In order to see something, the object either needs to emit light directly like the sun or a bulb or else light has to be reflected from an object into our eyes. An object will be invisible when it is not perceivable by the eye. For this to be possible either light ray should not enter the eye or even if they enter we should not be able to sense it. There are some scientific possibilities to become invisible. Although there are limitations in every process but there are really some promising and useful ways in the term of invisibility. Few possibilities are discussed below:

1. Move at great speed: 

There are few theories that say that an object can turn invisible when it travels at a speed greater than that of light because light cannot catch up with it. But Einstein’s special theory of relativity says that we need infinite amount of energy to get an object to reach the speed of light. This is not practically possible. What if we can make an object move at a very fast velocity but less than that of light so that we can’t see it? If we see the example of a bullet, when a bullet is fired off the gun, it becomes invisible because it is travelling at a great speed. Now, we understand that a bullet can become invisible. So, can a human become invisible in the same process?
To become invisible one must run so fast which will be practically impossible. There is no such technology invented so far that can move a human being at a great speed to become invisible. The speed should be approximately 12 times faster than a F1 car and the angling of the eye should be limited.

2. Bending of light: 

Here we try to take light pass it around an object as if it weren’t there. Just like a light from light source suddenly bends and passes the object and doesn’t fall on the object. In this process the object would be invisible. This is very simple in theoretical way but very complex in the practical way. To make this theory work, we need to make a setup. This setup should have reflectors which would reflect the background of the object into eyes. So, the object inside the setup would go invisible and only the background will be visible. But there is a limitation, the object or human inside the setup cannot see anything outside the setup. So, this process can be used only to hide something.

3. The invisible ink: 

A red colored object appears red because it absorbs all colors and reflect only red. So does every color. Only rays having a wavelength of 380-740 nanometers can be perceived by the human eye. What do infrared colored object looks like? Would they be invisible? If we paint a human with red ink, he will look red. But what if we paint him with an infrared paint? Would he be really invisible? No. As the infrared ink absorbs all colors except infrared it turns out that it absorbs all colors in the visible range. So, the paint will be perceived as black but not visible. Invisibility is not about absorbing the light but we should be able to see what is behind the object.

4. Using cameras and displays: 

In this process a camera captures the background of an object and projects it onto a screen creating an invisible region. If you look at the screen from a different direction you'll understand that this is only a projection. So, to make it look more real we have to track the eye of the observer and change the display screen accordingly. But if there are two people, there system would be failed.

5. Metamaterials: 

Scientists have developed an invisible cloak which can bend electromagnetic radiation around an object and it is known as Metamaterials. Metamaterials can bend light around an object, giving the appearance that it isn't there at all. But this technique only worked with microwave light so far. Scientists are trying to develop the concept to make it work with visible lights too. The invisible cloak is generally made out of silk. Materials are developed to bend light so that eyes can see what lies behind the object and make the object disappear. 
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Satellite is an space based object that orbits the planet. There are two types of satellites: Natural and Artificial. Moon is a natural satellite of earth that is continiously orbitting the earth. And arificial satellites also orbits earth. They are developed in the purpose of Communicating, Imging, Scientific surveying and Navigation. Satellites have made global communication system more easier, quicker and more realistic. It has connected the whole world with its bound. There are almost 2000 artificial satellites orbitting the world. These carry analog and digital signals such as voice, pictures, videos and data from one part of the world to another. There are two types of artificial satellites: Polar and Geosynchronous.
Polar: polar satellites doesn't rotate with the earth as it just float on the space. The earth rotates and the whole earth becomes subject to its perscrutation.
Geosynchronous: These satellites are associated with a particular  location and they rotate following the rotation of earth's rotation. Only a particular location becomes a subject to its scrutiny.
What do Satellites do for us?
Satellites are used for many purposes. They Are mostly used for communication, Photography, Imaging, Scientific surveys or researches and Navigation.
Communication: Communication  satellites use radio waves to communicate with the ground stations. It catches signals from the ground signals from the ground station and amplifying or modifying themtransmits them to another ground station. Television, Radio, Internet Cellular phones communicate via satellites. It is also ueful in the areas where typical wired or wireless systems doesn't work.
Photography, Imaging and Scientific Surveys: People all over the world use google map or Google Earth systems which work with the help of satellites. Satellites capture high quality pictures of the ground. It can monitor live activities such as live traffics and pedestrians on earth. They are also used for many purposes such as measuring sea levels, measuring plunktons and nutritional activity scanning the color of the ocean, Weather forecasting, Mapping and monitoring changes in land etc.
Navigation: People all over the world us GPS system to track the maps and location and to get street directions. People also use it as sky compass to get direction. These are possible by the help of Satellites.
What is inside a satellite?
A satellite is a very complex and expensive machine. But we will only discuss about the simple basics. There are a some essential parts of a satellite. Such as,
•             It has large and small parablic dish antennas for sending and receiving signals.
•             It has upper and lower solar batteries of four solar panels in each. The solar panels are the sorce of power for a satellite.
•             It has supports that hel to fold out solar panels.
•             It has main satellite rocket motor.
•             It has controll engines that keep the satellite in its precision, position, spin and orbit.



