyepiece with focal length fe focus nearly parallel light rays from a distant object into an inverted image of much smaller size than the object. 1. Assume that a and ß are small angles (much less than 1 radian). Using the ray diagram shown in Figure 2 and considering two appropriate triangles, show that M = -fo/ fe.

PHY.3   What is the focal length of a flint glass converging lens with radii of curvature 4cm and 5cm ? The lens from question 2 is used to produce the image of anobject 0.9 meters in front of the le a. At what distance from the lens is the object's image?