Working of Vision

How Vision Works

Although small in size, the eye is a very complex organ.

It's no accident that the main function of the sun at the center of our solar system is to provide light. Light is what drives life. It's hard to imagine our world and life without it.
The sensing of light by living things is almost universal. Plants use light through photosynthesis to grow. Animals use light to hunt their prey or to sense and escape from predators.
­Some say that it is the development of stereoscopic vision, along with the development of the large human brain  and the freeing of hands from locomotion, that have allowed humans to evolve to such a high level.

In this article, we'll discuss the amazing inner workings of the human eye!

 

Basic Anatomy

Although small in size, the eye is a very complex organ. The eye is approximately 1 inch (2.54 cm) wide, 1 inch deep and 0.9 inches (2.3 cm) tall.

The eye is unique in that it is able to move in many directions to maximize the field of vision.

The tough, outermost layer of the eye is called the sclera. It maintains the shape of the eye. The front sixth of this layer is clear and is called the cornea. All light must first pass through the cornea when it enters the eye. Attached to the sclera are the muscles that move the eye, called the extraocular muscles.
The choroid (or uveal tract) is the second layer of the eye. It contains the blood vessels that supply blood to structures of the eye. The front part of the choroid contains two structures:

• The ciliary body - The ciliary body is a muscular area that is attached to the lens. It contracts and relaxes to control the size of the lens for focusing.
• The iris - The iris is the colored part of the eye. The color of the iris is determined by the color of the connective tissue and pigment cells. Less pigment makes the eyes blue; more pigment makes the eyes brown. The iris is an adjustable diaphragm around an opening called the pupil.

The iris has two muscles: The dilator muscle makes the iris smaller and therefore the pupil larger, allowing more light into the eye; the sphincter muscle makes the iris larger and the pupil smaller, allowing less light into the eye. Pupil size can change from 2 millimeters to 8 millimeters. This means that by changing the size of the pupil, the eye can change the amount of light that enters it by 30 times.
The innermost layer is the retina -- the light-sensing portion of the eye. It contains rod cells, which are responsible for vision in low light, and cone cells, which are responsible for color vision and detail. In the back of the eye, in the center of the retina, is the macula. In the center of the macula is an area called the fovea centralis. This area contains only cones and is responsible for seeing fine detail clearly.
The retina contains a chemical called rhodopsin, or "visual purple." This is the chemical that converts light into electrical impulses that the brain interprets as vision. The retinal nerve fibers collect at the back of the eye and form the optic nerve, which conducts the electrical impulses to the brain. The spot where the optic nerve and blood vessels exit the retina is called the optic disk. This area is a blind spot on the retina because there are no rods or cones at that location. However, you are not aware of this blind spot because each eye covers for the blind spot of the other eye.
When a doctor looks at the back of your eye through an ophthalmoscope, here's the view:

Inside the eyeball there are two fluid-filled sections separated by the lens. The larger, back section contains a clear, gel-like material called vitreous humor. The smaller, front section contains a clear, watery material called aqueous humor. The aqueous humor is divided into two sections called the anterior chamber (in front of the iris) and the posterior chamber (behind the iris). The aqueous humor is produced in the ciliary body and is drained through the canal of Schlemm. When this drainage is blocked, a disease called glaucoma can result.
The lens is a clear, bi-convex structure about 10 mm (0.4 inches) in diameter. The lens changes shape because it is attached to muscles in the ciliary body. The lens is used to fine-tune vision.
Covering the inside surface of the eyelids and sclera is a mucous membrane called the conjunctiva, which helps to keep the eye moist. An infection of this area is called conjunctivitis (also called pink eye).
The eye is unique in that it is able to move in many directions to maximize the field of vision, yet is protected from injury by a bony cavity called the orbital cavity. The eye is embedded in fat, which provides some cushioning. The eyelids protect the eye by blinking. This also keeps the surface of the eye moist by spreading tears over the eyes. Eyelashes and eyebrows protect the eye from particles that may injure it.
Tears are produced in the lacrimal glands, which are located above the outer segment of each eye. The tears eventually drain into the inner corner of the eye, into the lacrimal sac, then through the nasal duct and into the nose. That is why your nose runs when you cry.
There are six muscles attached to the sclera that control the movements of the eye. They are shown here:

Muscle	Primary Function
Medial rectus	moves eye towards nose
Lateral rectus	moves eye away from nose
Superior rectus	raises eye
Inferior rectus	lowers eye
Superior oblique	rotates eye
Inferior oblique	rotates eye

Working of Hologram

How Holograms Work

­ Mass-produced holograms often look more like green photographs than 3-D images.