How does Satellite works?
As satellites receive signals from ground stations so there should be some ground stations to communicate with satellites. Earth or ground stations send information and data via radio waves in the form of high frequency signals to satellites. They receive and retransmit them back to earth. The retransmitted signals are received by other earth stations in the range of the satellite. The transmission system from the earth is known as uplink and the retransmission from the satellite is called downlink.
Who Invented satellite?

The idea of satellite technology was first revealed by a science fiction author Arthur C. Clarke. He proposed of geosynchronous satellite which can cover a third of the planet each. Although he didn't patent it, he is generally credited with its invention.
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Depending on where it originates from, the sun, the moon, lightning, certain types of glowing insects, worms and fishes etc. are all example of natural sources of light. While, a candle flame, various types of torches, lamps and light fixtures are amongst the artificial light sources.
·        Visible light
Our sun emits light. Really strong intense light or radiation called electromagnetic radiation or EMR. The radiation is called electromagnetic because it contains both electric and magnetic properties. Electromagnetic radiation is a large family consisting seven members known as The Electromagnetic Spectrum. Our particular family member which lies in the middle of the spectrums, falls on the earth and which our eyes detect is called visible light or the visible spectrum. Its fraction is so small that if we consider the entire EM spectrum to be around 2500 miles, the visible fraction would be just one inch! It is within this inch that we live in our world of natural and extra ordinarily high definition color. We enjoy this privilege of color due to the presence of certain specialized cells in our eyes called Rods and Cons.
·        Invisible light

 Now the other type of electromagnetic radiations is also called light but not visible light. So, automatically they are considered by us to be invisible. So, now we know the light which we have naturally been introduced to since birth and that we see throughout our life is actually radiation and radiation of a good type that an important source of energy for life on earth as we have seen earlier. Now, just as members of a family differ from one another and have their own unique identity, so also these fractions of radiation, although part of the same family of light, are slightly different from one another. These different are due to three factors that are independent and give each member, its individuality. These factors are Wavelength, Frequency and Energy. The shorter the wavelength is the higher the frequencies and energies are. And the longer the wavelength is the lower the frequency and energy is.

