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When a metre stick is immersed
in a bucket of water, the metre stick appears to bend. The
apparent bending of the stick is due to a phenomenon known as
refraction. Refraction of light occurs when light passes
between two materials that have different indices of
refraction. The path of the light that is reflected off the
metre stick to your eye is altered because the light changes
speed. When light rays change speed they also change
direction, or refract.
The speed of light in a vacuum
is 3.0x108 m/s and is generally considered to be a
universal constant. The speed of light does change though as
it passes through materials with different refractive indices.
The refractive index of a material is unique to that material
and can form the basis of identification. For example the
refractive index for a 30% solution of ammonia is 1.3502 while
the refractive index for methanol is 1.3284. Refractive index
determination for an unknown liquid can be accomplished using
a refractometer. A refractometer measures the difference
between the speed of light in the air and some other medium.
This device can provide a refractive index to four decimal
places. Once determined the value can be matched against
characteristic published values.
The ability of a solution to
bend light is affected by the solute concentration. Scientists
are investigating the relationship between the refractive
index of seawater and the level of salinity. The refractive
index of distilled water is 1.3325 at 298K, and typical
seawater is maybe 0.001 to 0.003 more than this. You obviously
need pretty sensitive equipment to measure this. The
refractometer can also be used as a tool to identify
gemstones. There are many types of gemstones and they each
have their own characteristic refractive index.
Table 1. Refractive Index of Some Common
Liquids
|
|
ND20 |
|
Methanol |
1.3288 |
|
Ethanol |
1.3611 |
|
1-propanol |
1.3850 |
|
2-propanol |
1.3776 |
|
Methyl acetate |
1.3593 |
|
Ethyl acetate |
1.3723 |
|
Acetone |
1.3588 |
|
2-butanone |
1.3788 |
|
Hexane |
1.3750 |
|
water |
1.3330 |
The index of refraction can be
determined by other means, but the resulting number may prove
to be less accurate. The refractive properties of a substance
may also be effected by a change in the temperature. Most data
is expressed at a standard temperature and conversion factors
are available to correct for temperatures above or below the
standard temperature. The values in the table above were
determined at 20° C.
Not only does light change
speed as it passes through various media, scientists
have now been able to freeze a beam of light. Nothing
travels faster than light in a vacuum, but scientists have
manipulated clouds of atoms with lasers so that pulses of
light travel through the clouds at one twenty-millionth of
their normal speed--slower than highway traffic. A similar
technique completely halts the pulses. The process of slowing
and stopping light has many research and technological
applications.
Links:
Is The Speed of Light Constant?
How do you test to see if gems are real without
damaging them?
Speed Of Light May Not Be Constant, Physicist
Suggests
Texas A&M University Physicists Have Devised A
Way To Stop Light |
