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Changing the Speed of Light

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Background

September 2001

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

Refraction at an air/water interface - follow this link to a Java simulation which shows the principle of refraction

 


Physicists say they can effectively catch a light pulse in a bottle, hold onto it and release it, in an operation described as slowing light to a dead stop

 


Abbe Refractometer
This instrument measures the refractive indices and mean dispersion of glass, as well as other transparent and translucent liquids and solids.

 

Last Modified 10/12/2001