Refraction

Refraction is the bending of light as it travels through the boundary of two mediums. How much the light bends depends on the refraction indices of the two mediums. The refraction index is simply the ratio between the speed of light in a vacuum and the speed of light in the medium. Consequently, the refraction index is also the ratio of the wavelength of light in a vacuum and the wavelength of light in the medium.

Index of Refraction:
n = c / v where c is the speed of light in a vacuum and v is the speed of light in the medium.
n = L1 / L2 where L1 is the wavelength in a vacuum and L2 is the wavelength in the medium.

Here are refraction index values for common materials:
Air 1.0003
Water 1.33
Glass 1.52
Diamond 2.42
Lead Sulfide 3.91

Light will travel slower in denser materials, this slowdown as the light crosses the boundary is what causes the bending.

To better understand how the light is bent, consider a plane flying. If one side of the wing is stalled, the plane will turn. Then, after some time, if both wings are stalled, the plane will follow a straight path one again, but towards a different direction.

As a real life example of refraction, when light hits the water at an angle, the beam of light will bend, its speed as well as its wavelength will also decrease.

The relationship between the angles are described with Snell's Law: sin 1 / sin 2 = V1 / V2 = constant
Modified form of Snell's Law: n1 sin 1 = n2 sin 2

Using the modified form of Snell's Law, simple algebra will reveal information about the beam of light and the properties of the two mediums the light is passing through.

In a special case, when light tries to travel from a dense medium to a medium of lesser density at a certain angle, the critical angle, the light will bend at 90° and travel along the boundary. If, however, the angle of incidence is past the critical angle, the beam of light will reflect within the denser medium. This is called "Total internal reflection." fiber-optics takes advantage of this property to transfer the beam of light along its length. No matter how the fiber twists and turns, the beam of light will follow.

The critical angle can be determined by using the following equation:
Sin c = (n1 / n2) sin 90° = n1 / n2 where n1 and n2 are the index of refraction of the less dense medium and the denser medium respectively.

See also: Interactive Virtual Experiments and Demonstrations

Next article: Lenses. More bending!