Reflection and Refraction

In this experiment the goal was to find the relationship between the two angles of incident and refraction.  For the experiment using the following setup:

we shined a beam of light first through the flat surface of the semicircular plastic prism and we measured the mentioned angles. Then we turned the prism and repeated the same experiment.
Here are the result

Flat surface first
Trial	θIncident	θrefraction	sin(θIncident)		sin(θrefraction)
1	0±0.5	0±0.5	0.000±0.009		0.000±0.009
2	5±0.5	4±0.5	0.087±0.009		0.070±0.009
3	10±0.5	7±0.5	0.174±0.009		0.122±0.009
4	15±0.5	10±0.5	0.259±0.009		0.174±0.009
5	20±0.5	13±0.5	0.342±0.009		0.225±0.009
6	25±0.5	16±0.5	0.423±0.009		0.276±0.009
7	30±0.5	19±0.5	0.500±0.009		0.326±0.009
8	35±0.5	22±0.5	0.574±0.009		0.375±0.009
9	40±0.5	26.5±0.5	0.643±0.009		0.446±0.009
10	50±0.5	31±0.5	0.766±0.009		0.515±0.009
11	60±0.5	35±0.5	0.866±0.009		0.574±0.009
12	70±0.5	39±0.5	0.940±0.009		0.629±0.009

and this is the graph of sin(incident) vs sin(refraction)

y = 0.6641x + 0.0024


Curved surface first

trial	θincident	θrefraction	sin(θincident )	Sin(θrefraction)
1	0±0.5	0±0.5	0.000±0.009	0.000±0.009
2	5±0.5	7.5±0.5	0.087±0.009	0.131±0.009
3	10±0.5	16±0.5	0.174±0.009	0.276±0.009
4	15±0.5	23±0.5	0.259±0.009	0.391±0.009
5	20±0.5	32±0.5	0.342±0.009	0.530±0.009
6	25±0.5	39±0.5	0.423±0.009	0.629±0.009
7	30±0.5	48.5±0.5	0.500±0.009	0.749±0.009
8	35±0.5	63±0.5	0.574±0.009	0.891±0.009
9	40±0.5	75±0.5	0.643±0.009	0.966±0.009
10	45±0.5	no refraction	0.707±0.009

and the graph is as follow

y = 1.5125x + 0.0026


in both cases the angle of refraction is on the y axis and angle of incident on the x axis 

n₁Sin(θ_incident) = n₂Sin(θ_refraction)

=> n₁ / n₂ = Sin(θ_refraction) / Sin(θ_incident) = m slope of the graph

however for the first graph n₁ is air and n₂ is the prism so the slope is reciprocal of n₂ so since the slope is 0.6641 the reciprocal is 1.506±0.01  which is close to 1.5125±0.01 of the second graph's slope
both these values represent the index of refraction of the prism.