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# U10 Cn2 Lens Intro

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Introduction to lenses and ray tracing

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### U10 Cn2 Lens Intro

1. 1. Introduction to Lenses E. Alexander Burt Unit 10 class 2
2. 2. General categorizations: <ul><li>Converging lenses: Parallel rays of light are brought together (“converge”) on a point – the focus. </li></ul><ul><li>Diverging lenses: Parallel rays of light are spread apart as if they had come from a single point – the focus. </li></ul>
3. 3. Converging and Diverging lenses <ul><li>Generally, convex lenses are converging lenses </li></ul><ul><li>Generally, concave lenses are diverging lenses. </li></ul>
4. 4. A few other lens shapes: Lenses come in other shapes, which we will not study in this course. Again, they can be categorized into converging or diverging, depending on the path of parallel rays of light through the lens.
5. 5. Images formed by lenses: <ul><li>Real Images </li></ul><ul><ul><li>If the image can be projected on a screen, it is a real image. </li></ul></ul><ul><ul><li>Real images form on the side of the lens which is opposite the object. </li></ul></ul><ul><li>Virtual Images </li></ul><ul><ul><li>If the image cannot be projected on a screen, the image is virtual </li></ul></ul><ul><ul><li>Virtual images form on the same side of the lens as the object. </li></ul></ul>
6. 6. Converging lenses: Ray Drawing Rules http://kottan-labs.bgsu.edu/teaching/workshop2001/chapter2_files/image006.gif
7. 7. Ray Drawing Rules, continued <ul><li>1: A ray of light parallel to the principal axis will refract through the focus on the opposite side of the lens </li></ul><ul><li>2: A ray of light passing through the focus on the same side as the object will refract parallel to the principal axis. </li></ul><ul><li>3: A ray of light passing through the center of the lens will not refract. </li></ul>
8. 8. Similarity to mirrors: <ul><li>We still use the same terminology to describe the images: virtual or real, upright or inverted, larger or smaller </li></ul><ul><li>The equations for image distance and magnification are still the same </li></ul><ul><li>Many (but not all) of the sign conventions are the same: Positive distances are real. The difference: there is no “positive side” for a lens as there was for a mirror. </li></ul>
9. 9. Diverging lens: Ray Drawing Rules http://kottan-labs.bgsu.edu/teaching/workshop2001/chapter2_files/image008.gif
10. 10. Same three rays, but use the focus on the opposite side <ul><li>A ray of light parallel to the axis will refract as if it had come from the near side focus. </li></ul><ul><li>A ray of light which would pass through the far side focus will refract parallel to the axis </li></ul><ul><li>A ray of light through the center of the lens will not refract. </li></ul>
11. 11. Sign conventions for diverging lenses <ul><li>The focal length of a diverging lens is negative </li></ul><ul><li>The image formed by a diverging lens is always virtual and small, so q will always be negative as well. </li></ul>