Cast Ray From Middle Of Screen
Vector3 rayOrigin = new Vector3(0.5f, 0.5f, 0f); // center of the screen
float rayLength = 500f;
// actual Ray
Ray ray = Camera.main.ViewportPointToRay(rayOrigin);
// debug Ray
Debug.DrawRay(ray.origin, ray.direction * rayLength, Color.red);
RaycastHit hit;
if (Physics.Raycast(ray, out hit, rayLength))
{
// our Ray intersected a collider
}