John, I clicked 'send' on my last message without adding the detail of the formula!
The basic formula is actually f=(s^2)/(L), where f is the focal length, s is the radius of the (infinitely thin!) hole and L is the wavelength of the light. Sunshine has a representative wavelength of 550 nanometres. so that the formula can be written: s=SQRT(f)*SQRT(550) So, D the diameter is given by D=2*SQRT(f) *23.452, or D= 46.904*SQRT(f) But all these are in nanometres so we need to multiply by 0.001 (actually SQRT(0.001^2) to get millimetres. Thus D=0.046904*SQRT(f) That's where the 0.047 comes from. It gives a way for calculating how a near-perfect shadow sharpener should work when used on sundials. Patrick ------------------------------------------------------------------------- E-Mail: [EMAIL PROTECTED] Web: http://ourworld.compuserve.com/homepages/Patrick_Powers/ Lat: N 51d. 49m. 09s: Long: W 00d. 21m. 53s -
