Answer:
The critical angle is 

Step-by-step explanation:
From the question we are told that 
 The index of refraction of the glass in the fiber is 

 The index of refraction of the glass in the cladding is 

The critical angle is mathematically evaluated as 
 
![\theta_c = sin^(-1)[(n_c )/(n_f ) ]](https://img.qammunity.org/2021/formulas/physics/college/s9z2zp57tzjfveeh9ope4q54dl2zbjzwrn.png)
substituting values 
 
![\theta_c = sin^(-1)[(1.50 )/(1.69 ) ]](https://img.qammunity.org/2021/formulas/physics/college/8ruqgk9bmh2sj78boux60ib1wafowwfy3v.png) 
 
 
