8- The critical angle of a certain glass is 36.0' when surrounded by air. Hence, the refractive index n of the glass is: (a) 1.45 (b) 1.60 1.70 -(d) 1.85 60 360 na. 9- What percent of a radioactive material remains after a time period equal to two times its half-life? (a) 50% (b) 0% (0) 25% (d) 12.5 %