What is Optics?

Optics is the field of science dealing with light propagation and behavior. It is a very well developed science with an amazing amount of every day application. The term light has a much broader meaning than what we think of when we flip a light switch or walk outside on a sunny day. In Optics, we deal with light that is visible (light bulbs or the sun), infrared, and ultraviolet. Optics plays an everyday role in your life. Holograms, telescopes, glasses, microscopes, lasers, and fiber optics are all consequences under the broad umbrella of Optics.

There are generally two branches of Optics. Physical Optics deals with the nature of light, its emission and its properties, while geometrical Optics is concerned with the path traveled by a ray of light in reflection, transmission, and refraction.

A Brief History

Optics is possibly the oldest discipline of physics. There is a long list of well-known names scattered along the historical path of Optics: Schroedinger, Huygens, Faraday, Newton, Michelson, Young, Fraunhofer and Rayleigh to name a few. These are people that, through years of research and study, have determined the laws governing the propagation of light.

Around 300 B.C., Euclid in his Optica noted that light appeared to travel in straight lines and described the basic law of reflection. He believed that vision involved rays going from the eyes to the object seen and he studied the relationship between the apparent sizes of objects and the angles that they subtend the eye. Optics developed for many centuries after this, and was mainly concerned with refraction and reflection. Somewhere around 1270 A.D. Witelo completed Perspectiva which remained a standard text in the field of Optics for several centuries. The major contribution Witelo made described a method of creating parabolic mirrors from iron and carried out careful observations on refraction. His work may have directly influenced the creation of lenses used in early microscopes, telescopes, and even glasses.

Around 1590 A.D., Zacharius Jensen constructed the first compound microscope, and in 1608 A.D., Hans Lippershey constructed the first telescope. Lippershey's telescope directly influenced Galileo to construct his own and begin astronomical viewing from his bedroom. This led to more investigation into the reflective, and refractive properties of light. Polarization, intensity, and eventually wave-particle duality aspects of light have developed and been utilized in modern science. What may now be considered the ultimate triumph in Optics, the laser, was developed in 1960. Lasers (Light Amplification by Stimulated Emission of Radiation) have become the cornerstone of many technically dynamic fields in modern physics and engineering.

Lasers are used everywhere today. A laser beam at the counter reads the prices of your groceries, your phone call is probably transferred via laser light through an optical fiber, a laser reads your CDs, and your vision can be improved using laser surgery.

The 1600s and 1700s led to rapid discovery of light phenomena, and the field has grown by many orders of magnitude ever since. For a more complete historical progression, see sources below.



What is Light?

So we've established that Optics deals with light. What exactly is light then? Light is fundamentally a form of energy, and simply stated, is a photon traveling through space. To humans and other animals gifted with sight, light is the visible part of the electromagnetic radiation (EMR) spectrum. Visible light has wavelengths that range from 400 to 760 nanometers (nm). This is a tiny part of the EMR spectrum. X-rays, radio waves, microwaves, infrared, and even gamma rays are all part of the same spectrum. They are only different in that they all have different wavelengths, and only those waves with wavelength 400 to 760 nm cause the sensation of vision. Waves with wavelengths between 400 and 760 nm are called visible light. You can see these in the form of color. Think of ROY G BIV. Red has longer wavelengths (around 760 nm), all the way down to violet (somewhere around 400 nm). And there is every other color in between.

There originally were two main theories describing light:

It has become known to physicists in the last 100 years, that light is really both a particle and a wave. This is termed wave-particle duality. If we follow this path of explanation, we quickly delve into quantum physics, but can be somewhat simply described:
Optical image formation displays both the wave and particle properties of light.
This really means that light is a definite particle moving in a wave motion. It's different from sound, which is energy moving in a wave in a separate medium. It's also different than a baseball, which is a particle of some size. It's both. Imagine a baseball moving with wave motion, then shrink it REALLY small. That's where we are today.



