My iMac (2011) keeps ejecting discs

Go to your local office supply store and buy a CD/DVD cleaning kit and use it on the SuperDrive. NEVER EVER blow air or anything else into your Mac or any other computer!!!!! If the cleaning kit does not work, then replace the SuperDrive by purchasing a USB external drive. You can find many, I am sure you will one that will suit your needs by navigating to Macsales.com. On my 2020 iMac I bought this one as I wanted some Blueray abilities:

https://eshop.macsales.com/item/OWC/MR3UBDRW16/ however there are much less expensive solutions that will work just fine.

Does it eject both CDs and DVDs.

It may have failed.

Or the lenses could be dirty.

Do you have a can of compressed air.

If so give it a blast to see if it removes any dirt,

remember hold the can upright so no liquid gets inside the iMac.

Clean the disks in question (is it all disks or just some?) with a soft damp cloth in similar fashion to how Apple advices you to clean a screen. They are plastic, so warm mild soapy water (Dawn?) can be used too. Just be sure they are dry before use.