15. Given that the work function for sodium metal is 2.8 ev, what is the threshold frequency, vo, for sodium? 16. When lithium is irradiated with light, the kinetic energy of the ejected electrons is 2.935x10-19 J for 2 = 300.0 nm and 1.28x10-19J for 2. = 400.0 nm. Calculate a) the threshold frequency, and b) the work function of lithium from these data. 17. (book 1-7) Sirius, one of the hottest known stars, has approximately a blackbody spectrum with Amax = 260 nm. Estimate the surface temperature of Sirius. 18. (book 1-8) The temperature of a thermonuclear explosion can reach approximately 10' K. What value of Amax does this correspond to? IN what region of the spectrum is this wavelength found (use figure 1.19)