Unity - Phyics.Raycast returns false

我创建一个emtpy游戏对象并附加.cs文件。我尝试在鼠标单击位置加载预制件(.obj 文件(。我的代码是：

    Ray ray;
    RaycastHit hit;
    public GameObject prefab;
    void Start()
    {
    }
    // Update is called once per frame
    void Update()
    {
        if (Input.GetMouseButtonDown(0))
        {
            ray=Camera.main.ScreenPointToRay(Input.mousePosition);
            if (Physics.Raycast(ray, out hit))
            {
                GameObject obj = Instantiate(prefab, new Vector3(hit.point.x, hit.point.y, hit.point.z), Quaternion.identity) as GameObject;
            }
            else
            {
                Debug.Log("Physics.Raycast returns false");
            }
    }

光线投射每次都返回 false。