#!/usr/bin/perl
# Varun Bhalerao
# http://www.astro.caltech.edu/~varun/ay20/ps2/photons_sun.pl

# Count the number of photons from a blackbody in a given band
#
# **********
# CGS units !
# **********

$h=6.63e-27;
$c=2.998e10;
$k=1.38e-16;
$t=5777;
$hckt=$h*$c/$k/$t;
$step=1000;		#step size as a fraction of wavelength, 1/$step = dlambda / lambda

sub count(){
	($lstart,$lend)=@_;
	print "Wavelength band: $lstart nm - $lend nm. ";
	$lstart*=1e-7;		#convert nm into cm
	$lend*=1e-7;		#convert nm into cm
	$dlambda=$lstart/$step;
	$lambda=$lstart;
	my $integral=0;
	while ($lambda<$lend){
		$dlambda=$lambda/$step;		#it is important that step size varies with the wavelength !
		$lambda+=$dlambda;
		$bldl=2*$c/$lambda**4/ (exp($hckt/$lambda) -1) *$dlambda;
		$integral+=$bldl;
	}

	printf("Number of Photons %E\n", $integral);
}

&count(300,1100);
&count(505,595);
&count(391,489);
&count(331,399);
&count(0.519,519000);#peak emission for 5777K is 519 nm. integrate from 519/1000 to 519*1000