It has to be used because you can not visually see them
        
             
        
        
        
Answer:
A. Materials with a low index of refraction cause light to refract very little.
 
        
                    
             
        
        
        
Answer:
Stress = F / A       force per unit area
A = 3.00 cm^2 = 3 E-4  m^2
F = 2.4E8 N/m^2 * 3E-4 m^2 = 7.2E4 N    max force applied
F/3 = 2.4E4 N  if force not to exceed limit   (= f)
f = M a 
a = 2.4 E4 N / 1.2 E3 kg = 20 m / s^2      about 2 g