The critical angle for diamond with respect to air is nearly (in $^{\circ}$)

In the figure,the optical fiber is $L = 2\,m$ long and has a diameter of $d = 20\,\mu m$. If a ray of light is incident on one end of the fiber at an angle $\theta_1 = 40^\circ$,the number of reflections it makes before emerging from the other end is close to:

What is the maximum acceptance angle at the entrance surface of an optical fiber,if $n_1$ and $n_2$ are the refractive indices of the core and cladding,respectively?

$A$ point source of light is kept below the surface of water $(n_{w} = 4/3)$ at a depth of $\sqrt{7} \ m$. The radius of the circular bright patch of light noticed on the surface of water is

With respect to air,the critical angle in a medium for red light of wavelength $\lambda_1$ is $\theta$. Other facts remaining same,the critical angle for yellow light of wavelength $\lambda_2$ will be

$A$ step index fibre has a relative refractive index difference of $0.88\%$. What is the critical angle at the core-cladding interface in degrees?