### Wavelet:

Wavelets are tiny parts of waves.

### Secondary wavefront:

The envelop of all the secondary wavelets in the forward direction is called the secondary wavefront. The backward envelop is not a secondary wavefront.

### Wavefront

A wavefront can be defined as the locus of all the points of the medium which are vibrating in phase and are also displaced at the same time.

If the distance of the source is small the wavefront is spherical. But when the distance is large then any small portion of the wavefront can be considered plane.

### Diffraction grating

Diffraction grating is an extremely useful device. In one of its form it consists of a very large number of narrow slits side by side. The slits are separated by opaque spaces.

### What do you mean by Fresnel and Fraunhofer class of diffraction?

Diffraction phenomena can conveniently divided into two groups.
1. Fresnel Diffraction Phenomena: In the Fresnel class of diffraction, the source or the screen or the both are at finite distances from the aperture or obstacle causing diffraction. In this case, the effect at a specific point on the screen due to the exposed incident wavefront is considered and no modification is made by lenses and mirrors.
2. Fraunhofer Diffraction Phenomena: In the Frauhofer class of diffraction phenomena, the source and the screen on which the pattern is observed are at infinite distances from the aperture or the obstacle causing diffraction. The incoming light is rendered parallel with a lens and the diffracted beam is focused on the screen with another lens.

### Diffraction

The rhythmic variations in intensity and the bending of light around the corners of an obstacle or the encroachment of light into the region of geometrical shadow constitute a class of phenomena known as the diffraction of light.

### Newton’s rings and its formation :

When a Plano-convex lens of long focal length is placed on a plane glass plate, a thin film of air is enclosed between the lower surface of the lens and the upper surface of the plate. The thickness of the air film is very small at the point of contact and gradually increases from the center outwards.

### Fringe width

The distance between any two consecutive bright or dark fringe is known as fringe width. Symbolically the fringe width is β and

### Why two independent source of light can not produce interference?

Two independent sources of light can not produce interference because a large number of difficulties are involved. The two sources may emit light waves of largely different amplitude and wave length and the phase difference between the two may change with time. That’s why interference of light is produced by two coherent sources which are virtual.

### Coherent sources

Two sources are said to be coherent if they emit light waves of same frequency, nearly equal amplitude and are always in phase with each other.

### Interference of light:

Wave emitted from two coherent sources having the same wavelength, equal or nearly equal amplitude when passes through a point in a medium, then superposition takes place.

During superposition, if the two waves reaching the point in the same phase then the intensity will be maximum that is bright. But if the two waves reaching the point in opposite phase then the point will have minimum intensity that is dark. This variation of intensity due to superposition is called interference of light.

The interference is said to be constructive or destructive when the intensity is respectively more or less than that given by one beam alone.

### Huygens principle

To explain the propagation of light through ether, Huygens proposed the following principles for the construction of a wavefront at a subsequent time from the known position of the wavefront at any given instance.

### Stationary waves:

When two simple harmonic waves of the same amplitude, frequency and time period travel in opposite directions in a straight line, the resultant wave obtained is called a stationary or a standing wave. The formation of stationary waves is due to the superposition of the two waves on the particles of the medium.

### Transverse waves:

In transverse wave motion, the particles of the medium vibrate at right angles to the direction of propagation of wave. Example: Light waves are transverse waves.

### Longitudinal wave motion:

In longitudinal wave motion, particles of the medium vibrate along the direction of propagation of the wave. Example: Sound waves are longitudinal waves.

### Wave motion:

Wave motion is a form of disturbance produced in the medium by the repeated periodic motion of the particles of the medium. Here, only the waves travel forward whereas the particles of the medium vibrate about their mean positions.

### Resonance:

When the forced frequency is equal to the natural frequency of vibration of the body, resonance takes place. Resonance is a vibration of large amplitude produced by a relatively small vibration near the same frequency of vibrations as the natural frequency of the resonating system.

### Undamped free vibrations:

When a bob of a simple pendulum (in vacuum) is displaced from its mean position and left, it executes simple harmonic motions. The pendulum will continue to oscillate with the same time period and amplitude for any length of time. In such cases there is no loss of energy by friction or otherwise. In all similar cases, the vibrations will be undamped free vibrations.

### Lissajous’ figure:

When a particle is influenced simultaneously by two simple harmonic motions at right angles to each other, the resultant motion of the particle traces a curve. These curves are called Lissajous’ figure.

### Simple harmonic motion:

The type of motion where the acceleration is directed towards a fixed point (the mean position of rest) and is proportional to the displacement of the vibrating particle is called simple harmonic motion.

