Air bubbles underwater appear silvery or highly visible because light passing from water into the gas experiences total internal reflection due to the significant difference in refractive indices; no citation is needed as this is a matter of basic physics and direct everyday observation.
The claim describes a fundamental, elementary principle of physical optics and everyday observation (total internal reflection at a gas-water boundary). According to the system instructions, claims that are trivially true by definition or direct everyday observation are classified as COMMON_KNOWLEDGE, requiring no citations or paper attachments.