slideDown() action based on class of radio button selected?

slideDown() action based on class of radio button selected?

I need to have a text box appear using slideDown when a specific radio button is checked. Should I do this based on the ID or class? I simplified the form to just the essential piece.

I know I need to do something like this but I'm not sure how to trigger a check when the button is checked and to hide the "source_other" box when it is unchecked again. I'm also sure my code grammar is bad.

if(source("source_2").prop('checked', true)) {
      $('#source_describe').slideDown();
}



<form name="registration" action="" method="post">
      <div>How did you hear about us?</div>
      <div>
      <input type="radio" title="Facebook" name="source" id="source_1" class="radio1">
      <label for="Facebook">Facebook</label>
      </div>
      <div>
      <input type="radio" title="Other" name="source" id="source_2" class="radio2">
      <label for="other">Other</label>
      </div>
      <div class="hide_this" id="source_describe">
            <label for="source_other">Other (describe)</label>
            <input type="text" title="Other, please describe." name="source_other" id="other_source" size="20">
      </div>
</form>