### Entropy:

The term ‘Entropy’ is a Greek word and means transformation. Entropy is a physical property or nature of a substance that remain constant in an adiabatic process. It is a quantity that increases with the application of heat in a reversible manner.

If, a process or cycle reject or absorb heat of amount dQ in T temperature, then entropy,

### Carnot’s reversible engine

In 1824, Sadi Carnot conceived a closed reversible cycle consisting two strokes. Since that Carnot’s cycle reversible because it consists two isothermals and two adiabatic, all of which are reversible process.

Carnot’s engine is free from all the defects of practical engine, its efficiency is maximum and it is an ideal heat engine. It has three essential requisites;

### Postulates of the kinetic theory of gases:

Following are the 13 postulates of kinetic theory of gases.

### Isothermal process:

If a system is perfectly conducting to the surroundings and the temperature remains constant throughout the process, then the process is called an isothermal process. In this process, although the temperature remains constant but the heat is variable.

From the given figure, from A to B, pressure decreased and work is done. There should be fall in temperature but the system absorbs heat and maintains a constant temperature.

From B to A, pressure increased and external work is done. There should be rise in temperature, but the system gives up extra heat to the surroundings and the temperature remain constant.

### Second law of thermodynamics

The second law of thermodynamics has Kelvin-Planck statement and Kelvin-Clausius statement. They are stated below.

### First Law of Thermodynamics:

The first law of thermodynamics states that the amount of heat given to a system is equal to the sum of the increase in the internal energy of the system and the external work done.

### What are the various types of video formats?

Following are various types video file formats with minimal amount of description.

### What are the various types of Audio formats.

Following are various types of Audio Formats with Minimal Descriptions.

### What are various types of picture formats.

Graphics file formats are mainly differ as raster formats and vector graphics. Below, some of the raster formats have been described.

### Why we need own website Or What is the necessity of personal websites?

There are probably as many reasons for wanting to publish a site on the web. Below, some types of web sites and their purposes have been given.

### Why we need Search Engines Or Search Engine vs Web Directories.

Necessity of having search engines : The main advantage of Web directories is that the categorize of web sites are creation of humans and human can go through human’s choice. When in a web directory to find something, one may have to click a lot. That’s why; it takes longer time to find necessary information. Web directories are great for browsing when user doesn’t know the exact term of the information we are looking for.

### Web directories

A web directory or link directory is a directory on the World Wide Web. It specializes in linking to other web sites and categorizing those links. Thus, in web directory, web sites are categorized by subject.

### URL (Uniform Resource Locator):

URL stands for Uniform Resource Locator, which is simply an address of a document on the web, more accurately, on the internet. Although a URL can look complex and long, it’s made up of four basic parts – protocols, host names, folder names and file name – each of which has a specific function. Depending on the application, a URL can include additional information, to specify user names or input to a server-side script, but fundamentally these four parts cover the basics.

### What are the various types of linking or anchor tags?

Link anchors are the glue that holds the web together. Link can be made to documents in the same folder, in different folder and on the web. They are described below.

Linking or Links : Links are the ‘hyper’ part of the hypertext – that is the part user uses to jump from one HTML page to another. Links, also called anchors, mark text or images as elements that points to other HTML documents, images, applets, multimedia effects or specific places within a HTML document. Links differentiate HTML documents from other electronic documents.

HTML : HTML is the short form of Hypertext Markup Language. HTML is normally used to create documents for the World Wide Web. When producing web pages, it allows using hypertext that can link a word or image to another page or web location. Using this language one can put audio-video clips, graphics etc in one’s documents. HTML is a special series of codes.

### Domain name selection guidelines:

Domain name should have to be easy to remember, such as names that reflect the name of the organization. Example : The domain of T&T Firm may be like, tandtfirm.com

Single letter domain names are not permitted. Example : x.com will not be a domain name.

Each top level domain name is reserved for a specific purpose. Such as, .com is for commercial purpose, .org for non-profitable organizations etc.

### Creating a web site:

Creating a web site consists several steps. But the most common and important steps to be followed can be shown by the following flowchart.

### Define Domain name with example. Write down the name of the top level domains with their purposes.

Domain name: Computers on the net has unique IP addresses. These addresses contain numbers which are very difficult to remember for us. That’s why, to make it easy to remember they are replaced by alias which are called domain name. These domain names are also unique for each computer. Example : funny.comedian-action.com

### What is Internet and IP address?

Internet : Internet is International Wide Area Network or International Network. In other words, internet is a network of networks that connects the computers of all over the world. It provides files and data transfer together with electronic mail functions for millions of users around the world. Anyone can use the internet.

