# Chapter 20 Photometry¶

## Example 20.1 Page no 1055¶

In :
#Given
E=2.5*10**5                             #lm/m**2
r=1.5*10**11                            #m

#Calculation
import math
l=E*r**2
a=4*math.pi*l

#Result
print"(i) Luminous intensity is", l,"cd"
print"(ii) Luminous flux of the sun is",round(a*10**-28,3)*10**28,"lm"

(i) Luminous intensity is 5.625e+27 cd
(ii) Luminous flux of the sun is 7.069e+28 lm


## Example 20.2 Page no 1055¶

In :
#Given
I2=150
I1=75.0
E1=20

#Calculation
E2=(I2*E1)/I1

#Result
print"Illumination is", E2,"lux"

Illumination is 40.0 lux


## Example 20.3 Page no 1056¶

In :
#Given
I=35
e=5.0                                #lumen/watt

#Calculation
import math
a=4*math.pi*I
P=a/e

#Result
print"Power of the lamp is", round(P,0),"Watt"

Power of the lamp is 88.0 Watt


## Example 20.4 Page no 1056¶

In :
#Given
a=1260
r=8                                   #m
a1=6

#Calculation
import math
I=a/(4.0*math.pi)
Ea=I/r**2
LB=math.sqrt(r**2+a1**2)
cos=r/LB
Eb=(I*cos)/LB**2

#Result
print"(i) The illumination at a point immediately below the lamp is", round(Ea,2),"lux"
print"(ii) The illumination on the working plane is",round(Eb,1),"lux"

(i) The illumination at a point immediately below the lamp is 1.57 lux
(ii) The illumination on the working plane is 0.8 lux


## Example 20.5 Page no 1056¶

In :
#Given
r=6.0                             #m
I=250                            #cd
PQ=8

#Calculation
Ep=I/r**2
LQ=math.sqrt(r**2+PQ**2)
cos=r/LQ
EQ=(I*cos)/LQ**2

#Result
print"(i) Illumination at a point P is", round(Ep,2),"lux"
print"(ii) illumination at a point Q is",EQ,"lux"

(i) Illumination at a point P is 6.94 lux
(ii) illumination at a point Q is 1.5 lux


## Example 20.6 Page no 1057¶

In :
#Given
t1=2.5                    #second
r1=0.5
r2=1

#Calculation
t2=(t1*r2**2)/r1**2

#Result
print"exposure time is",t2,"s"

exposure time is 10.0 s


## Example 20.7 Page no 1058¶

In :
#Given
i2=60
r2=105.0
r1=70

#Calculation
i1=(i2*r1**2)/r2**2

#Result
print"The luminous intensity of the first lamp is",round(i1,2),"cd"

The luminous intensity of the first lamp is 26.67 cd


## Example 20.8 Page no 1059¶

In :
#Given
ra=60
rb=45.0
a=40.0

#Calculation
ia1=(ra**2)/(rb**2)
ia=(ra**2)/(a**2)
i=ia-ia1
A=(i*100)/ia

#Result
print"percentage of light is absorbed by the glass is",round(A,0),"%"

percentage of light is absorbed by the glass is 21.0 %