Are lenses of film cameras compatible with slr digital cameras?

Not all lenses will fit.
Even Canon and Nikon have what is known as different "bayonet fittings".
A lens from one brand will not fit another brand.
There are companies which make lenses for different makes of cameras and these lenses will cost less.
Try Sigma or Tamron to start with.
Digital cameras tend to need higher quality lenses too so even if your old lens fits a new digital body the likelihood is that you won't get the quality from it.

For digital you need ED Extra Dispersal) lenses,
Extra-Low Dispersion Glass All photographs are made with light, composed of many colors of the spectrum. With panchromatic black-and-white and all color films, it is essential that both blue and red light rays be brought to focus at the same plane; otherwise, color "fringing" and unsharpness will be evident. Because different wavelengths of light are bent at slightly different angles as they pass through normal optical glass, the longer the focal length of the lens, the longer the individual light rays have to travel inside the lens and the more the discrepancy in focus. While modern techniques used to correct this "chromatic aberration" are effective with normal and wide angle lenses, telephoto lenses magnify even the slightest variation in focus between the red and blue light rays, leading many photographers to assume that no long telephoto or super-telephoto lens could equal a "shorter" lens in sharpness and color correction.
Also remember that with most digital cameras actually give a focal length different from marked on the lens.
This is because the sensor is smaller and the back of the lens is closer to it than it would be with say 35mm film.
as a general guide you multiply the focal lenth by 1.5 to give the correct focal length on a digital SLR. So 50mm would be 75mm.

SJB is totally wrong when he says they will fit. It is NOTHING to do with the focal length.

Yes they are. An SLR camera that has a lense that is 55mm will fit onto the same camera if it is digital.

55mm was just an example, there are other sizes too.

JusMe... When you address SJB. You start with Sir.

As long as your cameras are the same, I have a cannon eos rebel 35 millimeter and all the lenses I have for it also fit my cannon eos rebel digital camera.