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Unformatted text preview: Lesson 4.6 Properties of Light Until the middle of the 1800's, the generally accepted theory of light was the particle picture. In this viewpoint, advocated by Newton, light was considered to be a stream of tiny particles called corpuscles. However, in the late 1800's, the particle picture was replaced by the wave theory of light. This was because certain phenomena associated with light, namely diffraction, interference and polarization, could only be explained using the wave picture. Visible light is just one particular type of electromagnetic radiation. Other types of electromagnetic radiation include radio waves, infrared radiation (heat), ultraviolet radiation, x-rays and -rays. In the early 20th century, experiments revealed that there were some phenomena associated with light that could only be explained by a particle picture. Thus, light as it is now understood, has attributes of both particles and waves. In this Chapter we will deal mainly with the wave attributes of light. The present day understanding of light is that it is made up of tiny particles called photons which have wavelength and frequency. The energy E of a photon is directly proportional to its frequency f and is given by E = h f .(i) where h is called Plancks constant and has a value given by h = 6.626 10-34 J.s = 4.136 10-15 eV.s (1eV = 1.602 10-19 J) Light is a very complex phenomenon, but in many situations its behavior can be understood with a simple model based on rays and wave fronts. A ray is a thin beam of light that travels in a straight line. A wave front is the line (not necessarily straight) or surface connecting all the light that left a source at the same time. For a source like the Sun, rays radiate out in all directions; the wave fronts are spheres centered on the Sun. If the source is a long way away, the wave fronts can be treated as parallel lines. Rays and wave fronts can generally be used to represent light when the light is interacting with objects that are much larger than the wavelength of light, which is about 500 nm. In particular, we'll use rays and wave fronts to analyze how light interacts with mirrors and lenses. 1. Reflection of Light The figure shows a single ray of light striking a flat reflecting surface such as a plane mirror. The ray of light that strikes the surface is called the incident ray . The ray that leaves the surface is called the reflected ray. A perpendicular line drawn to the surface at the point of incidence is called the normal . The angle between the incident ray and the normal is called the angle of incidence. The angle between the reflected ray and the normal is called the angle of reflection. http://id.mind.net/~zona/mstm/physics/light/rayOptics/reflection/reflection1.html Laws of Reflection Reflected light obeys the law of reflection, that the angle of reflection equals the angle of incidence....
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