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

Gunpowder residue is most likely to show up where on a shooters hands

Considering only the earth's rotation, determine how much later the asteroid would have had to arrive to put the explosion above helsinki at longitude 25˚ e? this would have obliterated the city.

A1.5m wire carries a 2 a current when a potential difference of 52 v is applied. what is the resistance of the wire?