What is focus ?

What is the shape of the wavelength in case Light emerging out of a convex lens when a point source is placed at its focus.

If (2,0) is the vertex and y-axis is the directrix of a parabola then its focus is

The equation of the ellipse with its focus at (6, 2), centre at (1, 2) and which passes through the point (4, 6) is

Show that the locus of the foot of the perpendicular from the focus to the tangent of the parabola

The differential equation of the family of parabola with focus at the origin and the X -axis as axis is