RaycastHit.normal

説明

レイがヒットしたサーフェスの法線

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public Transform gunObj;
    void Update() {
        if (Input.GetMouseButton(0)) {
            RaycastHit hit;
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
            if (Physics.Raycast(ray, out hit)) {
                Vector3 incomingVec = hit.point - gunObj.position;
                Vector3 reflectVec = Vector3.Reflect(incomingVec, hit.normal);
                Debug.DrawLine(gunObj.position, hit.point, Color.red);
                Debug.DrawRay(hit.point, reflectVec, Color.green);
            }
        }
    }
}

関連項目: Physics.Raycast, Physics.Linecast, Physics.RaycastAll.