Validate require if checkbox option is X

Question

I am working on a form that requires validation. A small part of the form requires the user to choose between 2 options "Yes", "No". When no is selected I would need them to enter a reason, and otherwise if yes is selected, I don't need a reason (even the inputfield may be hidden then <- this is only doable via JavaScript I guess?)

How would I have to create this rule?

Let's say the part of the form looks like this

<form  id="Form" method="get" action="">
    <fieldset>
        <legend>form</legend>
    Yes:
        <input name="choice" type="radio" value="Yes"/>
        <br />
    No:
        <input name="choice" type="radio" value="No"/>
        <br />
        <label for="reaason">Reason</label>
        <input id="reason" name="reason"  />
    </fieldset>
</form>

Answer 1

http://jsfiddle.net/ZYqyC/

wrapping label and textbox in a div

<div id=reasondiv>
            <label for="reaason">Reason</label>
            <input id="reason" name="reason"  />
 </div>

then jquery:

$("#reasondiv").hide();
$("input[name=choice]:radio").click(function(){

   if($(this).val()=='Yes')
      $("#reasondiv").show();
   else
       $("#reasondiv").hide();

});