### What is Hypertext?

Hypertext: Hypertext is used to develop hyperlink. A hyperlink is a special string or phrases in a web page that point to another web page. That is, when we click on or select a hyperlink which may be a text or an image, we will be transported immediately to the page that is linked by the hyperlink. Web pages are often known as hypertext documents.

### Why search engine was created? Describe a search engine with its working procedure.

Search engine: A search engine is a software that carries out a search of a database when user asked it to do so. Back in the old days, internet was primarily a research medium. Files were stored separately, with no way to go from one file to another easily. In 1991, Tim Berner’s Lee developed a network of files with hypertext links.

### What is a web site?

Web site: A web site is a collection of web pages. There are various links on various topics & issues. On clicking them, user can move to the linked & related web pages. A web site can be accessed by a web browser. A web site can also contain downloadable audio-video, software, games, online games and many more. Example : www.yahoo.comwww.google.com etc.

### What is a web page? Classify web page and give definitions.

Web page: A web page is a single file stored on the server that may contain text, graphics, audio-video and hyperlinks to the other pages on the internet or within a web site. Web pages are written using HTML (Hypertext Markup Language). There are two types of web pages; they are given below.

### What is web browser and server?

Web browser: Web browser is a software program that is used to navigate through WWW pages stored on the internet. Users use this to request a web page to the server by giving the address and then the browser asks the server and the server returns the page which is then decoded and displayed by the web browser. That means, the web browser fills the gap between the user and the server. The browser displays any hotspot and will jump to another page if the user clicks on the hyperlink. Thus, It works as follows,

### Define World Wide Web. | HTML

World Wide Web (WWW): The world wide web is a network of computers. When on WWW, a cyberspace traveler can go through information stored in any computer of the world if the information is permitted to be explored on WWW. The information comes to the user in a web page or electronic page which contains hyperlinks. Using these links, user can move to another page. So, WWW is an application of hypertext with links.

### Commonly used scanf format codes:

In the following table the required format codes and their meanings are given.

### Character Test Functions:

In the following table the required functions and their tests are given.

### Input & output operations:

The operations which took place in order to take data and make the display of the processed information are known as input & output operations.

### Rules for ++ and – operators:

Following are the rules for the increment (++) and decrement (--) operators.

### Arithmetic operators:

The arithmetic operators are +, -, *, /. They work the same way as they do in other languages. There are three types of arithmetic operators which are as follows,

### Operator:

An operator is a symbol that tells the computer to perform certain mathematical and logical manipulations. Operators are used in programs to manipulate data and variables. Such as, ++, --, <, > etc.

### Data types:

C language is rich in its data types. ANSI supports three classes of data types.
1. Primary or fundamental data types.
2. Derived data types.
3. User-defined data types.

### Variables:

A variable is a data name that may be used to store data value. A variable may take different value at different times during execution. Some examples are, average, height, class_strength.

### Keywords:

Keywords have fixed meanings and these meanings can not be changed. There are 32 keywords. Some compiler may use additional keywords that must be identified from the C manual. Keywords serve as basic building block for a program statement.

### Character set:

The various category of characters are called the character set. In C, the characters are grouped into the following categories.

### Constants:

Constants are of fixed values that do not change during the execution of a program. There are various types of constants. The types are illustrated in the following figure.

### Information:

After converting or processing the data which may consists of numbers, characters, strings etc to something which is useful and make much sense to a person is known as Information. So, information is the converted or processed form of data.

### The else if ladder :

When a multipath decision is involved then we use else if ladder. A multipath decision is a chain of ifs in which the statement associated with each else is an if. It takes the following general form,

### Nested if...else statement:

When a series of decisions are involved, we may have to use more than one if-else statement in nested form as shown below,

### Write down about the if-else statement

The if-else statement : The general form of the if....else statement is as follows,

### Write down about the simple if statement

Simple if statement : The general form of a simple if statement is as follows,

### Write down about the decision making statement if.

The if statement : The if statement is a powerful decision making statement and is used to control the flow of execution of statements. It is basically a two way decision-statement. It takes the following form,

### What is decision making? What are the statements used in C for decision making purpose?

Decision making : In practice, users may have a number of situations where they may have to change the order of statements based on certain conditions. This is decision making. The decision making statements in C are as follows

