When does light refract? when it travels through gases when it crosses the boundary between two types of material light refracts every 2000 wavelengths. when it travels through a vacuum

Aclarinetist, setting out for a performance, grabs his 3.350 kg clarinet case (including the clarinet) from the top of the piano and carries it through the air with an upward force of 24.65 n. find the case's vertical acceleration. indicate an upward acceleration as positive and a downward one as negative.