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diff --git a/1535/CH11/EX11.4/Ch11Ex4.sci b/1535/CH11/EX11.4/Ch11Ex4.sci
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+// Scilab Code Ex11.4: Page-250 (2010)
+lambda = 500e-009;    // Wavelength of laser light, m
+f = 15e-02;    // Focal length of the lens, m
+d = 2e-02;    // Diameter of the aperture of source, m
+a = d/2;    // Radius of the aperture of source, m
+P = 5e-003;    // Power of the laser, W
+A = %pi*lambda^2*f^2/a^2;    // Area of the spot at the focal plane, metre square
+I = P/A;    // Intensity at the focus, W per metre square    
+printf("\nThe area of the spot at the focal plane = %4.2e metre square", A);
+printf("\nThe intensity at the focus = %4.2e watt per metre square", I);
+
+// Result
+// The area of the spot at the focal plane = 1.77e-010 metre square
+// The intensity at the focus = 2.83e+007 watt per metre square 
+\ No newline at end of file