### Draw the flowchart of a multifunction program

Multifunction program: A program, which has been written using a number of functions, is treated as a multifunction program. The flowchart of the flow of control in a multifunction program is as follows,

### What is recursion?

Recursion : Recursion is a special case where a function calls itself. A very simple example of recursion is presented below,

### What is nesting of functions?

Nesting of functions : In C, each function can contain one or more than one function in it. There is no limit as to how deeply functions can be nested. Consider the following example,

### What is function prototype?

Prototype : The declaration of a function is known as function prototype. The function prototype is coded in the following format,

### What are parameters in functions?

Function Parameter: Function parameters are variables of a function that we can use to send data to the function. Parameters are declared as follows:

### How to declare a function in C or C Plus Plus?

Function declaration : The program or a function that called a function is referred to as the calling function or calling program. The calling program should declare any function that is to be used later in the program. This is known as the function declaration.

### Explain or Discuss function call.

Function call : In order to use functions user need to invoke it at a required place in the program. This is known as the function call. A function can be called by simply using the function name followed by a list of actual parameters, if any, enclosed in parentheses.

### Function Definition:

The function definition is an independent program module that is specially written to implement to the requirements of the function. A function definition, also known as function implementation shall include the following elements

### What are the elements of user defined functions?

Elements of user defined functions: In order to make use of a user-defined function, we need to establish three elements that are related to functions.

Function definition
Function call
Function declaration

### What is multi-function program?

Multifunction program : A function is a self-contained block of code that performs a particular task. Once a function has been designed and packed, it can be treated as a ‘black box’ that takes some data from the main program and returns a value. Thus a program, which has been written using a number of functions, is treated as a multi-function program.

### What are the necessities or advantages of user defined functions?

Necessity of user defined functions : When not using user defined functions, for a large program the tasks of debugging, compiling etc may become difficult in general. That’s why user defined functions are extremely necessary for complex programs. The necessities or advantages are as follows,

### User defined functions:

C functions can be classified into two categories, namely, library functions and user-defined functions. The functions which are developed by user at the time of writing a program are called user defined functions. So, user defined functions are functions created and developed by user.

### 3 Differences between counter and sentinel controlled loops. Counter controlled loop VS Sentinel controlled loop

The differences between the counter and sentinel controlled loops are as follows.

### 3 Differences between entry and exit controlled loops with examples. Entry controlled loops VS Exit controlled loops

The differences between the entry and exit controlled loops are as follows.

### Comparison among the three loop:

The comparison among the three types of loops for loop, while loop and do....while loop is given below.

### What are the entry controlled and exit controlled loops?

Depending on the position of the control statement in the loop, a control structure can be classified into two types; entry controlled and exit controlled. They are described below.

### Write down the steps of looping process.

Steps of looping process: A looping process, in general, would include the following four steps.

### What are the counter controlled and sentinel controlled loops?

Based on the nature of the control variables and the kind of value assigned to, the loops may be classified into two general categories; counter controlled and sentinel controlled loops.

### Loop

A loop is a particular area of a program where some executable statements are written which gets execution by testing one or more conditions. So, in looping, a sequence of statements is executed until some conditions for termination are satisfied.

A program loop therefore consists of two segments; the body of the loop and the control statement.

### What are the static arrays and dynamic arrays

Static arrays : The process of allocating memory at compile time is known as static memory allocation and the arrays that receives static memory allocation are called static arrays. This approach works fine as long as user know that what the data requirements are.

### Write down the initialization of two dimensional array

Initialization of two dimensional array : Like the one dimensional arrays, two dimensional arrays may be initialized by following their declaration with a list of initial values enclosed in braces. For example,

### Write down about the initialization of one dimensional array

Initialization of one dimensional array :  After an array is declared, its elements must be initialized. An array can be initialized either of the following stages,

§  At compile time.
§  At run time.

Compile time initialization : User can initialize the elements of an array in the same way as the ordinary variables when they are declared. This is compile time initialization. The general form is as follows,

type array_name[size]={list of values};

The values in the list are separated by commas. For example,

int number[3]={0,0,0};

This will declare the variable number as an array of size 3 and will assign 0 to each element.

Run time initialization : An array can be explicitly initialized at run time. This approach is usually applied for initializing large arrays. For example,

The first 50 elements of the array sum are initialized to zero while the remaining 50 elements are initialized to 1.0 at run time.

### Write down about the declaration of one dimensional array.

Declaration of one dimensional array : Like any other variable, arrays must be declared before they are used. The general form of array declaration is,

### One dimensional array:

A list of items can be given one variable name using only one subscript and such a variable is called a one dimensional array.

