What is the approximate radius of the central bright differaction spot of light of wavelength `lambda=0.5 mum`, if focal length of the lens is 20 cm and radius of aperture of the lens is 5 cm ?

Explanation

`r_(0) = (0.61 lambda f)/(a)`
putting values,
`r_(0) = (0.61 xx 0.5 xx 10^(-6) xx 20 xx 10^(-2))/(5 xx 10^(-2))`
`= 1 . 22 xx 10^(-6) m`
= 1 . 22 ` mu m `
Note 2 : The argument we applied for telescope can be applied for objective lens of a microscope .In the case, the object is placed slightly beyond f, so that a real images is formed at a distance v . the
magnification `m~~ (v)/(u) ~~(v)/(f)`