­If you want to see a hologram, you don't have to look much farther than your wallet. Th­ere are holograms on most driver's licenses, ID cards and credit cards. If you're not old enough to drive or use credit, you can still find holograms around your home. They're part of CD, DVD and software packaging, as well as just about everything sold as "official merchandise."

­

Unfortunately, these holograms -- which exist to make forgery more difficult -- aren't very impressive. You can see changes in colors and shapes when you move them back and forth, but they usually just look like sparkly pictures or smears of color. Even the mass-produced holograms that feature movie and comic book heroes can look more like green photographs than amazing 3-D images.
­On the other hand, large-scale holograms, illuminated with lasers or displayed in a darkened room with carefully directed lighting, are incredible. They're two-dimensional surfaces that show absolutely precise, three-dimensional images of real objects. You don't even have to wear special glasses or look through a View-Master to see the images in 3-D.
If you look at these holograms from different angles, you see objects from different perspectives, just like you would if you were looking at a real object. Some holograms even appear to move as you walk past them and look at them from different angles. Others change colors or include views of completely different objects, depending on how you look at them.

If you tear a hologram in half, you can still see the whole image in each piece. The same is true with smaller and smaller pieces.

Holograms have other surprising traits as well. If you cut one in half, each half contains whole views of the entire holographic image. The same is true if you cut out a small piece -­- even a tiny fragment will still contain the whole picture. On top of that, if you make a hologram of a magnifying glass, the holographic version will magnify the other objects in the hologram, just like a real one.
Once you know the principles behind holograms, understanding how they can do all this is easy. This article will explain how a hologram, light and your brain work together make clear, 3-D images. All of a hologram's properties come directly from the process used to create it, so we'll start with an overview of what it takes to make one.
­

Making a Hologram

Transmission and Reflection There are two basic categories of holograms -- transmission and reflection. Transmission holograms create a 3-D image when monochromatic light, or light that is all one wavelength, travels through them. Reflection holograms create a 3-D image when laser light or white light reflects off of their surface. For the sake of simplicity, this article discusses transmission holograms viewed with the help of a laser except where noted.

It doesn't take very many tools to make a hologram. You can make one with:

• A laser: Red lasers, usually helium-neon (HeNe) lasers, are common in holography. Some home holography experiments rely on the diodes from red laser pointers, but the light from a laser pointer tends to be less coherent and less stable, which can make it hard to get a good image. Some types of holograms use lasers that produce different colors of light as well. Depending on the type of laser you're using, you may also need a shutter to control the exposure.
• Lenses: Holography is often referred to as "lensless photography," but holography does require lenses. However, a camera's lens focuses light, while the lenses used in holography cause the beam to spread out.
• A beam splitter: This is a device that uses mirrors and prisms to split one beam of light into two beams.
• Mirrors: These direct the beams of light to the correct locations. Along with the lenses and beam splitter, the mirrors have to be absolutely clean. Dirt and smudges can degrade the final image.
• Holographic film: Holographic film can record light at a very high resolution, which is necessary for creating a hologram. It's a layer of light-sensitive compounds on a transparent surface, like photographic film. The difference between holographic and photographic film is that holographic film has to be able to record very small changes in light that take place over microscopic distances. In other words, it needs to have a very fine grain. In some cases, holograms that use a red laser rely on emulsions that respond most strongly to red light.

There are lots of different ways to arrange these tools -- we'll stick to a basic transmission hologram setup for now.

1. The laser points at the beam splitter, which divides the beam of light into two parts.
2. Mirrors direct the paths of these two beams so that they hit their intended targets.
3. Each of the two beams passes through a diverging lens and becomes a wide swath of light rather than a narrow beam.
4. One beam, the object beam, reflects off of the object and onto the photographic emulsion.
5. The other beam, the reference beam, hits the emulsion without reflecting off of anything other than a mirror.

