why the eyes doesn't get burn when we used converging lens in the spectacle
Answers
In bright sunny conditions, the eye's pupil shrinks to about 1mm diameter. If you look straight at the Sun with the pupil in this state at noon, when intensity is of the order of 1000Wm−2, your eye will therefore focus about 0.7mW onto the retina.
This is actually considerably below the amount of heat the superbly densely envasculated retina can safely dump, so, were it not for the shorter wavelengths in sunlight (which leads to sunburn of the retina from photochemical damage), the Sun would be a safe (albeit unpleasant) light source to stare at. There are exceptions to this rule - such as macular degeneration - but a healthy retina can easily cope with this much heat.
The danger from viewing the Sun or any other beam with an optical instrument is that it may couple radiation from a larger area than a 1mm diameter pupil through the eye's pupil. This is why binoculars, which can couple the sunlight incident on a several centimeter diameter aperture into the 1mm diameter pupil, are extremely dangerous to the sight if you were to look at the Sun with them.
Eyeglasses are not this kind of instrument, if you wear them at the correct distance from your eye. They are meant to add or subtract a small amount of optical power / correct optical aberration and do not concentrate a beam as a magnifying instrument does. In your example, the eyeglass was held at its focal length - which is typically tens of centimetres - from the paper. In this configuration, they are playing a very different optical role from when you wear them 5 millimetres or so from your eye's pupil. Correctly fitted eyeglasses when worn as they are meant to be therefore do not raise the risk of eye damage from the Sun.