Borough of Manhattan Community college If a beam of parallel rays passes through a convex lens, refraction causes the rays to be bent The City University of New York (converged) toward a single point, F, as denoted in Figure 1. This point is known as the principle focus, or focal point, of the lens. The distance f, from the center of the lens to the focal SCIENCE DEPARTMENT point F, is called the focal length of the lens. Laboratory Experiment SIMPLE CONVEX LENSES AND TELESCOPES Objective: Focal Point . To determine the focal length of a simple convex lens. To study the images produced by the lens. . To construct a simple refractor telescope. Principal Axis Apparatus: Online simulator for double convex lenses, optical bench, screen and illuminated object Figure 1. ( http://dept.swccd.edu/hlee/content/simulation/simulation-geometric-optics/index.html ) and a A Converging Lens meter ruler. Focal Length Theory: Examine Figure 2 below. The relative distance of an object to a lens, p. to the distance of its image from the lens, q, is determined by the focal length of the lens. The LENS EQUATION provides the exact mathematical relationship among these distances: LENS EQUATION: Concave Convex [object distance (p) x image distance (q)] focal length (f) = ----- ---- [ or simply f = (p*q) / (p+q) ] [object distance (p) + image distance (q)] Simple Concave and Convex Lens: (1) Figure 2. (2 (3) Ray Diagram for a Convex Lens Object "Principal ...... mage Axis Double Convex Lens:Real image: Light rays pass through the image location (the right side of the lens). That is, the You will see a lens, an arrow on the left and the image of the arrow created by the lens on the image is on the other side of the lens that the object is on. right. You should also see a grid you can use to measure distances--the arrow begins at a distance of 200 units from the lens Virtual image: Light rays that appear to come from the location of the image form the image. That is, the image is on the same side of the lens that the object is on. Record procedures 3-7 on your data page (3) Measure the distance between the arrow and the lens (object distance p) and the distance Inverted image: The image is upside down. from the lens to the image of the arrow (image distance q), with the image in focus. (4) Using the lens equation, calculate the focal length (f) of the lens. Show all your work in your write up (data pages report file). The simplest astronomical telescope consists of two lenses, the objective lens and the eyepiece. Like those in Figures 1 and 2, the objective lens is a double convex lens of long focal length, f, (5) Move the arrow to a different distance (such as 250 or 350 or 50 units) from the lens and which forms a real inverted image of a distant object. The eyepiece is a double convex lens of repeat steps (3) and (4), using a different distance between the arrow and lens. That is, use a short focal length, f, used to magnify the image produced by the objective lens different object distance p, and see how the image distance, q, also changes (This is the 2nd Measurement). The eyepiece is used as a magnifying glass, by means of which an enlarged virtual image is formed of an object placed just inside its focus. In this instance the object used is the image (6) Move the arrow to another different distance of your choice from the lens and repeat steps formed by the objective lens. The eye looks directly at the enlarged visual image produced by (3) and (4), using a different distance between the arrow and lens. That is, use a different object the eyepiece. Consider Figure 3. distance p. and see how the image distance, q, also changes (This is the 3"d Measurement) Cardboard (7) Move the arrow to another different distance of your choice from the lens and repeat steps Screen (3) and (4), using a different distance between the arrow and lens. That is, use a different object Long Focal Short Focal Length Lens Length Lens Observer's distance p. and see how the image distance, q, also changes (This is the 4th Measurement) Eye CO (8) Move the arrow to another different distance of your choice from the lens and repeat steps (3) and (4), using a different distance between the arrow and lens. That is, use a different object Optical Bench distance p. and see how the image distance, q, also changes (This is the 5th Measurement). WARNING: Each of you MUST have different lengths in steps 4-8. NO SAME data/lengths from two (or more) students will be accepted, and submitting Figure 3. Refracting identical data points (p and q measurements) in steps 4-8 will result in a zero Telescope grade. (9) Calculate the average value of the focal length (f) of your lens for the two sets of Procedure: measurements. (1) Go to: http://dept.swccd.edu/hlee/content/simulation/simulation-geometric- optics/index.html (10) Using the average value of the focal length calculated for your lens, predict the characteristics (real or virtual, erect or inverted, larger or smaller than the actual object) of the (** You may need to click enter twice after you copy and paste the link into your browser in image formed for the following object distances: order to see the images in the page.) 0.75 times the focal length. 1.0 times the focal length. (2) Scroll down to the simulator labelled 'Converging and Diverging Lens: Finding the Image'. 2.0 times the focal leng 4.0 times the focal length. Now, actually place the arrow at each distance in the simulator to check your predManhattan Community college Step 9: The City University of New York Average value of focal length = (Average = Sum of all 5 values of focal length you calculated in steps 3-8 and divide by 5) SCIENCE DEPARTMENT Show work here: Laboratory Experiment LAB DATA PAGE SIMPLE CONVEX LENSES Step 10: NAME: Object Distances Is it real or virtual? Is it erect or inverted? |Is it larger or smaller than actual Step 3 and 4 - First measurement: object? P = q= (a) 0.75 times the Focal length of lens = focal length Show work here: (b) 1 times the focal length (c) 2 times the focal length Step 5 - Second measurement with different value of p: (d) 4 times the focal P = 9= length Focal length of lens = Show work here: Step 11: Step 6 - Third measurement with different value of p: * For questions 1, 2 and 3, you can zoom-in and zoom-out the virtual lens set-up by hitting the P = Bird and 4th buttons from the right beneath the simulator ("four-arrows-out" button for zooming Focal length of lens = out & "four-arrows-in" button for zooming-in). Show work here: 1- What happens to the thickness of the lens as you increase F? (Default value of F in the simulator is 2 unit/box length from the lens. You can click on F Step 7 - Fourth measurement with different value of p: and hold it with the mouse. You then move it closer to and away from the lens, and P = observe the thickness of the lens. Your answer should be "it gets thicker" or "it gets 9= Focal length of lens = thinner".) Show work here: 2- Look at the rays from the blue arrow where they enter the lens from the left--what Step 8 - Fifth measurement with different value of p: happens to them as you move the arrow farther away--do they become more parallel or p = do they spread out? q= Focal length of lens = Show work here: 3- If you could keep moving the arrow away from the lens, to an enormous distance (like an astronomical object), what would happen to the rays from the arrow