RaycastHit.normal

Description

The normal of the surface the ray hit.

using UnityEngine;

public class Example : MonoBehaviour
{
    // Calculate the reflection of a "laser beam" off a clicked object.

    // The object from which the beam is fired. The incoming beam will
    // not be visible if the camera is used for this!
    Transform gunObj;

    void Update()
    {
        if (Input.GetMouseButton(0))
        {
            RaycastHit hit;
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);

            if (Physics.Raycast(ray, out hit))
            {
                // Find the line from the gun to the point that was clicked.
                Vector3 incomingVec = hit.point - gunObj.position;

                // Use the point's normal to calculate the reflection vector.
                Vector3 reflectVec = Vector3.Reflect(incomingVec, hit.normal);

                // Draw lines to show the incoming "beam" and the reflection.
                Debug.DrawLine(gunObj.position, hit.point, Color.red);
                Debug.DrawRay(hit.point, reflectVec, Color.green);
            }
        }
    }
}

Additional resources: Physics.Raycast, Physics.Linecast, Physics.RaycastAll.