RaycastHit.normal

Description

The normal of the surface the ray hit.

The normal of the surface the ray hit on the collider where the ray intersects, which may or may not match the original mesh surface depending on the collider type and settings. For primitive colliders such as BoxCollider or SphereCollider, the normal is calculated based on their simple geometric shape. For MeshCollider, if convex is set to false (non-convex), Unity can return the actual interpolated normal from the mesh surface at the hit point. If convex is true, the normal is instead approximated from the convex hull of the mesh.

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 Start()
    {
        gunObj = this.GetComponent<Transform>();
    }
    
    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.