what are the properties of light?

Light

Light is electromagnetic radiation that has properties of waves.  The electromagnetic spectrum can be divided into several bands based on the wavelength of the light waves.  As we have discussed before, visible light represents a narrow group of wavelengths between about 380 nm (1 nm = 10^-9 m) and 730 nm. 

Interaction of Light with Matter

Velocity of Light and Refractive Index

The energy of light is related to its frequency and velocity as follows:

The velocity of light, C, in a vacuum is 2.99793 x 10¹⁰cm/sec.  Light cannot travel faster than this, but if it travels through a substance,  its velocity will decrease.  Note that from the equation given above- 

The frequency of vibration, n, remains constant when the light passes through a substance.  Thus, if the velocity, C, is reduced on passage through a substance, the wavelength, l, must also decrease.

We here define refractive index, n, of a material or substance as the ratio of the speed of light in a vacuum, C,  to the speed of light in a material through which it passes, C_m.

Note that the value of refractive index will always be greater than 1.0, since C_m can never be greater than C.  In general,  C_m depends on the density of the material, with  C_m decreasing with increasing density.  Thus, higher density materials will have higher refractive indices.

The refractive index of any material depends on the wavelength of light because different wavelengths are interfered with to different extents by the atoms that make up the material. In general refractive index varies linearly with wavelength. 

Materials can be divided into 2 classes based on how the velocity of light of a particular wavelength varies in the material.

1. Materials whose refractive index not depend on the direction that the light travels are calledisotropic materials.  In these materials the velocity of light does not depend on the direction that the light travels.  Isotropic materials have a single, constant refractive index for each wavelength.  Minerals that crystallize in the isometric system, by virtue of their symmetry, are isotropic.  Similarly, glass, gases, most liquids and amorphous solids are isotropic.
    
2. Materials whose refractive index does depend on the direction that the light travels are calledanisotropic materials.  These types of materials will have a range of refractive indices between two extreme values for each wavelength.  Anisotropic materials can be further divided into two subclasses, although the reasoning behind these subdivisions will become clear in a later lecture.
   
   
   1. Minerals that crystallize in the tetragonal and hexagonal crystal systems (as well as some plastics) are uniaxial and are characterized by 2 extreme refractive indices for each wavelength. 
       
   2. Minerals that crystallize in the triclinic, monoclinic, and orthorhombic crystal systems arebiaxial and are characterized by 3 refractive indices, one of which is intermediate between the other two.

Air, since it is a gas, is isotropic.  The refractive index of air is usually taken as 1.0, although its true value is 1.0003.

When light strikes an interface between two substances with different refractive indices, two things occur. An incident ray of light striking the interface at an angle, i, measured between a line perpendicular to the interface and the propagation direction of the incident ray, will be reflected off the interface at the same angle, i.  In other words the angle of reflection is equal to the angle of incidence.

n_i sin (i) =  n_r sin (r)

Note that if the angle of incidence is 0^o (i.e. the light enters perpendicular to the interface) that some of the light will be reflected directly back, and the refracted ray will continue along the same path.  This can be seen from Snell's law, since sin(0^o) = 0, making sin (r) = 0, and resulting in r = 0.

The fact that refractive indices differ for each wavelength of light produces an effect calleddispersion.  This can be seen by shining a beam of white light into a triangular prism made of glass. White light entering such a prism will be refracted in the prism by different angles depending on the wavelength of the light. 

When light enters a transparent material some of its energy is dissipated as heat energy, and it thus looses some of its intensity.  When this absorption of energy occurs selectively for different wavelengths of light, they light that gets transmitted through the material will show only those wavelengths of light that are not absorbed.  The transmitted wavelengths will then be seen as color, called the absorption color of the material.

Normal light vibrates equally in all direction perpendicular to its path of propagation.  If the light is constrained to vibrate in only on plane, however, we say that it is plane polarized light. The direction that the light vibrates is called the vibration direction, which for now will be perpendicular to the direction.  There are two common ways that light can become polarized.

• The first involves reflection off of a non-metallic surface, such as glass or paint.  An unpolarized beam of light, vibrating in all directions perpendicular to its path strikes such a surface and is reflected.  The reflected beam will be polarized with vibration directions parallel to the reflecting surface (perpendicular to the page as indicated by the open circles on the ray path).  If some of this light also enters the material and is refracted at an angle 90^o to the path of the reflected ray, it too will become partially polarized, with vibration directions again perpendicular to the path of the refracted ray, but in the plane perpendicular to the direction of vibration in the reflected ray (the plane of the paper, as shown in the drawing).

• Polarization can also be achieved by passing the light through a substance that absorbs light vibrating in all directions except one.  Anisotropic crystals have this property in certain directions, called privileged directions, and we will discuss these properties when we discuss uniaxial and biaxial crystals.  Crystals were used to produce polarized light in microscopes built before about 1950. The device used to make polarized light in modern microscopes is a Polaroid, a trade name for a plastic film made by the Polaroid Corporation.  A Polaroid consists of long-chain organic molecules that are aligned in one direction an placed in a plastic sheet.  They are placed close enough to form a closely spaced linear grid, that allows the passage of light vibrating only in the same direction as the grid.  Light vibrating in all other directions is absorbed.  Such a device is also called a polarizer.

Polaroid sunglasses use these same principles.  For example, incoming solar radiation is reflected off of the surface of the ocean or the painted hood of your car.  Reflected light coming off of either of these surfaces will be polarized such that the vibration directions are parallel to the reflected surface, or approximately horizontal (as in the first method of polarization discussed above).  Polaroid sunglasses contain polarizers with the polarization direction oriented vertically.  Wearing such glasses will cut out all of the horizontally polarized light reflecting off the water surface or hood of your car.

The Polarizing Microscope

In optical mineralogy we use a microscope called a polarizing microscope. Such a microscope is equipped with two polarizers that are normally oriented so that their polarization directions are perpendicular to one another. 

Mounted within the microscope tube is a second polarizer, called the analyzer, that can be rotated or pushed so that in can be in the light path (inserted position) or not in the light path (analyzer out position).  The analyzer has a polarization direction exactly perpendicular to that of the lower polarizer These directions are usually referred to as North - South.  If the analyzer is in, then the plane polarized light coming from the lower polarizer will be blocked, and no light will be transmitted though the ocular lens above.

If the analyzer is out, so that it is not in the light path, then the polarized light will be transmitted through the ocular lens.