2.8 A point source of light is 12 cm below the surface of a large body of water (n = 1.33 for water). What is the radius of the largest circle on the water surface through which the light can emerge?
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2.10 Show that the critical angle at an interafce between doped silica with n1 = 1.460 and pure silica with n2 = 1.450 is 83.3 degree.
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2.13 Consider a dielectric slab having a thickness d = 10 mm and index of refraction n1 = 1.50. Let the medium above and below the slab be air, in which n2 = 1. Let the wavelength be λ = 10 mm (equal to the thickness of the waveguide).
(a) What is the critical angle for the slab waveguide?
(b) Solve Eq. (2.26b) graphically to show that there are three angles of incidence which satisfy this equation.
(c) What happens to the number of angles as the wavelength is decreased?
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(a) critical angle for the slab waveguide

(b) The Equation :

2.18 A step-index multimode fiber with a numerical aperture of 0.20 supports approximately 1000 modes at an 850-nm wavelength.
(a) What is the diameter of its core?
(b) How many modes does the fiber support at 1320 nm?
(c) How any modes does the fiber support at 1550 nm?
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(a) Number of modes is given by,

(b)

(c)