How to alert if the form field is empty

How to alert if the form field is empty

I have a form and email address field I want to prompt jquery alert when
the user not input the field of email address?
<form method="post" id="form1" target="frame_result3">
<input id="email" type="text" name="email" placeholder="email" />
<br />
<input id="blast" type="button" value="GET FREE QOUTE" name="blast" />
</form>