Surface Normal

If we were standing on the surface of a plane, the normal points straight up:

Ground Flat on X-Axis

If the ground is flat along the x-axis, then the y-axis becomes the surface normal, because a flat ground along the x-axis points straight up the y-axis:

plane { y, 0
  pigment { Blue }
}

Adjust the height of the ground

Sink the ground 20 metres below sea level, and give it a different colour:

plane { y, -20
 pigment { checker Red, Blue }
}

Challenges

1. make a ground and a sky (so use 2 planes), and adjust the camera so it looks at both meeting in the horizon: