Read the passage below and answer the following questions in this context.

YDSE is abbreviation used for Young's Double Slit Experiment. YDSE shows interference of light. Interference of light is the phenomenon where two light waves meet each other while travelling in the same medium. A light source is kept behind a wall where two slits are made : S1 and S2. On the screen, there appears a pattern of alternate bright and dark fringes. The central fringe is always bright. 

Two slits are illuminated by light of wavelength 5890 Aº and the distance b/w the fringes obtained on screen is 0.2º. If the whole apparatus is immersed in water then what will be the angular fringe width considering the refractive index of water is 4/3?

0.15º

Underwater, the wavelength of the light will change. If  λ_a is the wavelength in air and  λ_c is the wavelength in water, then using : 

μ = 4/3 = λ_a/λ_w , $\lambda_w = \frac{5000}{\frac{4}{3}} = 3750A^0$

θ = n λ/ d

$\theta = \frac{3}{4}\times 0.2 = 0.15A^0$