Types of light:

1.       Radio wave: It has the longest wavelength with lowest frequency and energy. They are used for broadcasting TV and radio signals because they can cover a greater distance through the atmosphere due to their large wavelength. They are usually emitted and received by antennas.
2.       Microwaves: It has shorter wavelength than radio waves and higher frequencies and energy. These waves are used in the appliances like the microwave oven. They are also used in channels of telecommunication like mobile networks, Bluetooth and wireless LAN protocols.
3.       Infrared: This region of spectrum is more known for the aspect of heat associated with it as they have higher energy as compared to radio waves and microwaves. They are used by appliances like grills and electric stoves for heating and cooking food. IR is also used as a part of heat therapy to relieve aches and pains.
4.       Visible: Visible light has the wavelength in the range of 380-780 nanometers. Visible ray are simply of those wavelengths which our eyes can detect. It has colors and that is what we see.
5.       UV or Ultra-Violate: The uses of ultra violate radiations are also widespread in various aspects of life. They are used as sterilizing agents by the medical industry and by water purifiers to get rid of harmful bacteria.    It is used by dentist to harden teeth filling and by banks to detect fake currency notes.
6.       X-Ray: As we are aware, x-ray is also used to a great extent by the medical industry to examine the state of bones or foreign object in the body. It is also used by airport security to scan luggage for partially dangerous objects.
7.       Gamma: Gamma rays are a type of radiation with the lowest wavelength and the highest energy. They are the exact opposite of radio waves. They are emitted by radioactive materials, given off during nuclear explosions and during certain types of thunder storm. But despite of the fearful reputation they are used to kill cancer cells for medical and security scanning and also by space observatories for astronomical observations.
Light
Wavelength
Radio wave
5m- 1m
Microwave
1 m-1 mm
Infrared
1 mm-780nm
Visible
780nm-380nm
Ultra Violate
380nm-10nm
X-Ray
10nm-0.01 nm
Gamma Ray
           0.01 nm-0.00005nm
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4G stands for fourth-generation wireless cellular service that is almost 10 times faster than 3G network. The 1G or 1st generation was the old analog cellular system. That was obviously super slow. 2G or 2nd generation moves to digital but they were still very much slow, somewhere around 10-200 kbps. The first iPhone was 2G. 3G is a much faster digital and it starts around 384 kbps+. 4G systems like LTE and WiMAX claim to initially start around 5 mbps which is a lot higher than all the previous systems. But 4G isn’t just faster than 3G, the 4G network also tend to have a lot more capacity than 3G network. On 4G, there should be a lot more data plans. But sometimes people argue that there is no such difference in 3G and 4G as if they both serve the same service. Because, 3G networks sometimes are really faster than the beginning of 4G. For example, HSPA+, is 3G basically but it is really fast 3G somewhere around 7 mbps.
4G was first experimented in the U.S. beginning in 2009. Smart phones or recent digital devices are functioned to stream videos and music and so internet speed became so important. Whenever someone is moving with high mobility, it will require huge data rate. So, in order to make the requirement of high speed mobility like trains, bullet trains, super cars, we need 4G. 4G is developed quite highly that it can be compared with broadband connections where previous cellular speeds were much slower than high speed broadband connections to computer.
Requirements: To have access to 4g network, a 4G capable device is required. Devices with 4G capabilities are available widely these days. 4G compatible devices usually have larger screens, extremely fast RAM, great processors and powerful batteries.
There are several types of 4G such as 4G LTE, WiMAX. But LTE is much faster as it can reach up to 100 mbps while WiMAX can reach up to 70 mbps.
Why 4g?
4G makes the data transmitting or accepting rate real faster. When a device send a picture to another via wireless system, it disassembly converts into separated codes and when the other device receives the codes it reassembles and converts them into a picture. So, the converting and assembling rate depends upon the networking system. 4G is developed for doing these things real faster. It saves time and makes things quicker. Communications between 4G networks becomes smoother and faster.

Why 4G is faster?
The modulation and coding scheme of 4G network contains a lot more symbols or bits. It also can have a bandwidth of 20 MHz in a single carrier while a 3G network can have only 5MHz.
 So, 4G is an evolution of mobile networks. As we approach to the future, we will be getting faster networks with the development of science and technology.
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LiFi or light fidelity is a high speed and fully networked wireless communication technology similar to WiFi. It uses visible light rays to transfer data and is 100 times faster than WiFi. This is because the light has the capacity to transfer vastly more amount of data in a less time compared to radio waves.
Method: The ceiling LEDs encode data from internet and decode it into light. The LED light is switched ON and OFF at a very high rate, too quick to be noticed by human eye. This process works like a binary code. This light is then received by a photon detector which converts the variation in the intensity of light into electrical charges. These electrical charges are then converted to binary data and send to computer or mobile devices.

