Time slowing down near massive objects was verified accurately near the Earth: second (time unit) is about nanosecond dilated (longer) near the Earth than away from the Earth. Even in GPS satellites that use atomic clocks for geolocation, such clocks are almost half a nanosecond per second faster on the orbit than the same atomic clocks on the Earth. (And it is not because GPS satellites move fast: contrary to time slowing down inside fast-moving objects, time in GPS satellites speeds up because they are on high orbit.)
Time dilation causes two interesting optical effects: refraction (change in light beam direction) and redshifting (change in light color — wavelength change). Here we need refraction only: light beam bends when travelling in space where time speed varies, literally by the same Snell’s law that we learned in school: