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//To calculate the spot size
n = 1; //for air
lamda = 650; //wavelength, nm
lamda = lamda*10^-9; //wavelength, m
bs = 1; //beam size, mm
bs = bs*10^-3; //beam size, m
fl = 1; //focal length of lens, mm
fl = fl*10^-3; //focal length of lens, m
tan_theta = fl/(2*bs); //value of tan_theta
theta = atand(tan_theta);
NA = n*sind(theta);
ss = 0.6*lamda/NA; //spot size, m
ss = ss*10^6; //spot size, micro metre
printf("spot size is %5.3f micro metre",ss);
//answer given in the book is wrong
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