snell's law

surface air physics derivation snell's law formula velocity boundary educational reflection optical incidence refractive index nature normal medium line template icon sign snell law refraction speed light angle graphic refracted ray element chemistry phase velocities set sines angles indices reflection water chemical science reflected boundary substance object black beam symbol education sines arrow concept isolated students incident chart math background measure diagram graph draw equation refractive snell art wavelength direction vector definition angle drawing design flat school optics