call a class in another class

i am a beginner in unity and c#, as well as oop, i am since two days to study for this proble but i cannot find the solution, and das problem: there is a class which has just start and update:using UnityEngine;
using System.Collections;
public class White : MonoBehaviour {
public float duration = 1.0F;
public Light lt;
void Start() {
lt = GetComponent();
void Update() {
if(Input.GetKeyDown(KeyCode.W)) {

		float phi = Time.time / duration * 2 * Mathf.PI;
		float amplitude = Mathf.Cos(phi) * 3F + 3F;
		lt.intensity = amplitude;}}}

i need to call this funtion, which is for adjusting the light in another class, with a if statment such as:
public White whitelight;//there is in this script too
//if the ray cast, then call the function light adjusting , i must stop here?
if (Physics.Raycast (ray, out hits, 100f)) {

		if (hits.collider.tag == "Light") {
			//Debug.Log ("Raycast hit a light");

how i can call this function, because the first script has just state and update method, i dont know, how ? or must i write the code again under if statment?
Thank you very much for your response, please note i ma absolute beginner, if you want to answer for beginner please, thank you again.

To call a class in another class you can

1: Make an instance of the class you wish to call

public class SOME_CLASS : MonoBehaviour {

public static SOME_CLASS instance;

public void Move()
do something...

Then from there you can simply call it

public class MY_CLASS : MonoBehaviour {

void Start()

2: The other way would be to reference the class like:

public class CLASS : MonoBehaviour {

public SOME_CLASS someclass;

void Start()

Cheers @jedimar!
-PlayWell Studios

Hi, thank you very much for your post, i have tried both option, i have deleted Update, and rewrite it as public void Move(), as well as instance, is there something wrong that i do not have the Move() for calling in other class, i mean, it is not blue as you have, thank you again, you do not know how much you help me with your respond, i appreciate your advice. Thank you.