Kegs is right, you should never ever be throwing things at your cat. Even if it's not something dangerous, that's not going to do anything but stress out your older cat even more and make her relationship more negative with both the younger cat and with you. Breaking up the bullying behavior in the moment might be good, but please don't do it that way.
Adding to the above post, how long have you had the younger cat? How were they introduced? It can take a very long time for a resident cat to warm up to a new housemate, especially another cat. In either case, what you need to work on doing is building positive associations between the two. Feed them near each other (depending on how bad the bullying is, you might want to keep them separate but where they can still smell each other for a while), reward your older cat when she's not bothering the younger one. Take very small steps and consider any non-aggressive interaction a foot forward.
Also make sure there are enough litter boxes, hiding places, and high vantage points for both cats. I don't know how many of these things you have, but if the answer to any of them is "one", that's something to strongly consider changing as soon as possible. Where they are may also make a difference.
If this wasn't a problem for a long time and now you see your older cat targeting the younger one unprovoked (actually seeking the other cat out, not just reacting when it gets too close to her), try to pinpoint any changes in/around the house, your routine, etc that might be affecting her mood and behavior. Are there any other issues or signs?