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meaning When a ray of light strikes a plane mirror, the light ray reflects off the mirror. Reflection involves a change in direction of the light ray. The convention used to express the direction of a light ray is to indicate the angle which the light ray makes with a normal line drawn to the surface of the mirror. The angle of incidence is the angle between this normal line and the incident ray; the angle of reflection is the angle between this normal line and the reflected ray. According to the law of reflection, the angle of incidence equals the angle of reflection. These concepts are illustrated in image 1.
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Image Fig. 1. Reflection (#2.)
Image Fig. 2. Reflection (#3.)
In image 2, the ray of light approaching the mirror is known as the incident ray (labeled I in the diagram). The ray of light that leaves the mirror is known as the reflected ray (labeled R in the diagram). At the point of incidence where the ray strikes the mirror, a line can be drawn perpendicular to the surface of the mirror. This line is known as a normal line (labeled N in the diagram). The normal line divides the angle between the incident ray and the reflected ray into two equal angles. The angle between the incident ray and the normal is known as the angle of incidence. The angle between the reflected ray and the normal is known as the angle of reflection. The law of reflection states that when a ray of light reflects off a surface, the angle of incidence is equal to the angle of reflection.In fact, reflection of light may occur whenever light travels from a medium of a given refractive index into a medium with a different refractive index. In the most general case, a certain fraction of the light is reflected from the interface, and the remainder is refracted.
To view an image of a pencil in a mirror, you must sight along a line at the image location. As you sight at the image, light travels to your eye along the path shown in the diagram below. The image 3 shows that the light reflects off the mirror in such a manner that the angle of incidence is equal to the angle of reflection.
Image Fig. 3. Reflection of Light (#3.)
It just so happens that the light that travels along the line of sight to your eye follows the law of reflection. If you were to sight along a line at a different location than the image location, it would be impossible for a ray of light to come from the object, reflect off the mirror according to the law of reflection, and subsequently travel to your eye. Only when you sight at the image, does light from the object reflect off the mirror in accordance with the law of reflection and travel to your eye. This truth is depicted in image 4 below.
Image Fig. 4. True Image (#3.)
For example, in image 4, the eye is sighting along a line at a position above the actual image location. For light from the object to reflect off the mirror and travel to the eye, the light would have to reflect in such a way that the angle of incidence is less than the angle of reflection. In Diagram B above, the eye is sighting along a line at a position below the actual image location. In this case, for light from the object to reflect off the mirror and travel to the eye, the light would have to reflect in such a way that the angle of incidence is more than the angle of reflection. Neither of these cases would follow the law of reflection. In fact, in each case, the image is not seen when sighting along the indicated line of sight. It is because of the law of reflection that an eye must sight at the image location in order to see the image of an object in a mirror.
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Where n2 is index of refraction of the medium with higher index.
Image Fig. 5. Multiple total internal reflections (#4.)
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The index of refraction of medium is dependent on the frequency of incoming light ray. Therefore if light with with wide frequency spectrum (such as daylight) enters for example glass prism, the component frequency bands (for visible colors) will diffract under slightly different angles. The result is light divided into colorful "strip" for each of basic colors. The very same effect is behind rainbow which is natural occurrence of dispersion.
Image Fig. 6. 5 Diffraction of light in prism (#5.)
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Refraction is described by Snell's law, which states that for a given pair of media and a wave with a single frequency, the ratio of the sines of the angle of incidence θ1 and angle of refraction θ2 is equivalent to the ratio of phase velocities (v1 / v2) in the two media, or equivalently, to the opposite ratio of the indices of refraction (n2 / n1)(#6.):
(#6.)
Fig. 7. Refraction (#6.)
Common Example:
Going from a medium of lower refractive index, to a medium of higher refractive index, the beam bends toward the normal. In opposite case, when light travels from a higher refractive index to lower refractive index, the beam bends away from the normal. 
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Fig. 8. Refraction of light in water (#8#7.)
Related Video:
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<ac:structured-macro ac:name="anchor" ac:schema-version="1" ac:macro-id="e7f447ccbf6263eb-a51eaf6d-4821432c-9cf5b3a1-dfc61a308b999ddbb2405d42"><ac:parameter ac:name="">3.</ac:parameter></ac:structured-macro> 3. _Reflection and Its Importance_. (n.d.). Retrieved December 10, 2012, from Physics Classroom: \[http://www.physicsclassroom.com/class/refln/u13l1c.cfm\] |
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9. Lally, S. (2012, October 25). Reflection and Refration . Retrieved December 10, 2012, from How Things Work - Towson University Physics 103: http://howthingsworkclass.blogspot.fi/2012/10/reflection-and-refration-reflection.html