20. consider two stars a and b with temperatures ta and tb and radii ra and rb respectively. if ta
= 2 tb and ra= 12 rb, which statement about the stars' luminosities, la and lb, is true?
using this formula: l=(r^2)(t^4)
a) la =lb
b) la = 2 lb
la=4 lb
d) la = 12 lb
e) la = 14 lb

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Glass is transparent to visibile light under normal conditions; however, at extremely high intensities, glass will absorb most of the light incident upon it. this works through a process known as multiphoton absorption. in this process, several photons are absorbed at the same time. if very intense light whose photons carry 2ev of energy is shined onto a material with a band gap of 4ev, that light can be absorbed through two-photon absorption, because two photons have the right amount of energy to bridge the band gap. what is the minimum number of photons of 800-nm light that are needed to equal or exceed the band gap of fused silica glass

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