Break into small particles. The heat causes the rock to break up and form pebbles or sand. Hope that helped. Have a nice day
The speed of a wave in a uniform medium doesn't depend on its wavelength.
The rays of light coming from the Sun are parallel to each other, so when they are reflected by the concave piece of glass (which acts as a concave mirror) they converge into the focus of the mirror, which is

The radius of curvature of a concave mirror is twice its focal length, so in this case it is: