Making a sprite and collider desipere on collision and back on exit

Hey guys. Im trying to make so if my player runs behind a hill e.g then it will turn down the transparency to 50% and up to 100% again when it leaves the hill obejct. the game is in 2D.

Here is my script (Don't mind the comments they are on danish):

public class FrontObjectDesipereTest : MonoBehaviour
{

private SpriteRenderer sR;
private Collider2D c2D;


void Awake()
{
sR = gameObject.GetComponent<SpriteRenderer>();
c2D = gameObject.GetComponent<Collider2D>();
}

public void OnTriggerEnter2D(Collider2D bakke)
{
if (bakke.gameObject.tag == "FrontObjects")
{
sR.color = new Color(1f, 1f, 1f, 0.5f);
c2D.enabled = true;
}

else
{
sR.enabled = false;
c2D.enabled = false;
}
}

public void OnTriggerExit2D(Collider2D bakke)
{
if (bakke.gameObject.tag == "FrontObjects")
{
sR.color = new Color(1f, 1f, 1f, 1f);
c2D.enabled = true;
}

else
{
sR.enabled = false;
c2D.enabled = false;
}
}
}

The hill got the tag "Frontobjects" and the script is on the hill and the trigger is on a box collider 2D on my player.

 

at this point the hill desiperes on collision.