1 PHYS 101 - Module 11 1 Summary: • Light propagation, ray model • Reflection (flat surfaces) • Refraction • Total internal reflection • Reflection (curved surfaces); concave and convex mirrors Physics 101 Module 11: Geometrical Optics PHYS 101 - Module 11 2 Light propagation: Phenomena from everyday experience shows us that light usually travels in straight lines: • the sharpness of shadows • beams such as headlights, laser pointers, etc Although this is not always true (remember the goldfish in the fishbowl) this ray model of light is what we often use to interpret the world around us. When we use the ray model to describe reflection, refraction, and the behavior of light in lenses and mirrors, we talk about geometrical optics .
2 PHYS 101 - Module 11 3 Reflection : We’ve already seen the law of reflection. This law works for light in exactly the same way as we’ve seen it work for other wave phenomena: angle of incidence Θ i = angle of reflection Θ f where the angles are measured from the normal to the surface. surface normal Θ i Θ f PHYS 101 - Module 11 4 Let’s look in more detail at what’s happening when we look in a plane (flat) mirror . The light rays emitted from the object (or reflected from the object) strike the mirror and are reflected into the observer’s eyes. She extrapolates these rays backwards and perceives an image “behind the mirror”. The image is called a virtual image (it cannot be imaged onto film or paper, for example) and appears at the image distance d i behind the mirror. Here the image distance d i is equal to the object distance d o . d o d i d i = d o
3 PHYS 101 - Module 11 5 Example of plane mirror geometry : Giancoli Ex 23-2. Minimum height of mirror necessary for 1.60 m person to see himself? Assume eyes are 10 cm below the top of his head. mirror d Independent of the distance from the person to the mirror! PHYS 101 - Module 11 6 Refraction
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- Spring '08
- Light, Total internal reflection, Geometrical optics, rays