Working of Xbox 360

How Xbox 360 Works

Microsoft's first video game console, the Xbox, has sold more than 20 million units worldwide since its introduction in 2001. Despite the Xbox's impressive power, the list of big-name video game titles to support it and the success of the Xbox's online component, Xbox LIVE, Sony's PlayStation 2 still outsold it.

As the game industry moved into the next generation of video game technology, Microsoft was determined to dethrone Sony's PlayStation. Enter the Xbox 360.

Photo courtesy Microsoft Corp. Xbox 360.

Microsoft rebuilt the Xbox from the ground up. From the name to the look to hardware and features, the Xbox 360 is a radically different and more powerful machine than its predecessor. Far more than a video game console, the Xbox 360 is a total media center that allows users to play, network, rip, stream and download all types of media, including high-definition movies, music, digital pictures and game content.
In this article, we will learn about the hardware and features that make the Xbox 360 a leap forward into the next generation of gaming consoles.
The Xbox 360, like all video game consoles, is just a computer with hardware and software dedicated to the function of running video game software. The original Xbox was essentially a Windows PC with a modified Pentium III processor, some relatively powerful graphics and audio hardware and a modified version of the Microsoft operating system Windows 2000, all packaged in that distinctive black box. The Xbox 360 is also a specially packaged computer, but once you look inside, you realize that this console has quite a bit under the hood:

• Custom IBM Power PC-based CPU with three 3.2 GHz cores
• Custom ATI graphics processor with 10 MB embedded DRAM
• 512 MB 700 MHz GDDR3 RAM
• Detachable and upgradeable hard drive -- all models except the Core system
• 12x dual-layer DVD-ROM
• Support for up to four wireless game controllers
• Three USB 2.0 ports
• Two memory unit slots

As you can see, Microsoft intends the Xbox 360 to be a serious game machine. The company is also serious about reaching more audiences with the Xbox 360. On the next , we'll look at variations of the Xbox 360 that are marketed to different kinds of gamers.

Xbox 360 Consoles

Microsoft released two versions of the Xbox 360 in November 2005: the Xbox 360 Premium Package and Xbox 360 Core System. Since then, the lineup has undergone some changes. The Premium Package is now known simply as the Xbox 360 console. A new Elite system hit store shelves in April 2007. Microsoft announced another new system, the Xbox 360 Arcade, in October 2007.

The Xbox 360 debuted at the 2005 E3 Expo Microsoft booth.

The Core System is "plug and play" -- in addition to the console, it includes a wired controller and an AV cable. The Xbox 360 comes with a wireless controller, an HD AV cable, an Ethernet connectivity cable, a headset and a removable 20-GB hard drive. Initially, it also included a DVD remote, but this is no longer available as part of the package.
The Xbox 360 Elite is similar to the main Xbox 360, with a black case, matching wireless controller and headset. It also includes a larger 120-GB hard drive and an HDMI cable.
To combat Nintendo's surprise powerhouse, the Wii, Microsoft announced the Xbox 360 Arcade in October 2007. Aimed at casual gamers, the console will come with between three and five Xbox LIVE Arcade games and will probably include "Pac-Man," "Uno" and "Luxor 2."
Microsoft has also prepared subtle variations of the console for marketing tie-ins. To commemorate the release of "The Simpsons Movie," Microsoft created a run of 100 limited-edition Simpsons Xbox 360s, which were given away in promotions. Fans of Bungie's "Halo" game series can purchase the "Halo 3" limited edition Xbox 360, which comes in "Spartan green and gold" and features a matching controller.

On the next page we will see what makes the Xbox 360 tick -- the central processing unit, or CPU.

Xbox SalesBecause of delays in manufacturing, there were not enough Xbox 360s to meet the demand of the 2005 holiday season. As a result, Xbox 360s were selling for as much as $2,000 on Web sites like eBay, and initial sales figures were poor. However, Microsoft still got a jump on its competitors, as the PlayStation 3 and the Nintendo Wii did not release until 2006. According to industry analyst the NPD Group, as of August 2007, the Xbox 360 sold 6.3 million Xbox 360s in the United States, to 4 million of Nintendo's Wii and 1.75 million Sony Playstation 3s.

