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+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Chapter 4:LASERS "
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "##Example 4.1, Page number 4.32"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Divergence = 0.5 *10**-3 radian\n"
+     ]
+    }
+   ],
+   "source": [
+    "import math\n",
+    "from __future__ import division\n",
+    "\n",
+    "#variable declaration\n",
+    "r1 = 2;                #in radians\n",
+    "r2 = 3;                #in radians\n",
+    "d1 = 4;                #Converting from mm to radians\n",
+    "d2 = 6;                #Converting from mm to radians\n",
+    "\n",
+    "#calculations\n",
+    "D = (r2-r1)/(d2*10**3-d1*10**3)   #Divergence\n",
+    "\n",
+    "#Result\n",
+    "print \"Divergence =\",round(D*10**3,3),\"*10**-3 radian\""
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "##Example 4.2, Page number 4.32"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Frequency (V) = 4.32 *10**14 Hz\n",
+      "Relative Population= 1.081 *10**30\n"
+     ]
+    }
+   ],
+   "source": [
+    "import math\n",
+    "from __future__ import division\n",
+    "\n",
+    "#variable declaration\n",
+    "C=3*10**8                    #The speed of light\n",
+    "Lamda=6943                   #Wavelength\n",
+    "T=300                        #Temperature in Kelvin\n",
+    "h=6.626*10**-34              #Planck constant \n",
+    "k=1.38*10**-23               #Boltzmann's constant\n",
+    "\n",
+    "#Calculations\n",
+    "\n",
+    "V=(C)/(Lamda*10**-10)       #Frequency\n",
+    "R=math.exp(h*V/(k*T))       #Relative population\n",
+    "\n",
+    "#Result\n",
+    "print \"Frequency (V) =\",round(V/10**14,2),\"*10**14 Hz\"\n",
+    "print \"Relative Population=\",round(R/10**30,3),\"*10**30\""
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "##Example 4.3, Page number 4.32"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      " Frequency= 4.74 *10**14 Hz\n",
+      "no.of photons emitted= 7.322 *10**15 photons/sec\n",
+      "Power density = 2.3 kWm**-2\n"
+     ]
+    }
+   ],
+   "source": [
+    "import math\n",
+    "from __future__ import division\n",
+    "\n",
+    "#variable declaration\n",
+    "C=3*10**8                     #Velocity of light\n",
+    "W=632.8*10**-9                #wavelength\n",
+    "P=2.3\n",
+    "t=1\n",
+    "h=6.626*10**-34              #Planck constant \n",
+    "S=1*10**-6\n",
+    "\n",
+    "#Calculations\n",
+    "V=C/W                        #Frequency\n",
+    "n=((P*10**-3)*t)/(h*V)       #no.of photons emitted\n",
+    "PD=P*10**-3/S                #Power density\n",
+    "\n",
+    "#Result\n",
+    "print \"Frequency=\",round(V/10**14,2),\"*10**14 Hz\"\n",
+    "print \"no.of photons emitted=\",round(n/10**15,3),\"*10**15 photons/sec\"\n",
+    "print \"Power density =\",round(PD/1000,1),\"kWm**-2\"\n"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "##Example 4.4, Page number 4.33"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Wavelenght = 8628.0 Angstrom\n"
+     ]
+    }
+   ],
+   "source": [
+    "import math\n",
+    "from __future__ import division\n",
+    "\n",
+    "#variable declaration\n",
+    "h=6.626*10**-34              #Planck constant \n",
+    "C=3*10**8                    #Velocity of light\n",
+    "E_g=1.44                     #bandgap \n",
+    "\n",
+    "#calculations\n",
+    "lamda=(h*C)*10**10/(E_g*1.6*10**-19)       #Wavelenght\n",
+    "\n",
+    "#Result\n",
+    "print \"Wavelenght =\",round(lamda),\"Angstrom\"\n"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "##Example 4.5, Page number 4.33"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 25,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Band gap = 0.8 eV\n"
+     ]
+    }
+   ],
+   "source": [
+    "import math\n",
+    "from __future__ import division\n",
+    "\n",
+    "#variable declaration\n",
+    "W=1.55                       #wavelength\n",
+    "\n",
+    "#Calculations\n",
+    "E_g=(1.24)/W                 #Bandgap in eV           \n",
+    "\n",
+    "#Result\n",
+    "print \"Band gap =\",E_g,\"eV\""
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 2",
+   "language": "python",
+   "name": "python2"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 2
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython2",
+   "version": "2.7.9"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}