Total Internal Reflection

Every object can be classified as Transparent: Objects that light to pass through completely, Translucent: Objects that allow light to pass through partially and Opaque: Objects that do not allow light to pass through them.  

    


Transparent objects have a special property called Internal Reflection. This property is because of the fact that light bends when it travels from one medium to another or simply, when it refracts. When a light ray travels form a optically rarer medium to a optically denser medium, it bends towards the normal. In the same way, when light travels from and optically denser medium to optically rare medium, then it bends away from the normal.

  When you keep increasing the angle of incidence, the ray of light keeps bending away from the normal and a point comes when the ray travels along the surface of the object or the medium. This angle is called as the critical angle. If you increase the angle further, then the ray of light wont refract but simply reflects. The type of reflection is called as the Total Internal Reflection. Its called as the Total Internal Reflection because the light ray that strikes it is reflected completely without 0.0001% of energy being absorbed.

                                                    Differences between TRI and Reflection

	Total Internal Reflection	Reflection
1.	The Light ray bounces back completely	Some of the light ray that strikes is absobred
2.	This takes place at particular angles only(Θ>C)	This takes place at all angles.
3.	Possible only in transparent and translucent objects.	Possible in all objects. In fact, the reason we see them is because they reflect light!

The critical angle of an object calculate with the equation given below: 
sin i_c = n₁ / n<sub>2,
where ic is critical angle, n1 = refractive index of vacuum and n2 = refractive index of material [n1=1(always) because we are checking the critical angle with respect to air, i.e a light ray is travelling from a given medium to air.]


Here is a list of critical angles of a few materials:</sub>

Material	Critical Angle
Glass(Acrylic)	42.155°
Crown Glass	41.810°
Flint Glass	38.682°
Pyrex(a borosilicate glass)	42.86°
Rock Salt	41.271°
Diamond	27.407°
Acetone	47.332°
Water	49.761°

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Reflection

Reflection is a phenomenon in which the wave that strikes a material, returns back to the same medium. The fact that we are able to see ourselves in the mirror is because the light ray that is incident on it, reflects back. Reflection is seen not only in electromagnetic radiations, but also in sound. Echoes are the best example for reflection of sound.

Reflection of light: Refraction of light is of two types. Either specular or diffuse.

Specular

Specular: is the mirror-like reflection of light (or of other kinds of wave) from a surface, in which light from a single incoming direction is reflected into a single outgoing direction. Such behavior is described by the law of reflection, which states that the direction the incident ray), and the direction of the reflected ray make the same angle with respect to the surface normal, thus the angle of incidence equals the angle of reflection (θ_i = θ_r in the figure), and that the incident, normal, and reflected directions are coplanar.




Laws of Reflection: There are three laws of reflection

• The incident ray, the reflected ray and the normal to the reflection surface at the point of the incidence lie in the same plane. 
• The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes to the same normal.
• The reflected ray and the incident ray are on the opposite sides of the normal.

Diffuse: is the reflection of light from a surface such that an incident ray is reflected at many angles rather than at just one angle as in the case of specular reflection. An illuminated ideal diffuse reflecting surface will have equal luminance from all directions in the hemisphere surrounding the surface (Lambertian reflectance).

A surface built from a non-absorbing powder such as plaster, or from fibers such as paper, or from a polycrystalline material such as white marble, reflects light diffusely with great efficiency. Many common materials exhibit a mixture of specular and diffuse reflection.The visibility of objects is primarily caused by diffuse reflection of light: it is diffusely-scattered light that forms the image of the object in the observer's eye.

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Light

Light is a form of energy is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Light itself is invisible but other things visible. The range of visible light in terms of wavelength is from 400nm to 800nm and in terms of frequency is from 400 THz to 800 THz. Light is emitted as tiny packets of quanta. This quanta is called photon. Light behaves as a wave as well as a particle and this property is known as wave-particle duality. Light obeys laws of reflection, refraction, defraction and interference.


The Electromagnetic Spectrum

Generally, EM radiation (the designation 'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave, infrared, the visible region we perceive as light,ultraviolet, X-rays and gamma rays.

