Be cool, don't be awkward, let your curiosity/interest guide your questions, get a feel for the optometrist's openness, patience, and willingness to teach you, consider your bedside manners, don't assume that every patient is cool with you being a wallflower while they're doing their exam, i.e. ensure you get the OK from the doctor before you put yourself in the exam room w/the patient. Smile, be cool.
How did you end up being an optometrist, how long have you been practicing, why this practice, did you consider other environments? What's one thing you enjoy about this job? What are some of its biggest challenges? What percentage of the practice's revenue comes from the dispensary vs. the exam room? How many patients do you normally see daily? What's the split between medical and vision patients?
Business casual is a safe assumption. Arrive on-time, even if the optometrist is relaxed. You want to demonstrate you care about this and that you know that the privilege of observing is due to their generosity.
