12: Lasers

Example number 12.1, Page number 360

In [1]:
#importing modules
import math
from __future__ import division

#Variable declaration
e=1.6*10**-19;     #charge(coulomb)
c=3*10**8;   #velocity of matter wave(m/s)
h=6.62*10**-34;    #plank's constant(Js)
lamda=6328*10**-10;    #wavelength(m)

#Calculation
E=h*c/(lamda*e);    #energy of photon(eV)
p=h/lamda;      #momentum of photon(kg m/s)

#Result
print "energy of photon is",round(E,2),"eV"
print "momentum of photon is",round(p*10**27,2),"*10**-27 kg m/s"

energy of photon is 1.96 eV
momentum of photon is 1.05 *10**-27 kg m/s

Example number 12.2, Page number 360

In [2]:
#importing modules
import math
from __future__ import division

#Variable declaration
c=3*10**8;   #velocity of matter wave(m/s)
h=6.62*10**-34;    #plank's constant(Js)
lamda=7000*10**-10;    #wavelength(m)
n=2.8*10**19;       #number of ions

#Calculation
E=n*h*c/lamda;    #energy of laser pulse(joule)

#Result
print "energy of laser pulse is",round(E,2),"joule"

energy of laser pulse is 7.94 joule

Example number 12.3, Page number 361

In [3]:
#importing modules
import math
from __future__ import division

#Variable declaration
c=3*10**8;   #velocity of matter wave(m/s)
l=2.945*10**-2;
lamda=5890*10**-10;    #wavelength(m)

#Calculation
n=l/lamda;       #number of oscillations
tow_c=l/c;       #coherence time(s)

#Result
print "number of oscillations is",int(n/10**4),"*10**4"
print "coherence time is",round(tow_c*10**11,2),"*10**-11 s"

number of oscillations is 5 *10**4
coherence time is 9.82 *10**-11 s

Example number 12.4, Page number 361

In [4]:
#importing modules
import math
from __future__ import division

#Variable declaration
P=10*10**-3;     #power(W)
d=1.3*10**-3;      #diameter(m)

#Calculation
I=4*P/(math.pi*d**2);      #intensity of beam(W/m**2)

#Result
print "intensity of beam is",round(I/10**3,1),"kW/m**2"

intensity of beam is 7.5 kW/m**2

Example number 12.5, Page number 361

In [5]:
#importing modules
import math
from __future__ import division

#Variable declaration
c=3*10**8;   #velocity of matter wave(m/s)
h=6.62*10**-34;    #plank's constant(Js)
lamda=6940*10**-10;    #wavelength(m)
P=1;     #power(J)

#Calculation
n=P*lamda/(h*c);    #number of ions

#Result
print "number of ions is",round(n/10**18,2),"*10**18"

number of ions is 3.49 *10**18

Example number 12.6, Page number 362

In [8]:
#importing modules
import math
from __future__ import division

#Variable declaration
c=3*10**8;   #velocity of matter wave(m/s)
h=6.62*10**-34;    #plank's constant(Js)
lamda=6*10**-7;    #wavelength(m)
e=1.6*10**-19;     #charge(coulomb)
k=8.6*10**-5;
T=300;        #temperature(K)

#Calculation
E=h*c/(lamda*e);    #energy(eV)
N=-E/(k*T);            #population ratio

#Result
print "population ratio is e**",int(N)

population ratio is e** -80

Example number 12.7, Page number 362

In [9]:
#importing modules
import math
from __future__ import division

#Variable declaration
lamda=10.66*10**-6;    #wavelength(m)
delta_lamda=10**-5*10**-9;      #line width(m)

#Calculation
cl=lamda**2/delta_lamda;      #coherence length(m)

#Result
print "coherence length is",round(cl/10**3,2),"km"
print "answer varies due to rounding off errors"

coherence length is 11.36 km
answer varies due to rounding off errors

Example number 12.8, Page number 362

In [10]:
#importing modules
import math
from __future__ import division

#Variable declaration
lamda=7000*10**-10;    #wavelength(m)
d=5*10**-3;        #aperture(m)
f=0.2;       #focal length(m)
P=50*10**-3;      #power(W)

#Calculation
d_theta=1.22*lamda/d;       #angular speed(radian)
A=(d_theta*f)**2;           #areal speed(m**2)
I=P/A;                #intensity of image(watt/m**2)

#Result
print "areal speed is",round(A*10**8,3),"*10**-8 m**2"
print "intensity of image is",round(I/10**5,2),"*10**5 watt/m**2"
print "answer given in the book is wrong"

areal speed is 0.117 *10**-8 m**2
intensity of image is 428.48 *10**5 watt/m**2
answer given in the book is wrong