how to add an down arrow sign image in the navigation bar to indicate sub menu opening down`

```
<ul class="navigation">
    <li> 
    <a href="#"> parent </a> 
            <ul>  
            <li> <a href="#"> child 1 </a></li>
            <li> <a href="#"> chil2 </a></li>
            <li> <a href="#"> child3 </a></li>
            <li> <a href="#"> child4</a></li>
            <li> <a href="#"> child5</a></li>
            <li> <a href="#"> child6 </a></li>
            <li> <a href="#"> child7 </a></li>
            </ul>
     </li>`

```

my list is made like this and i want parent to show with an down arrow image indication it has child list below