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Physics 1230: Light and Color Lecture 17: Lenses and Ray Tracing

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Physics 1230: Light and Color Lecture 17: Lenses and Ray Tracing Physics 1230: Light and Color Chuck Rogers, [email protected] Ryan Henley, Valyria McFarland, Peter Siegfried physicscourses.colorado.edu/phys1230 Lecture 17: Lenses and ray tracing Online and Written_HW9 due TONIGHT EXAM 2 is next week Thursday in-class 1 Last Time: Refraction all the way through block What was happening in Activity 8? U2L05 3 LastConvex Time: Concaveand concave and convex lenses lenses • Each of the two surfaces has a spherical shape. • Light can penetrate through the lenses and bend at the air-lens interface. 4 We build lenses out of glass with non-parallel sides Glass If slabs aren’t parallel - lens Glass A B C Which ray of light will have changed direction the most upon exiting the glass? We build lenses out of glass with non-parallel sides Put film, Retina here! 7 We build lenses out of glass with non-parallel sides Put film, Retina here! • Light rays bent towards each other… CONVERGING LENS. • The less parallel the two sides, the more the light ray changes direction. • Rays from a single point, converge to a single point on the other side of the lens (and then start diverging again). 8 Converging (convex) lens Light rays coming in parallel focus to a point, called the focal point optical axis Focus f Light focusing properties of converging lens a good light collector or solar oven; can also fry ants with sunlight (but please don’t do that) 10 Light focusing properties of converging lens The “backwards” light collector: create a collimated light beam 11 Ray tracing to understand lenses and images Ray tracing: 1. As long as ray stays in same medium, it goes straight. n>1 n=1 2. At each interface to a different medium, calculate how it will bend. Go back to 1. Rays entering “slower” material Rays entering bend toward “faster” material This gets tedious! normal bend away from normal 12 Thin convex (converging) lens focal length If the glass surface is nearly a section of a } sphere, it will FOCUS parallel rays. A THIN LENS is very thin F F compared to the focal length. Then we can simplify the treatment with THREE SIMPLE RULES. foci 13 Group Activity! • Come on down and get a handout. • Your group should have a small focusing lens • You should have a ruler or come get one. • IT IS AN EXCELLENT IDEA TO HAVE A RULER FOR THE COMING EXAM 2. Please get started on Part 1. A Question: You hold up a focusing lens and look through it at an image of a distant object that is far on the other side of the lens. You find that the image is located at: A)The object side of the lens B)On your side of the lens C)Exactly inside the lens The image is: A) Virtual B) Real Group Activity! Please get continue on Part II. Thin convex (converging) lens: Ray tracing rules focal length 1) A ray parallel to the axis is deflected through the } focus on the other side 1 2) A ray through the center 2 of the lens continues F’ undeviated 3 F 3 3) A ray coming from the focus on one side goes out parallel to the axis on the other foci 17 Ray tracing Where will this ray go? foci (focuses?) 18 RayRay Tracingtracing Where will this ray go? Suppose it’s emitted from this object foci (focuses?) 19 RayRay Tracingtracing We know where these 3 rays go, using the simple ray rules foci (focuses?) 20 RayRay Tracingtracing Amazing property of this lens: all rays from the tip of the arrow will converge to the same point We know where these 3 rays go, using the simple ray rules foci (focuses?) 21 Ray tracing: Thin lens, object outside focus See how the rays emerge from this point (the image)? Amazing property of this lens: all rays from the tip of the arrow will converge to the same point (the image) 22 Ray tracing: Thin lens, object outside focus Amazing property of this lens: all rays from the tip of the arrow will converge to the same point (the image) Eye sees an image The Lens acts as our here. “Magic Ray Machine”, creating the rays to produce an image. 23 Clicker question In this case, the image is: A)Virtual B)Real Eye sees an image here. 24 Clicker question In this case, the image is: A)Virtual B)Real Real because the light rays really go through the image. You can put Eye sees an image a screen there to see it. here. 25 Remember the Pinhole? Pinhole = selecting one set of rays. Dim but always in focus! Many pinholes = many images Lens gathers and bends light from multiple rays to focus to one image Demo and PHET Geometric-optics http://phet.colorado.edu/en/simulation/geometric-optics Idea of magnification and focused image Group Activity! Please get continue on Part II and start to get to G. Ray tracing a convex lens: object inside focus 29 Ray tracing a convex lens: object inside focus The image appears larger (and farther away) than the object. This is a magnifying glass. (Remember: a magnifying glass is a convex lens.) Aside: near-sighted people need concave/diverging lenses; can a marooned myopic start a fire with his eye-glasses? 30 Converging/convex lens: Near objects are magnified and upright. Far objects are upside down and smaller Far object Near object Magnifying glass Group Activity! Please get continue on Part 3. Good place for a break! Enjoy your day. 33 Ray tracing exercise Ray tracing exercise Come get some handouts!.
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