Working of Routing Algorithms

How Routing Algorithms Work

Think you know how routers work? These devices use intricate formulas to figure out exactly where to send a packet and how to get it there.

If you have read the article How Routers Work, then you know that a router is used to manage network traffic and find the best route for sending packets. But have you ever thought about how routers do this? Routers need to have some information about network status in order to make decisions regarding how and where to send packets. But how do they gather this information?
In this article, we'll find out precisely what information is used by routers in determining where to send a packet.

The Basics

Routers use routing algorithms to find the best route to a destination. When we say "best route," we consider parameters like the number of hops (the trip a packet takes from one router or intermediate point to another in the network), time delay and communication cost of packet transmission.




Based on how routers gather information about the structure of a network and their analysis of information to specify the best route, we have two major routing algorithms: global routing algorithms and decentralized routing algorithms. In decentralized routing algorithms, each router has information about the routers it is directly connected to -- it doesn't know about every router in the network. These algorithms are also known as DV (distance vector) algorithms. In global routing algorithms, every router has complete information about all other routers in the network and the traffic status of the network. These algorithms are also known as LS (link state) algorithms. We'll discuss LS algorithms in the next .

LS Algorithms

In LS algorithms, every router has to follow these steps:

1. Identify the routers that are physically connected to them and get their IP addresses
   When a router starts working, it first sends a "HELLO" packet over network. Each router that receives this packet replies with a message that contains its IP address.
   
2. Measure the delay time (or any other important parameters of the network, such as average traffic) for neighbor routers
   In order to do that, routers send echo packets over the network. Every router that receives these packets replies with an echo reply packet. By dividing round trip time by 2, routers can count the delay time. (Round trip time is a measure of the current delay on a network, found by timing a packet bounced off some remote host.) Note that this time includes both transmission and processing times -- the time it takes the packets to reach the destination and the time it takes the receiver to process it and reply.
   
3. Broadcast its information over the network for other routers and receive the other routers' information
   In this step, all routers share their knowledge and broadcast their information to each other. In this way, every router can know the structure and status of the network.
   
4. Using an appropriate algorithm, identify the best route between two nodes of the network
   In this step, routers choose the best route to every node. They do this using an algorithm, such as the Dijkstra shortest path algorithm. In this algorithm, a router, based on information that has been collected from other routers, builds a graph of the network. This graph shows the location of routers in the network and their links to each other. Every link is labeled with a number called the weight or cost. This number is a function of delay time, average traffic, and sometimes simply the number of hops between nodes. For example, if there are two links between a node and a destination, the router chooses the link with the lowest weight.

The Dijkstra algorithm goes through these steps:

1. The router builds a graph of the network and identifies source and destination nodes, as V1 and V2 for example. Then it builds a matrix, called the "adjacency matrix." In this matrix, a coordinate indicates weight. For example, [i, j] is the weight of a link between Vi and Vj. If there is no direct link between Vi and Vj, this weight is identified as "infinity."
   
2. The router builds a status record set for every node on the network. The record contains three fields:
   
   
   • Predecessor field - The first field shows the previous node.
   • Length field - The second field shows the sum of the weights from the source to that node.
   • Label field - The last field shows the status of node. Each node can have one status mode: "permanent" or "tentative."
   
   
   
   
3. The router initializes the parameters of the status record set (for all nodes) and sets their length to "infinity" and their label to "tentative."
   
4. The router sets a T-node. For example, if V1 is to be the source T-node, the router changes V1's label to "permanent." When a label changes to "permanent," it never changes again. A T-node is an agent and nothing more.
   
5. The router updates the status record set for all tentative nodes that are directly linked to the source T-node.
   
6. The router looks at all of the tentative nodes and chooses the one whose weight to V1 is lowest. That node is then the destination T-node.
   
7. If this node is not V2 (the intended destination), the router goes back to step 5.
   
8. If this node is V2, the router extracts its previous node from the status record set and does this until it arrives at V1. This list of nodes shows the best route from V1 to V2.

