Introduction

To love beauty is to see light. Victor Hugo

All of us enjoy the beauty. For example, a beautiful piece of art, a beautiful bloomed flower, nature, etc. Of course,  beauty can be appreciated only if we have eyes. But above that,  even if we have good vision, it can be only seen if there is light. If there’s no light, beauty becomes redundant. 

What is Light? 

Light is  a kind of energy called "electromagnetic (EM) radiation" (but this kind of radiation is not harmful, except for an occasional sunburn). There are other kinds of Electro-Magnetic radiation too like  (radio waves, microwaves, x-rays, etc.), but light is the part we can see, the part that makes the rainbow.

How does light travel?

Light travels in a perfectly straight line until something obstructs it. This straight path of light is called LIGHT RAY.

How FAST does light travel?

About 1,86,000 miles per second that is 300,000 kilometers per second], so the light from the sun, takes about 8 minutes to go 93 million miles [149 million kilometers] to earth. 

Does this seem SLOW? Well, if you could DRIVE to the sun at 60 mph [100 kph], it would take you 177 years to get there! 

In one second, light can go around the earth 7 times!

Concept 1: Light

Light as said above is an electromagnetic wave. 

There are THREE basic ways to control light.

• Block it ... with something. (this makes a shadow)

• Reflect it (change its path with a mirror). This is called a REFLECTION.

• Bend it. Change its direction, by making it pass through, other transparent materials of different densities, like glass or water. This is called REFRACTION,

All these are the properties of light. 

This is shadow which is most common

This is reflection, also commonly observed.

The next property is refraction.

Refraction: Light travels in a vacuum and in a medium. Refraction as said earlier is the bending of light wave when it enters a medium where its speed is different. When it passes from a fast medium into a slow medium the light ray bends towards the normal. The phenomenon of a ray of light deviating from its original path, while travelling from one optical medium to another optical medium is called refraction.

In the above diagram, normal is the imaginary line drawn perpendicular to the boundary between the two media. Light travels faster in air than in water. Hence, as said before the light ray bends towards the normal. 𝜃i and 𝜃 r are the angle of incidence and angle of refraction. As shown the angles are between the light rays and the normal.

During Refraction: Except velocity and wavelength, no other characteristics of light wave changes during refraction.
Phase of vibration of particles remains constant
Frequency remains constant
Speed or velocity changes
Wave length changes

It is because v=ν λ, where v is velocity, 𝜈 is frequency and 𝜆 is the wavelength. With frequency remaining constant, wavelength changes, if velocity changes. 

Consequences of total internal reflection: 

a) A shining air bubble appears in water. The speed of light is faster inside the air bubble than in the water. Thus the light gets reflected away from the bubble around the outside, looking like a mirror. This is only along the outer circle,  in the direction you are looking.

b ) A diamond glitters. Internal reflection occurs when the angle of refraction is equal to or greater than 90 degrees. This occurs more in the diamond because the critical angle depends upon the ratio of the indices of refraction of the air to the stone. The diamond has a very high refraction index.

c) Mirages formed in deserts. Mirages are formed when the ground is very hot and the air is cool. The hot ground warms the layer of air just above the ground. When the light moves through the cold air and into the layer of hot air, it is refracted (bent). A layer of very warm air near the ground refracts the light from the sky almost into a U-shaped bend.

d) Looming is also an optical illusion that occurs usually in very cold regions. In looming, a distant object such as a ship moving in polar areas appears to be hanging in midair due to atmospheric refraction and the total internal reflection of light rays. 

e) The rays of light entering the water from below, suffer refraction. If these rays strike the water-air surface at an angle which is greater than 48°, they get totally internally reflected. These rays on emerging out of the water, appear to come from the upper surface of the water, which in turn appears silvery. Although it is a common phenomenon, we fail to notice it.

Optical Fiber: Total internal reflection (TIR) is a powerful tool since it can be used to confine light. One of the most common applications of total internal reflection is fibre optics. An optical fibre is a thin, transparent fibre, usually made of glass or plastic, for transmitting light.

Total internal reflection is often demonstrated through various experiments. 

In one such demonstration, a beam of laser light is directed into a coiled plastic thing-a-ma jig. The plastic serves as a light pipe, directing the light through the coils,  until it finally exits out of the opposite end. When the light entered the plastic, it was in the denser medium. Every time the light approached the plastic-air boundary, it approached at angles greater than the critical angle. The two conditions necessary for TIR are met, and all of the incident light at the plastic-air boundary stays internal to the plastic and undergoes reflection. 

This demonstration helps to illustrate the principle by which optical fibers work. The use of a long strand of plastic (or other materials such as glass) to pipe light from one end of the medium to the other is the basis for modern-day use of optical fibers. Optical fibers are used in communication systems and micro-surgeries. Since total internal reflection takes place within the fibers, no incident energy is ever lost due to the transmission of light across the boundary. The intensity of the signal remains constant.

Total internal reflection is the basic principle of a very useful branch of physics known as Fiber optics. An optical fiber is a very thin fiber made of glass or plastic having a radius of the order of a micrometre 10 the power of minus 6 m. A bundle of such thin fibres forms a light pipe.

NGSS

MS-PS4-2.

MS-PS4-3.

PS4.A

PS4.B

PS4.C

4.PS3.A (MS-PS4-1); 4.PS3.B (MS-PS4-1); 4.PS4.A (MS-PS4-1); 4.PS4.B (MS-PS4-2); 4.PS4.C (MS-PS4-3); HS.PS4.A (MS-PS4-1),(MS-PS4-2),(MS-PS4-3); HS.PS4.B (MS-PS4-1),(MS-PS4-2); HS.PS4.C (MS-PS4-3); HS.ESS1.A (MS-PS4-2); HS.ESS2.A (MS-PS4-2); HS.ESS2.C (MS-PS4-2); HS.ESS2.D (MS-PS4-2)

Common Core State Standards Connections: