How To Validate Letters Or Numbers Only For Inputs

I have this java script code in my php file wherein I used this as my validator..for the inputs in different fields in my textbox. How to create a validation that only accepts letters... in my first name and last name field. I also wanted to know how about accepting numbers only in my field.

Here's the code:
<script type='text/javascript'>
validation and submit handling
$(document).ready(function(){
$('#employee_form').validate({
submitHandler:function(form){
$(form).ajaxSubmit({
success:function(response){
tb_remove();
post_person_form_submit(response);
},
dataType:'json'
}); .....