A)Sun appears round all the time.during evening , as the light rays have to travel more distance through the atmosphere, the intensity of the rays are reduced . So we are able to look at the sun and see it. But during noon time , the light rays have to travel less distance to reach the earth. So intensity of the light is high and we cannot look at the sun directly.
b)The matchstick would burn because a convex lens converges the rays on the focus.