Working of Fiber Optics

How Fiber Optics Work

You hear about fiber-optic cables whenever people talk about the telephone system, the cable TV system or the Internet. Fiber-optic lines are strands of optically pure glass as thin as a human hair that carry digital information over long distances. They are also used in medical imaging and mechanical engineering inspection.







Parts of a single optical fiber In this article, we will show you how these tiny strands of glass transmit light and the fascinating way that these strands are made.

What are Fiber Optics?

Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter of a human hair. They are arranged in bundles called optical cables and used to transmit light signals over long distances.
If you look closely at a single optical fiber, you will see that it has the following parts:

• Core - Thin glass center of the fiber where the light travels
• Cladding - Outer optical material surrounding the core that reflects the light back into the core
• Buffer coating - Plastic coating that protects the fiber from damage and moisture

Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. The bundles are protected by the cable's outer covering, called a jacket. Optical fibers come in two types:

• Single-mode fibers
• Multi-mode fibers

Single-mode fibers have small cores (about 3.5 x 10^-4 inches or 9 microns in diameter) and transmit infrared laser light (wavelength = 1,300 to 1,550 nanometers). Multi-mode fibers have larger cores (about 2.5 x 10^-3 inches or 62.5 microns in diameter) and transmit infrared light (wavelength = 850 to 1,300 nm) from light-emitting diodes (LEDs).
Some optical fibers can be made from plastic. These fibers have a large core (0.04 inches or 1 mm diameter) and transmit visible red light (wavelength = 650 nm) from LEDs.
Let's look at how an optical fiber works.

How Does an Optical Fiber Transmit Light?

Suppose you want to shine a flashlight beam down a long, straight hallway. Just point the beam straight down the hallway -- light travels in straight lines, so it is no problem. What if the hallway has a bend in it? You could place a mirror at the bend to reflect the light beam around the corner. What if the hallway is very winding with multiple bends? You might line the walls with mirrors and angle the beam so that it bounces from side-to-side all along the hallway. This is exactly what happens in an optical fiber.

Diagram of total internal reflection in an optical fiber

The light in a fiber-optic cable travels through the core (hallway) by constantly bouncing from the cladding (mirror-lined walls), a principle called total internal reflection. Because the cladding does not absorb any light from the core, the light wave can travel great distances.
However, some of the light signal degrades within the fiber, mostly due to impurities in the glass. The extent that the signal degrades depends on the purity of the glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibers show much less signal degradation -- less than 10 percent/km at 1,550 nm.

working of Routers

How Routers Work

Fujitsu GeoStream R980 industrial strength router.

The Internet is one of the 20th century's greatest communications developments. It allows people around the world to send e-mail to one another in a matter of seconds.
We're all used to seeing the various parts of the Internet that come into our homes and offices -- the Web pages, e-mail messages and downloaded files that make the Internet a dynamic and valuable medium. But none of these parts would ever make it to your computer without a piece of the Internet that you've probably never seen. In fact, most people have never stood "face to machine" with the technology most responsible for allowing the Internet to exist at all: the router.

Keeping the Messages Moving

When you send e-mail to a friend on the other side of the country, how does the message know to end up on your friend's computer, rather than on one of the millions of other computers in the world? Much of the work to get a message from one computer to another is done by routers, because they're the crucial devices that let messages flow between networks, rather than within networks.



Let's look at what a very simple router might do. Imagine a small company that makes animated 3-D graphics for local television stations. There are 10 employees of the company, each with a computer. Four of the employees are animators, while the rest are in sales, accounting and management. The animators will need to send lots of very large files back and forth to one another as they work on projects. To do this, they'll use a network.
When one animator sends a file to another, the very large file will use up most of the network's capacity, making the network run very slowly for other users. One of the reasons that a single intensive user can affect the entire network stems from the way that Ethernet works. Each information packet sent from a computer is seen by all the other computers on the local network. Each computer then examines the packet and decides whether it was meant for its address. This keeps the basic plan of the network simple, but has performance consequences as the size of the network or level of network activity increases. To keep the animators' work from interfering with that of the folks in the front office, the company sets up two separate networks, one for the animators and one for the rest of the company. A router links the two networks and connects both networks to the Internet.

