Spherical Refracting Surfaces

Why when given a question to find the image of a point object in a spherical refracting surface do you have to consider that there are two surfaces refracting the light?

I assumed that one could simply use the equation,

$frac{n_a}{s}$ + $frac{n_b}{s’}$ = $frac{n_b – n_a}{R}$

and solve for $s’$ to get the image distance, but why do I also have to calculate

$frac{n_b}{-(s’-d)}$ + $frac{n_a}{s_2′}$ = $frac{n_a – n_b}{-R}$

and then when I have solved for $s_2’$ add R and that is the image distance.

Example

A candle is placed $60 cm$ from a glass sphere with a $5cm$ radius. The index of refraction of the glass sphere is $n_b = 1.5$. How far from the center of the glass sphere will the image of the candle be?

My answer

$s= 60 -5 = 55 cm$

$R = 5 cm$

$n_a= 1$ and $n_b=1.5$

$frac{1}{55}$ + $frac{1.5}{s’}$ = $frac{1.5 – 1}{5}$ $=>$ $s’ =+18.33 cm$

Answer: The image will be $18.33 cm$ from the center

Answer provided as correct

$frac{1}{55}$ + $frac{1.5}{s’}$ = $frac{1.5 – 1}{5}$ => $s’ =18.33 cm$

$frac{1.5}{-(18.33-10)}$ + $frac{1}{s_2′}$ = $frac{1 – 1.5}{-5}$ $=>$ $s’_2 = 3.6 cm$

$s’_3 = 3.6 +5 = 8.6 cm$

Answer: The image will be $8.6 cm$ from the center of the glass sphere