(1) to calculate the refractive index. According to the refraction law n=sinisinr, it is necessary to determine the incident angle and refraction angle. To determine the combination of incident angle and refraction angle, the angle can be measured with a protractor, so

The experimental steps are as follows: (1) Through P 1 and P2, a straight line and a circle intersect at point A,

(2) The straight line passing through P3 and P4 intersects the circle at point B;

(3) normals OA and OB refracted twice through points A and B, respectively;

(4) Measure the incident angle I and the refraction angle R with a protractor, and calculate the refractive index of the glass according to the refraction law.

② Refractive index formula of glass: n = sinisinr.

Answer: ① Write the following experimental steps in combination with drawings.

(2) Write the formula for calculating the refractive index of glass according to the marks in your drawing, where n = sinisinr.