49. How does the spread of different wavelengths of light by a diffraction grating depend on the number of slits in the grating? (10.3) 50. Determine the ratio of the speeds of light rays of wavelengths 400 nm and 800 nm in a vacuum. (10.4) 51. Identify three methods of polarizing light. (10.5) 52. State how the optical activity of molecules in a solution can be detected using two polarizers. (10.5) 53. What is Brewster's angle? Describe the polarization of the reflected light when the angle of incidence is equal to Brewster's angle. (10.5)