Optical Phenomena

The most commonly understood optical phenomena are:


Reflection and refraction are really two different phenomena resulting from the same occurence. When a ray of light is incident (collides) at a boundary between two mediums, part of the ray is reflected back into the first medium. This phenomenon is called reflection. The remainder of the ray is bent in its path as it enters the second medium. This phenomenon is called refraction. Think of this as light hitting a pool of water. Some of the light reflects back off the water and creates the mirror image you see reflecting off the water. The rest of the light refracts into the water. That is, some of the light passes into the water but is bent as it passes from air to water (one medium to another). This is a major principle of Optics.

Refraction and reflection are both described by Snell's Laws.

Reflection:

Snell's law of reflection follows:

Θi = Θr
Where Θi is the angle of incidence, and Θr is the angle of reflection. So, employing the vernacular, the angle at which the light comes in is the same as the angle it leaves. So the light won't get "bent." Reflection looks something like this:



v               ^
 \              /
  \            /                     
   \          /
    \        /
     \      /       (Air)
      \    /
Θi     \  /   Θr
========\/=============== (Mirror)
========================
So that's how the mirror in your bathroom works every morning. How 'bout that?

Refraction:

Remember, refraction is where light hits a medium (say water in a lake) and some of it is reflected, while some of it penetrates the new medium. Most often, light travels differently in the new medium, and tends to "bend." This bending simply means that the direction the light will travel in the new medium will be different than the direction it was originally travelling.

Snell's law of refraction describes this:

sin(Θt) / sin(Θi) = n1 / n2

n1 and n2 are the indices of refraction for the two mediums. The index of refraction, n, is basically a ratio of the speed of light in a vacuum, to the speed of light in the medium (light slows down in different mediums). This is determined experimentally for the most part, and these values can be looked up. Θt is the angle that the light bends to when it enters the second medium.

So this equation explains why light refracts in water. Stick your arm in a pool and you'll see that it looks like your arm is closer under the water than out of the water. That's due to refraction. Check out this diagram for a little clarity:


      v           
       \        
        \                Medium 1 (Air)           
         \      
          \     
           \ 
            \    
          Θi \  
==============\================
          Θt  /
            /
          /
        /
      /                  Medium 2 (Water)
    /
  /
v
Notice the light in Medium 2 travels at a different angle and direction than the original light.

So, refraction can describe how lenses work to some degree, but lenses also depend on how concave and convex shapes of refractive mediums can either magnify or decrease the size of an image, as well as invert images.

Rainbows are also caused by another aspect of refraction, namely dispersion. Light waves of different frequencies (colors) bend different amounts when refracted. In most cases, this is not noticeable, but prisms make use of this to spread out the spectrum so we can see all of the colors. Violet light bends the most, with each color bending a little less up to red, which bends the least.



Why is Optics Important?

Optics are used everywhere today. Like I mentioned before, they are used in CD players, surgery, communications, delicate cleaning procedures for ancient artifacts, vision, detection, and tons more.

Fiber optic cables use very reflective mirrors to transmit light long distances. This allows for fantastic new communications abilities. There are some great advantages to fiber optic cables over traditional copper cables:

So you can see that fiber Optics is definitely going to be a big player in the future of communications.

Understanding Optics allows us to understand much more of our physical world. This is important for so many reasons. Optics is quickly becoming it's own field of science, by growing out from under the traditional umbrella of physics. So go check it out, there is a lot more to it than this, but I hope this was a good start.



Sources:

  • http://imagers.gsfc.nasa.gov/ems/visible.html
  • http://members.aol.com/WSRNet/D1/hist.htm
  • http://micro.magnet.fsu.edu/optics/timeline/1600-1699.html
  • Ulaby, F.T., "Fundamentals of Applied Electromagnetics." Prentice Hall, Upper Saddle River, New Jersey 07458

Op"tics (?), n. [Cf. F. optique, L. optice, Gr. (sc. ). See Optic.]

That branch of physical science which treats of the nature and properties of light, the laws of its modification by opaque and transparent bodies, and the phenomena of vision.

 

© Webster 1913.

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