Why the dia of lens of digital camera is small than of 35mm film camera?

digital camera has few F No. where as Film camera is more e.g film camera has F No. 1.7 to 16 where as Digital camera has F No. 2.8 to 5.6

I think the overall lesser diameter of the digital lenses has to do with the smaller sensor in most models. Also, the digital SLRs have a pretty good range, so it is the function of the type of camera (point n shoot vs. SLR) rather than film vs. digital that is at the core of the differecen. In any case, don't forget what an F number represents. It is the ratio of lens' focal length to the diaphragm's opening diameter. So a lens with overall bigger diameter and overall bigger range of focal lenghts has a greater combination of Fs.

I think you may be mistaken. There are digital camera's with F/1.8 (Olympus 5050) and many Digital SLR's can use the same lens as their film counterparts. This means you could use the same EF 50mm/F1.0 on a film and digital canon slr.

If you're referring to lenses specifically for digital, e.g the Canon Ef-S or Nikon DX, it's probably not that it can't be made, but more likely that the companies haven't been convinced on the profitability of making them. F-stop is the ratio of the lens' focal lens to its' diameter of entrance pupil (see http://www.thefreedictionary.com/f-number). On digital P&S really short lenses are used (e.g. Nikon's Coolpix 5400 is 5.8 mm to ~24 mm). Thus at the maximum F-stop of 2.8 the lens needs to have only ~2mm entrance pupil. In 35 mm photography a lens with equivalent field of view would have to have 28 mm focal lens. In order to have the same F2.8 maximum aperture it needs to have 10mm entrance pupil. Hence differences in lens' girth ;)

Cheers,

Stas.