Difference between revisions of "Reflection"

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==Basic==
 
 
===Basic===
 
  
 
[[image:reflection.png|right|thumb|300px|Law of reflection (click to enlarge)]]
 
[[image:reflection.png|right|thumb|300px|Law of reflection (click to enlarge)]]
  
  
Reflection is the return by a surface of some of the light which falls on that surface (other parts get absorped or refracted).
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Reflection is the return by a surface of some of the light which falls on that surface. (Other parts get absorbed or refracted).
  
There are two main laws that play a role in this (called "Snell's Laws").
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There are two main laws (called "the laws of reflection") that play a role in this:
# The angle of reflection of a ray (r) of light is equal to the ray of incidence (i).
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# The angle of reflection of a ray ('''r''') of light is equal to the angle of the ray of incidence ('''i''').
# The incident ray (i), the reflected ray (r) and the normal (NO) all lie in the same plane.
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# The incident ray ('''I'''), the reflected ray ('''R''') and the normal ('''NO''') all lie in the same plane. In this case it is the plane of your computer monitor.<br clear=all>
  
<br clear=all>
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The angle of incidence ('''ION''') and the angle of reflection ('''NOR''') are both measured by reference to an imaginary line called the normal ('''NO'''), which is perpendicular to the reflecting surface ('''HP'''), at the point of incidence ('''O'''). The point of incidence is the point at which the incident ray strikes the reflecting surface.
 
 
The angle of incidence (ION) and the angle of reflection (NOR) are both measured by reference to an imaginary line called the normal (NO), which is perpendicular to the reflecting surface (HP), at the point of incidence (O).<br>
 
The point of incidence is the point at which the incident ray strikes the reflecting surface.
 
  
 
Understanding these basic principles are important in order to understand the many gemological terms that are used to describe optical effects caused by reflection.<br>
 
Understanding these basic principles are important in order to understand the many gemological terms that are used to describe optical effects caused by reflection.<br>
  
 
Amongst those effects are:
 
Amongst those effects are:
* [[Lustre]]
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* [[Luster]]
 
* [[Sheen]]
 
* [[Sheen]]
 
* [[Brilliance]]
 
* [[Brilliance]]
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* [[Interference]]
  
===Advanced===
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==Advanced==
  
====Brewster Angle====
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===Fresnel reflection===
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For light that is incident close to the normal (to about 10°), the amount of reflection can be calculated by the Fresnel equation for reflectivity.
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 +
:<math>Amount\ of\ reflection = \frac{(n_2 - n_1)^2}{(n_2 + n_1)^2}</math><br />
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For light traveling from air to diamond that translates to:<br />
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:<math>\frac{(2.417 - 1)^2}{(2.417 + 1)^2} \approx 0.17 = 17%</math>
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===Brewster Angle===
  
 
[[image:brewsterangle.png|right|thumb|300px|Brewster Angle]]
 
[[image:brewsterangle.png|right|thumb|300px|Brewster Angle]]
  
In 1812 Sir David Brewster (1781-1868) deposited a new phenomenom which showed that, at a certain angle, light falling on an optically denser object (like water) the reflected component of the light would be completely polarised in the plane of of the surface it reflects of. In this case horizontal.<br>
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In 1812, Sir David Brewster (1781-1868) described a new phenomenon of light. He found that, when light falls on an optically denser object (like water) at a certain angle, the reflected component of the, unpolarized, light will be completely polarized in the plane of the surface off which it reflects. In the case of water, it would be the horizontal plane.
 +
 
 
He also observed that the angle of the refracted ray was at 90° to the reflected ray (at this specific angle).
 
He also observed that the angle of the refracted ray was at 90° to the reflected ray (at this specific angle).
  
This angle was named ''The Brewster Angle'' and it varies for every material, relating to the refractive index of the material.
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This angle is named ''The Brewster Angle''. It varies for every two materials that are in contact, relating to the refractive indices of the materials.
  
When the refractive indices of both materials are known, one can [[Math |calculate]] the Brewster angle for the reflective material.
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When the refractive indices of both materials are known, one can [[Math |calculate]] the Brewster angle.
  