Example : int number[5];

Here in the example, 5 value of the variable number can be kept under the single variable number.

### What do you mean by arrays? Write different types of arrays.

Array : Array is a derived data type. When it is necessary to store more than one value under a variable, user can make use of array. An array is a fixed-size sequence collection of elements of the same data type. It is simply a grouping of like-data type.

Different types of arrays : There are three types of arrays. They are,

1. One dimensional array.
2. Two dimensional array.
3. Multidimensional array.

### Draw the flow chart of the process of compiling and running a C program

The flow chart of process of compiling and running a C program is as follows,

### What is operating system (Definition)

Operating system: An operating system is a program that controls the entire operation of a computer system. All input/output functions are channeled through the operating system. The operating system which is the interface between the hardware and the user, handles the execution of user programs.

### Executing a C program :

Executing a C program involves a series of steps. They are,

1. Creating the program.
2. Compiling the program.
3. Linking the program with functions that are needed from the C library.
4. Executing the program.

Although these steps remain the same irrespective of the operating system, system command for implementing the steps and conventions for naming files may differ on different system.

Must Read: Draw the flowchart of the process of compiling and running a C Program.

### What is #define directive?

#define directive:
1. A #define is a preprocessor compiler directive and not a statement.
2. #define should not end with a semicolon.
3. Symbolic constants are generally written in uppercase so that they can be easily distinguished from the lowercase variable names.
4. #define instructions are usually placed at the beginning before the main() function.

### Basic structure of C programming:

To write a C program, we first create functions and then put them together. A C program may contain one or more sections. They are illustrated below.

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### #include directive:

C programs are divided into modules or functions. Some functions are written by users and some are stored in C library. Library functions are grouped category wise and stored in different files known as header files. To access the file stored in the library, it is necessary to tell the compiler about the files to be accessed. This is achieved by the pre-processor directive #include as follows,

#include<filename>

Filename is the name of the library file that contains the required function definition. Pre-processor directives are placed at the beginning of the program.

### Define preprocessor, function, compile time, run time.

Preprocessor: It is a program that process the source code before it passes through the compiler.

Function: A function is a subroutine that may include one or more statements designed to perform specific tasks. For C programming functions are as building blocks.

Compile time: Compile time refers to the event that occur during the compilation process.

Run time: Run time refers to the event that occur while the program is actually executing.

### History of C:

C language is one of the most popular computer languages today because it is a structured, high level, machine independent language. It allows software developers to develop software without worrying about the hardware platforms where they will be implemented.

### Think Slow and Stay Steady

Human brain, an amazing thing with amazing features installed. We are not that much aware of how fast it process data that it receives from enormous number of sensors. It keeps going on, even when we were in the womb of our mother. From that very starting point it keeps running until we die, well that’s how far we know.

### Operations of Data Structures:

The particular data structure that one chooses for a given situation depends largely on the frequency with which specific operations are performed. Following are some most frequent operations.

### Explain different types of data structures.

Data structure means how the data is organized in memory. There different kind of data structures. Some are used to store the data of same type and some for different types of data. Different types of data structures help different types of operations of data structures too. Data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.

### Data Structure:

Data structure means how the data is organized in memory. There different kind of data structures. Some are used to store the data of same type and some for different types of data. Different types of data structures help different types of operations of data structures too. Data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.

# Memcache

Memcached is a general-purpose distributed memory caching system and memcache is the top level application or module that uses memcached service. It is often used to speed up dynamic database-driven websites by caching data and objects in RAM to reduce the number of times an external data source (such as a database or API) must be read.

### Why I would choose PhpStorm over Adobe Dreamweaver?

Okay, first of all guys, I have been using PHPStorm for the last project I am working on and its really awesome, but the following list awesomeness is obviously incomplete.

### ফহিন্নির ঘরের ফহিন্নি সব

ফহিন্নির ঘরের ফহিন্নি সব। রাস্তার পাশে শীতে কুঁকড়ে পড়ে থাকা বাপের বয়সী একটা ভিক্ষুকের ভিক্ষার থালায় ২ টাকা দিতে যে পারে না, সে আবার অন্য কেউ সেটা দিলে বলে 'হ্যারে (!) যে টাকাটা দিলেন, এইডা দিয়া দিয়াই তো তাগো অভ্যাসডা খারাপ কইরা দিতেছেন', সুন্দর হাসি-মাখা্ মুখে বললেন ভদ্রলোক (!)