The behaviour of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behaviour depends on the amount of energy per quantum it carries.

Refraction

Refraction

When a ray of light passes from a denser medium to a rarer medium, it undergoes refraction. Refraction is the bending of a wave with respect to the medium's optical density( not its density ). 

Laws of Refraction: 

1. The incident ray, the refracted ray, and the normal all lie in the same plane.
2. The Snell's law : the ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of phase velocities in the two media, or equivalent to the opposite ratio of the indices of refraction                                           

                                                           

                                
Refraction takes place because the speed of light is different in different mediums. In vacuum or air, the speed of light is 299,792,458 m/s or simply 3*10^8 m/s. But as it enters a different medium, the speed of light decreases because of absorbtion and remission of photons. The speed of light in water is 2.25*10^8.


        Refractive index: Refractive index is the ratio of speed of light in vacuum to the speed of light in the respective medium. It is denoted by μ. μ =   Speed of light in vacuum 
                                                                 Speed of light in the medium.


A list of Refractive index are given below:

Material	μ(Refractive Index)
Vacuum	1
Air at STP	1.000277
Helium	1.000036
Hydrogen	1.000132
Carbon dioxide	1.00045
Benzene	1.501
Water	1.3330
Acetone	1.36
Diamond	2.419

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Pulsars

 Pulsars are highly magnetized, rotating neutron stars that emit a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name. Because neutron stars are very dense objects, the rotation period and thus the interval between observed pulses is very regular. For some pulsars, the regularity of pulsation is as precise as an atomic clock. The observed periods of their pulses range from 1.4 milliseconds to 8.5 seconds. A few pulsars are known to have planets orbiting them


Formation
The events leading to the formation of a pulsar begin when the core of a massive star is compressed during a supernova, which collapses into a neutron star. The neutron star retains most of its angular momentum, and since it has only a tiny fraction of its progenitor's radius (and therefore its moment of inertia is sharply reduced), it is formed with very high rotation speed. A beam of radiation is emitted along the magnetic axis of the pulsar, which spins along with the rotation of the neutron star. The magnetic axis of the pulsar determines the direction of the electromagnetic beam, with the magnetic axis not necessarily being the same as its rotational axis. This misalignment causes the beam to be seen once for every half rotation of the neutron star, which leads to the "pulsed" nature of its appearance. The beam originates from the rotational energy of the neutron star, which generates an electrical field from the movement of the very strong magnetic field, resulting in the acceleration of protons and electrons on the star surface and the creation of an electromagnetic beam emanating from the poles of the magnetic field. This rotation slows down over time as electromagnetic power is emitted. When a pulsar's spin period slows down sufficiently, the radio pulsar mechanism is believed to turn off (the so-called "death line"). This turn-off seems to take place after about 10–100 million years, which means of all the neutron stars in the 13.6 billion year age of the universe, around 99% no longer pulsate. To date, the slowest observed pulsar has a period of 8 seconds.

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Black Hole

A black hole is a region of space from which nothing, not even light, can escape.  It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics.Black holes of stellar mass are expected to form when heavy stars collapse in a supernova at the end of their life cycle. After a black hole has formed it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, super massive black holes of millions of solar masses may be formed. Quantum mechanics predicts that black holes emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of stellar mass or greater.

Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter. Astronomers have identified numerous stellar black hole candidates in binary systems, by studying their interaction with their companion stars. There is growing consensus that supermassive black holes exist in the centers of most galaxies. In particular, there is strong evidence of a black hole of more than 4 million solar masses at the center of our Milky Way.
The theory of general relativity predicts that a sufficiently compact mass will deform space time to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return.
Time travel: Scientists predict that if anything is able to just escape form the event horizon, then that thing will not be sucked into a black hole. Time travelling that has been thought of from long times, is said to be possible for a person who has escaped the event horizon. A body of huge amount of mass in it is said to slow time because of its own gravitation. So for a person in the gravitational field of the black hole, the time will run slower. So he might as well make time travel possible because he'd reach the past. This has only been though of and not proved in reality.


A body of huge mass

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