 
<math>\arctan \theta_B = \frac{n_2}{n_1} </math> (where &Theta;<sub> B</sub> is the angle between the incident ray and the normal).
 
<math>\arctan \theta_B = \frac{n_2}{n_1} </math> (where &Theta;<sub> B</sub> is the angle between the incident ray and the normal).
 
<br clear=all>
 
<br clear=all>
The latter makes it possible to use reflection as an aid of measuring the refractive index of polished stones.<br>
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The latter makes it possible to use reflection as an aid to determine the refractive index of polished stones.
As the reflected ray is completely polarised in the horizontal plane at that specific angle, one could insert a vertically orientated polarizing filter to block all the reflected light (similar to how sunglasses work).
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 +
As the reflected ray is completely polarized in the horizontal plane at that specific angle, one could insert a vertically oriented polarizing filter to block all the reflected light (similar to how sunglasses work).
 +
By measuring the angle of the incident light, at which point no light passes through the polarizing filter, one may calculate the refractive index.  
  
By measuring the angle of the incident light, at which there passes no light through the polarizing filter, you could calculate the refractive index. This is the principle of "The Brewster Angle Meter" developed by Peter Read.
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If one were to use monochromatic light that is polarized in the vertical plane, there would be no need for a horizontal orientated polarization filter as at the Brewster Angle that light will not be reflected.
 +
This is the principle of "The Brewster Angle Meter" developed by Peter Read.
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<br />
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<br />
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'''Next: [[Polarization| Polarization]]'''
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<br /><br />
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'''[[Table_Of_Contents| Return to the Table of Contents]]'''

Latest revision as of 12:18, 30 May 2009

Basic

Law of reflection (click to enlarge)


Reflection is the return by a surface of some of the light which falls on that surface. (Other parts get absorbed or refracted).

There are two main laws (called "the laws of reflection") that play a role in this:

  1. The angle of reflection of a ray (r) of light is equal to the angle of the ray of incidence (i).
  2. The incident ray (I), the reflected ray (R) and the normal (NO) all lie in the same plane. In this case it is the plane of your computer monitor.

The angle of incidence (ION) and the angle of reflection (NOR) are both measured by reference to an imaginary line called the normal (NO), which is perpendicular to the reflecting surface (HP), at the point of incidence (O). The point of incidence is the point at which the incident ray strikes the reflecting surface.

Understanding these basic principles are important in order to understand the many gemological terms that are used to describe optical effects caused by reflection.

Amongst those effects are:

Advanced

Fresnel reflection

For light that is incident close to the normal (to about 10°), the amount of reflection can be calculated by the Fresnel equation for reflectivity.

<math>Amount\ of\ reflection = \frac{(n_2 - n_1)^2}{(n_2 + n_1)^2}</math>

For light traveling from air to diamond that translates to:

<math>\frac{(2.417 - 1)^2}{(2.417 + 1)^2} \approx 0.17 = 17%</math>

Brewster Angle

Brewster Angle

In 1812, Sir David Brewster (1781-1868) described a new phenomenon of light. He found that, when light falls on an optically denser object (like water) at a certain angle, the reflected component of the, unpolarized, light will be completely polarized in the plane of the surface off which it reflects. In the case of water, it would be the horizontal plane.

He also observed that the angle of the refracted ray was at 90° to the reflected ray (at this specific angle).

This angle is named The Brewster Angle. It varies for every two materials that are in contact, relating to the refractive indices of the materials.

When the refractive indices of both materials are known, one can calculate the Brewster angle.

<math>\arctan \theta_B = \frac{n_2}{n_1} </math> (where Θ B is the angle between the incident ray and the normal).
The latter makes it possible to use reflection as an aid to determine the refractive index of polished stones.

As the reflected ray is completely polarized in the horizontal plane at that specific angle, one could insert a vertically oriented polarizing filter to block all the reflected light (similar to how sunglasses work). By measuring the angle of the incident light, at which point no light passes through the polarizing filter, one may calculate the refractive index.

If one were to use monochromatic light that is polarized in the vertical plane, there would be no need for a horizontal orientated polarization filter as at the Brewster Angle that light will not be reflected. This is the principle of "The Brewster Angle Meter" developed by Peter Read.

Next: Polarization

Return to the Table of Contents