Optics • Reflection • Diffuse reflection • Refraction • Index of refraction • Speed of light • Snell’s law • Geometry problems • Critical angle • Total internal reflection • Brewster angle • Fiber optics • Mirages • Dispersion • Prisms • Rainbows • Plane mirrors • Spherical aberration • Concave and convex mirrors • Focal length & radius of curvature • Mirror / lens equation • Convex and concave lenses • Human eye • Chromatic aberration • Telescopes • Huygens’ principle • Diffraction
Reflection Most things we see are thanks to reflections, since most objects don’t produce their own visible light. Much of the light incident on an object is absorbed but some is reflected. the wavelengths of the reflected light determine the colors we see. When white light hits an apple, for instance, primarily red wavelengths are reflected, while much of the others are absorbed. A ray of light heading towards an object is called an incident ray . If it reflects off the object, it is called a reflected ray . A perpendicular line drawn at any point on a surface is called a normal (just like with normal force). The angle between the incident ray and normal is called the angle of incidence , i , and the angle between the reflected ray and the normal ray is called the angle of reflection , r . The law of reflection states that the angle of incidence is always equal to the angle of reflection.
Law of Reflection i r i = r Normal line (perpendicular to surface) incident rays reflected rays
Diffuse Reflection Diffuse reflection is when light bounces off a non-smooth surface. Each ray of light still obeys the law of reflection, but because the surface is not smooth, the normal can point in a different for every ray. If many light rays strike a non-smooth surface, they could be reflected in many different directions. This explains how we can see objects even when it seems the light shining upon it should not reflect in the direction of our eyes. It also helps to explain glare on wet roads: Water fills in and smoothes out the rough road surface so that the road becomes more like a mirror.
Speed of Light & Refraction As you have already learned, light is extremely fast, about 3 10 8 m/s in a vacuum. Light, however, is slowed down by the presence of matter. The extent to which this occurs depends on what the light is traveling through. Light travels at about 3/4 of its vacuum speed (0.75 c ) in water and about 2/3 its vacuum speed (0.67 c ) in glass. The reason for this slowing is because when light strikes an atom it must interact with its electron cloud. If light travels from one medium to another, and if the speeds in these media differ, then light is subject to refraction (a changing of direction at the interface).
- Fall '09
- Light, Total internal reflection, Geometrical optics, rays