1Chapter 28Reflection and Refraction22Reflection and Fermat’s Principle• Reflection is the process of atoms in a substance absorbing and reemitting light• Light ordinarily travels in straight lines• We also know that, almost always, a straight line is the shortest distance from one point to another• Light, however, prefers to minimize its travel time (which is not always a straight line)33Law of Reflection• A beam of light incident to a reflective surface will bounce off of the surface with the same angle that it hit the surfaceThe animation to the right here shows a ray in incident light reflecting off of a reflective surface. It then shows how the angles to the perpendiculars are the same between the incident and reflected light.44Plane Mirrors• When you see your reflection in a mirror, it is almost like it is a parallel universe behind it (but its not) because of the law of reflection• If you stand 1 m in front of the mirror, your image will appear to be 1 m behind the mirror• All of your skin tones and clothes are also the right color, so that means that reflected light has the same frequency as the incident light55The Law of Reflection and the MirrorThis is an animation of how the rays of light that hit our eye are reflected from a mirror by the law of reflect. Our eyes will trace back the rays behind the mirror, and that is why an image appears to be behind the mirror.66Me in the Mirror77Multiple Reflections88Diffuse Reflection• If a surface is not perfectly flat like a mirror, light will be reflected off of it at all angles (according to the law of reflection)• It is because of this, the reflection of the setting sun on the ocean stretches the sun out into a column instead of a circle99Refraction• The speed of light is slower in a medium than in a vacuumThis is an animation comparing how long light that interacts with material take to reach the right side of the screen compared to light traveling through a vacuum. The light that is transmitted through a material takes longer because of the absorption and emission process of light in the material.1010Cause of Refraction• When part of a wave front encounters a medium, it slows down•The slowing causes a bending of light towards the normalWe almost always define angles with respect to the normal when we are talking about light and optics.1111Total Internal Reflection• When moving from a medium with a slower speed of light to a higher speed, there is a special situation where the emerging light is bent to the surface• Any larger angle of an incidence angle will result in the light being reflected off of the surface back into the medium• This is called total internal reflection1212Lenses• Lenses refract light to change the path of the light• There are two types of lenses:– Converging: bring rays of light together at a focus point– Diverging: spreads rays of light so that they never come together1313Images• There are two different types of images:– Real: an image that can be focused onto a piece of paper– Virtual: an image that appears to be in a location, but isn’t really there• We see virtual image all the time– The image we see in the mirror really isn’t behind the mirror, it just seems to be• We commonly see real images as well– This slide projected onto a screen is real image1414Ray