American Board of Ophthalmology (ABO) Practice Exam

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What effect does heating a Zyl frame and then quenching it in cold water have?

It makes the frame more flexible
It causes the frame to shrink
It increases the lens curvature
It changes the color of the frame

The correct answer is: It causes the frame to shrink

Heating a Zyl (zylonite or cellulose acetate) frame followed by quenching in cold water leads to an effect known as thermal shock. When the heated material is suddenly cooled, it contracts rapidly, causing the frame to shrink. This process is utilized to alter the fit of eyewear, making it tighter or adapting it to the wearer's face shape. In the context of eyewear adjustment, this shrinkage can be beneficial for achieving a snugger fit on the wearer’s face. The other effects listed, such as flexibility, lens curvature, or color change, do not typically result from the heating and quenching process for Zyl frames. Instead, those effects would require different methods or materials to be realized. The frame's primary response to this thermal treatment is the reduction in size due to the rapid cooling, which is why the answer is accurate regarding the behavior of Zyl frames under these conditions.