1. Locus of all points having same phase of vibration is called wavefront.
2. Spherical wavefront, cylindrical wavefront plane wave front
3. Wave frond through a thin convex lens:
Consider a plane wave passing through a thin convex lens. The central part of the incident plane wave travels through the thickest portion of lens. Hence central part get delayed. As a result the emerging wave frond has a depression at the centre. Therefore the wave front becomes spherical and converges to a point F.