Directing Traffic

The router is the only device that sees every message sent by any computer on either of the company's networks. When the animator in our example sends a huge file to another animator, the router looks at the recipient's address and keeps the traffic on the animator's network. When an animator, on the other hand, sends a message to the bookkeeper asking about an expense-account check, then the router sees the recipient's address and forwards the message between the two networks.
One of the tools a router uses to decide where a packet should go is a configuration table. A configuration table is a collection of information, including:

• Information on which connections lead to particular groups of addresses
• Priorities for connections to be used
• Rules for handling both routine and special cases of traffic

A configuration table can be as simple as a half-dozen lines in the smallest routers, but can grow to massive size and complexity in the very large routers that handle the bulk of Internet messages. A router, then, has two separate but related jobs:

• The router ensures that information doesn't go where it's not needed. This is crucial for keeping large volumes of data from clogging the connections of "innocent bystanders."
• The router makes sure that information does make it to the intended destination.

In performing these two jobs, a router is extremely useful in dealing with two separate computer networks. It joins the two networks, passing information from one to the other and, in some cases, performing translations of various protocols between the two networks. It also protects the networks from one another, preventing the traffic on one from unnecessarily spilling over to the other. As the number of networks attached to one another grows, the configuration table for handling traffic among them grows, and the processing power of the router is increased. Regardless of how many networks are attached, though, the basic operation and function of the router remains the same. Since the Internet is one huge network made up of tens of thousands of smaller networks, its use of routers is an absolute necessity.

Working of Nintendo 64

How N64 Works

History

Just as Atari ushered in the dawn of the home video game, Nintendo is largely considered to be the company that revolutionized the industry with the introduction of the Nintendo Entertainment System (NES) in 1985. An 8-bit system based on the 6502 processor and some custom chips, the NES came together with Super Mario Brothers; this inclusion of an accurate home version of one of the most popular arcade games at the time turned out to be pure genius. Sales of the NES were phenomenal. This established Nintendo as the dominant home video game manufacturer until the late '90s, when it was eclipsed by the rival Sony PlayStation.

Working of Dreamcast

How Dreamcast Works

Sega Dreamcast hit the market in 1999 and was hailed as an innovative video game system.

With good reason, Popular Science magazine recognized the Sega Dreamcast as one of the most important and innovative products of 1999. Impressive technical specifications, great games and an imaginative advertising campaign heralded the arrival of the latest system from a company known for groundbreaking Video game systems.­

In this edition , you will learn about this phenomenal system, the revolutionary controller that it uses and the proprietary optical storage method chosen by Sega. You will also learn about the VMS, a versatile option that can be used all by itself!

Working of GameCube

How GameCube Works

In the United States, the Nintendo GameCube is the undeniable underdog of the "console wars." Sony's PlayStation 2 and Microsoft's Xbox certainly sell better, and they tend to get more media attention. Toward the end of 2002, for example, the PlayStation 2 stirred up a lot of controversy with its new game Grand Theft Auto : Vice City, and Microsoft had a hit with Xbox Live, its online gaming program. Once the undisputed king of home video games, Nintendo seems to be struggling just to hold its own.

The GameCube comes in a variety of colors like its predecessor Nintendo 64.

But if you've actually spent any time with a GameCube, you may be confused by its reputation as a third-rate system. It's hard to see ground-breaking GameCube games like Metroid Prime and Super Mario Sunshine as anything less than state of the art. No matter how it fares in sales, this console is definitely a remarkable technological achievement.

we'll find out what the GameCube has under the hood, and we'll see how it stacks up to the competition.

Working of Cable Modems

How Cable Modems Work

Motorola SURFboard modem

For millions of people, television brings news, entertainment and educational programs into their homes. Many people get their TV signal from cable television (CATV) because cable TV provides a clearer picture and more channels. (See How Cable TV Works for details.)

Many people who have cable TV can now get a high-speed connection to the Internet from their cable provider. Cable modems compete with technologies like asymmetrical subscriber lines (ADSL). If you have ever wondered what the differences between DSL and cable modems are, or if you have ever wondered how a computer network can share a cable with dozens of television channels, then read on. In this article, we'll look at how a cable modem works and see how 100 cable television channels and any Web site out there can flow over a single coaxial cable into your home.