1- I : - ._. . ACI'OSS 2. instrument for collecting visible-light or other electromagnetic radiation 6. electromagnetic radiation of wavelengths longer than infrared but shorter than radio waves 8. the number of waves that cross a given point per unit time (in radiation) 10. the process of giving an atom or an ion an amount of energy greater than it has in its lowest energy (ground) state 12. separation of different wavelengths of white light through refraction of different amounts 14. any of two or more forms of the same element whose atoms have the same number of protons but different numbers of neutrons 16. the distance from crest to crest or trough to trough in a wave 17. an instrument for obtaining a spectrum 18. type of telescope in which the principal light collector is a concave mirror Down 1. an idealized object that absorbs all electromagnetic energy that falls onto it 3. point where the rays of light converged by a mirror or lens meet 4. velocity motion toward or away from the observer 5. the process by which an atom gains or loses electrons 7. an atom that has become electrically charged by the addition or loss of one or more electrons 9. type of telescope which the principal light collector is a lens or system of lenses 11. electromagnetic radiation of wavelengths longer than the longest (red) that can be perceived by the eye, but shorter than radio wavelengths 13. diameter of the primary lens or mirror of a telescope 15. a discrete unit (or "packet") of electromagnetic energy