Friday 26 December 2008

Physics- light, reflection and refraction

Before you reading it, here’s the instruction: This note may contains something out of the textbook but useful, and most inside the bracket. If you’re not interested, just skip them. Necessary information in bracket will be bold in the doc. verision. Also, the table about "light and shadow" and "lens" will not be shown here. Please download the doc. version. Physics - Light, reflection and refraction Sight and light 1) We can an object because the light beams from luminous object or reflected from non-luminous object strike into our eyes, and the messages send to the brain. 2) Luminous object emits light but non-luminous object can’t. Note that those object emit light under some conditions are not a good example of luminous object. For example, “candle” is not a good example of luminous object. “Burning candle” is better. Properties of light -- there are 4 properties 1) Light is a form of energy. (We can found this through the converting of energy in light bulb.) 2) Light can travel in empty space. (No medium, ether, is necessary.) 3) Light travels in a very high speed, 3*105 kms-1 or 3*108ms-1 (the highest speed available, by the theory of relativity.) 4) Light travels in straight lines. (At least, in a normal condition, e.g. normal gravity field) Application on shadows Sundial in ancient China and shadow puppet.. Law of reflection They refer to 2 rules: AD- incident ray; BD- normal which is a imaginary line perpendicular to the plane mirror, CD; reflected ray Angle ABD is the angle of incident, ai, and the angle of reflection, ar as angle BDC. 1) Incident ray, reflected ray and normal all lies on the same plane. 2) ai,= ar Regular reflection occurs in smooth surface and the image is clear. Diffuse reflection occurs in a rough surface and the image is not clear (we can see it in some different direction) The 4 properties of image that formed by plane mirror is having the same distance to mirror, same size, erect and laterally inverted. There are various application on plane mirrors, for example, the use of plane mirror in interior design increases the spatial feel, and periscopes. The refraction The name and arrangement of light rays are the reflection. FE is the refracted ray, and angle DFE is the angle of refraction, af. (The relation between ai and af is that refers the refractive index of the 2 material, n1 and n2, we have n1sinai=n2sinaf.) This explains why if the density of original material is bigger than the new material, refracted ray bent away from the normal while in opposite, it’ll bent towards the normal.

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