Invention: The LiFi technology was founded by a University Professor, Herald Haas. Haas envisioned light bulbs that could act as wireless routers. Subsequently, in 2012 after four years of research, Haas set up a company called pureLiFi with the aim to be the world leader in visible light communications technology.
How does LiFi work?
LiFi technology is basically based on the idea of Solar Panel system. LiFi and WiFi are quite similar as both transmit data electromagnetically. However, WiFi uses radio waves while LiFi runs on visible light. LiFi use a photon detector to receive light signals and signal processing elements to convert the data into ‘stream-able’ content. An LED light bulb is a semi-conductor light source meaning that the constant current of electricity supplied to an LED light bulb can be modulated at very high speed without being visible to human eye. For example, data is fled into an LED light bulb with signal processing technology. It often sends data which is embedded in its beam at very high speeds to the photon detector. The tiny changes in the rapid dimming of LED bulbs are then converted by the receiver into electrical signal. The signal then converted back into a binary data stream that we could recognize as web, video and audio applications that run on internet enabled devices. The converted binary codes look like this,
Binary= ‘101101001101001’
LiFi features- benefits to the capacity, energy efficiency, safety and security of a wireless system with a number of key benefits over WiFi. But it’s worth mention that LiFi is not aiming to be direct replacement for WiFi and cellular networks, it’s a complementary technology that will work well in certain situations.
Professor Herald Haas, the person who coined the term LiFi, expect LiFi to unlock the third industrial revolution. The technology seems promising and many major key players are considering it. Microsoft is implementing the lucibel’s LiFi solution at its innovation center in France. There have been news reports that Apple is experimenting with LiFi wireless data for future iPhones and iPads.

Limitations of LiFi: LiFi is undoubtedly a promising and revolutionary invention but it also has some limitations.

·         LiFi might not be able to work smoothly in direct sunlight or harsh lighting. They might create problems in detecting the modulating light waves
·         LiFi may cover only a single room as light cannot pass the walls. If anyone is connect to the LiFi and want to move to another room, he better have another light bulb set up there.
·         Light bulb has to be on all the time, even during night.

LiFi technology does offer numerous benefits, but there are still important challenges that must be overcome before it becomes an essential part of everyday wireless communications.

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Since the beginning of the industrial revolution, the demand for energy has increased dramatically year after year. As the world is developing and population is increasing, the rising rate for the demand of energy is very logical. Nuclear power system is one of the widely used energy source in the world.

History: 

It all started in the 1940s. After the shocking disasters by the use of atomic bomb, nuclear energy was promised to be used peacefully by the new technology. So, the world can be back on its feet. Everyone’s imagination was running wild. Would electricity become free? Could nuclear power help settle the Antarctic? Would there be nuclear powered cars, airplanes or houses? It seemed this was just a few years of hard work away. One thing was certain that the future would be atomic. Just a few years later, there was a sort of atomic age hangover. As it turned out, nuclear power was a very complicated and very expensive. Turning physics into engineering was easy on paper but hard in real life. Also private companies thought that nuclear power was much too risky for investment. Most of them would rather stick to Gas, Coal and Oil. But there were many people who didn’t just want to abandon the promise of the atomic age and exciting new technology. The prospect of cheap electricity, the prospect of being independent of Oil and Gas and in some cases, a secret desire to possess atomic weapons provided a strong motivation to keep going. Nuclear power started to become popular in the early 1970s when oil price in the Middle East increased. Now, commercial interest and investment picked up at a dazzling pace. More than half of all the nuclear reactors in the world ware build in between 1970 and 1985.

