Refraction


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.

 

Law of Reflection - q1' = q1

Law of Refraction - n1sinq1 = n2sinq2      (Snell's Law) 
Why Snell's Law holds - very neat proof!

 

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

 

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
*0oC and 1 atm
Note that all the above are for a wavelength of 589 nm (yellow sodium Light)

 

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.
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.

1 800 295 5693 

Data from Fourth Edition of Fundamentals of Physics