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The figure at the bottom shows a beam of light
intercepted by a plane glass surface. Part of the incident
light is reflected by the surface; that is, it travels as a beam away
from the surface as if it bounced from the surface. The rest of the
light is refracted by the surface; that is, it travels as a beam
through the surface into the glass. Unless the incident beam is
perpendicular to the glass, the light always changes its direction of
travel when it crosses through the surface ; for this reason, the incident
beam is said to be "bent" at the surface.
Note that the angles of the incident,
reflected and refracted rays are measured from the normal of the
surface. Experiments show that these laws are applicable to reflection
and refraction.
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Law of Reflection
- q1'
= q1
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Law of Refraction -
n1sinq1
= n2sinq2
(Snell's Law)
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Why
Snell's Law holds - very neat proof!
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Here the "n's" are constants
called the index of refractions. These constants are obtained by the
dividing the speed of light (c) by the actual velocity of the light traveling
in the medium (v), in other words ( c / v ). Below is a
table of some common indices of refraction.
| Name |
n
at 20oC |
-dn/dt |
Dispersion |
| Water |
1.333 |
Slight |
…. |
| Acetone |
1.357 |
…. |
Slight |
| Ethyl
Alcohol |
1.362 |
0.0004 |
Slight |
| Ethyl
Butyrate |
1.381 |
…. |
Slight |
| Methyl
Butyrate |
1.386 |
…. |
Slight |
| Ethyl
Valerate |
1.393 |
…. |
Slight |
| Amyl
Alcohol |
1.409 |
0.00042 |
Slight |
| Kerosene |
1.448 |
0.00035 |
Slight |
| Petroleum
Oil |
1.470
to 1.477 |
0.0004 |
Slight |
| Monochlor-naphthalene |
1.626 |
…. |
Moderate |
| Monobrom--naphthalene |
1.658 |
0.00048 |
Moderate |
| Methylene
Iodide |
1.737
to 1.741 |
0.0007 |
Strong |
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| Medium |
Index |
Medium |
Index |
| Vacuum |
exactly 1 |
Typical Crown Glass |
1.52 |
| Air (STP) * |
1.00029 |
Sodium Chloride (salt) |
1.54 |
| Water (20o) |
1.33 |
Polystyrene |
1.55 |
| Acetone |
1.36 |
Carbon Disulfide |
1.63 |
| Ethyl Alcohol |
1.36 |
Heavy Flint Glass |
1.65 |
| Sugar Solution (30%) |
1.38 |
Sapphire |
1.77 |
| Fused Quartz |
1.46 |
Heaviest Flint Glass |
1.89 |
| Sugar Solution (80%) |
1.49 |
Diamond |
2.42 |
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*0oC and 1 atm
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Note that all the above are for a wavelength of
589 nm (yellow sodium Light)
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An important thing to remember is that glass for
instance will not always have the same n values, the differences
occur when different wavelengths of light are used, as in graph below.
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The index of refraction as a
function of wavelength for fused quartz. Light with a short
wavelength, corresponding to a higher index of refraction, is bent more upon
entering quartz than light with a long wavelength.
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