[Concept] : Optics

Ever wondered why the beautiful rainbow occurs?  Have you ever seen water on road (and it being disappearing when you go close) on a hot summer day? Ever wondered how our eye works? Heard the stories of Archimedes destroying enemy ships by focusing sunlight using lenses over enemy ship?

Well, all the answers lie in optics- one of the most fascinating topics in physics.

What is light?

Well in common terms, anything visible to our eyes is called light. Technically light is a part of a very wide electromagnetic spectrum of which a very small part which our eyes can sense is termed as visible light.

Also light has a very interesting history on what it is. First newton showed that it was a particle (photon) and he and other scientists proved the laws  that a ray of light follows. But later in the nineteenth century English scientist Thomas Young and French scientist Fresnel showed that light is a wave by showing the interference. Later Maxwell proved that light is an electromagnetic wave by calculating  velocity of electromagnetic wave and showing that it is same as velocity of light. By then it was thoroughly established that light is a wave. But at the end of nineteenth century neared some serious problems came up with the wave nature of light- it couldn’t explain some phenomenon like the black body radiation and the photoelectric effect. ____ explained the black body radiation and photoelectric effect by treating light as a particle. Simultaneously a new field of physics -quantum physics emerged. Now scientists were facing a serious dilemma- light showed interference which was explained  by the wave nature of light while the blackbody radiation and photoelectric effect were explained only by the particle theory- then what was light. Here quantum theory helped which stated that light( this may be extended to matter also )in nature has dual character- it is both simultaneously wave and particle. So finally we came to know what is light- it is both

Some basic laws followed by light:

Light is a ray i.e, it travels in a straight line.

• Laws of reflection
  • Angle of incidence is equal to the angle of reflection.
  • The incident ray, the reflected ray and the normal to the surface, all lie in the same plane.

As shown in the figure  is the angle of incidence and  is the angle of reflection where PO is the incident ray and QO is the reflected ray.

• Laws of refraction
  •  The incident ray, the reflected ray and the normal to the surface, all lie in the same plane
  •  Product of refractive index and sine of angle of ray with the normal of interface is constant. This is called Snell’s  law. This law has many implications. If there are several medium side by side and you want to find out the angle of refraction of any medium (lets say the fifth medium) you can use this method easily instead of finding each angle sequentially. Just compare with the product of sine angle and refractive index of  any known medium.

The refraction of light is a consequence of light changing its velocity in different medium. also the product of velocity of light in a medium and its refractive index in constant.

You might have heard that the wavelength of light changes in a medium. This is  to change  its velocity. Velocity is the product of wavelength and the frequency.

Have you ever wondered why the wavelength of light changes to change the velocity y and not the frequency. This is due to fact that energy of light  is the product of planck’s constant and frequency. As the energy of light is constant in any medium the frequency remains constant.

The reflection and refraction has  a widespread application in our daily life. It can be seen almost everywhere- from simple mirrors in our houses to huge ones used in astronomical telescopes, from optical fibre cable to microscopes and to polaroid glasses with which you watch 3d movies. All of these are based on one or more of many effects that light follows. Lets see some of them briefly:

Total Internal Reflection: It is based on the Snell’s law . When a light ray moving from an optically denser to rarer  medium is greater than certain angle called critical angle the whole ray is reflected with no transmission hence there is no loss of light( In normal conditions light is always partially reflected or refracted). Due to this, optical fibres are used in high speed data transmission as there is minimal loss of data.

 is the critical angle

Mirage:  Almost all of you must have seen a mirage on a road on a hot day. It is formed as the layers of air near the road are hotter than the air above hence light is refracted sequentially and you see water on road. You must have also heard of stories of travelers seeing water in a desert. They actually see the reflection of blue sky on the desert due to same effect as on road.

Rainbow: Rainbow is also formed due to total internal reflection of the sun rays and hence dispersion by the water droplets presents in the sky after the rain. Have you ever noticed that rainbow is always formed in the direction against the sun?

Interference: If i ask you a question that if you superimposed two light rays over each other what’ll happen? You might think that ( and it happens in general conditions) the intensity will double and there’ll be more light. But you’ll be amazed to know  that under proper conditions two light sources can add up to darkness!!!!(called destructive interference). It can also add up to more than twice the intensity(constructive interference)

To achieve this the two light sources must be in same phase. You might understand it better it by the following figure:

At the last last the story that Archimedes burned down ships of enemy might not be true as many scientists believe that it is not possible. As to achieve this mirrors of very high quality would be required which was not made that time. Also both the ships would have been in constant motion and focussing the sunlight over one point  would have been extremely difficult!!!!!!!! Infact flaming arrows or catapult throwing fire would have been much easily burned out the ships. He actually built many other things like ship shaker to destroy enemy ships.