Buildings: 

Nuclear power plants are thermal power stations which generate electrical energy from it. They consist of numerous buildings and facilities. The most important of which are here:
  1. ·       In Turbine buildings there are several turbines as well as generator necessary for electrical         power generation.
  2. ·       There is a containment building where the nuclear reactor is housed.
  3. ·       Inside the Cooling tower, nuclear reactions take place where water is heated up. In this               building, hot water is cooled.


Process: 

In order to easily understand the underlying principles, the following is a description of the most important components of a nuclear power plant that uses a pressurized water reactor (PWR). In the reactor pressure vessel, the nuclear reaction and the associated release of thermal energy takes place. Inside a pressurized water reactor, the fuel assemblies can be found. About 150 such assemblies are installed in it. A single fuel assembly is composed of many fuel rods. The actual nuclear fuel is found inside of each fuel rod. Small nuclear fuel pallets are composed of enriched Uranium or Plutonium that make nuclear fission chain reaction possible. In the fission chain reaction, thermal energy is released.

Water is needed in order to absorb the thermal energy and keep the chain reaction going. Inside the vessel the water is heated to over 570 degrees Fahrenheit. The water does not boil however since the pressurizer maintains the water pressure constant. The heated water is eventually pumped to a heat exchanger also called steam generator. Any water inside the heat exchanger begins to boil. The resulting steam is eventually fled through a set of pipes to the turbine building. The steam first arrive high pressure turbine and then is typically fled to two low pressure turbines. All of the turbines are connected by a spinning shaft through the electrical generator which in turn produces AC electricity from the shaft’s rotational energy. The steam is converted into liquid form in a condenser and then returned back to the steam generator.

The water needed for this often comes from a near river or is cooled in a cooling tower. The water circulation systems are always kept separate from one another.
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Wifi stands for wireless fidelity, a popular technology used to connect to the internet without hardware cables. It also allows devices like laptop, Computer, mobile phones to exchange information. It can also be taken as a synonym of WLAN.
Wifi network is built with the help of several hardware components like wireless access points or routers and user devices. Wifi router is the main hub of wifi network. It is usually connected to a high speed modem. The modem is connected to an Internet Service Provider (ISP) from where it gets hardware signals. The modem sends hardware signals to wifi router and the router converts them into radio waves. The radio waves can be accessed through any wifi supported devices.
Wifi signals are transmitted into certain frequencies, either 2.4 Gigahertz or 5.0 Gigahertz. But other usual devices like walky-talkies, radios, car radios etc use frequencies in kilohertz and megahertz range.
Wifi is a network carrying instructions which tell the connected devices what to do in each pixel on the screen. The instructions are converted into codes which only need two different modes- On and OFF. For example, a code might be this-
-OFF-OFF-OFF-OFF-OFF-OFF=A
OR
-ON-OFF-OFF-OFF-OFF-OFF=B
OR
-ON-ON-ON-OFF-OFF-OFF=C
To transmit a picture, we’ll need huge amount of ON-OFF signals and for video, even more. Luckily for us, electro-magnetic radiations travel in the speed of light. So, even something really so complex can be transmitted super quickly.
The area where wifi access is available is called hotspot. Any digital device within the hotspot range can connect to the wifi network. The range is not possible to be defined accurately but a typical wifi network can cover a whole family apartment.   Business networks can cover a whole office building. Even there are examples of setting wifi covering several square miles in some cities. The network speed varies from device to device and depending on distance from router. Wireless range is possible to be effected by surroundings such as natural elements and obstacles. Physical obstacles like walls or concretes, water and even human beings can slow down wifi. Sometimes microwave ovens can create disturbance in the network because it also uses the frequency range of 2.4GHz.

There are several types of wifi network depending on the speed limits. The costs for the network also depend on the speed limit of wifi.
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