Refraction of light — practice question

A student carries out an investigation into the reflection and refraction of light with a semi-circular glass block. • He puts a glass block on a sheet of paper. • He traces the outline of the block with a pencil. • He uses a ray box to send a ray of light to the centre C of the flat face AB of the block. • He marks AB and point C. Fig. 3.1 shows the observation made by the student. • He uses a pencil to put two crosses $X_1$ and $X_2$ on the incident ray. • He uses a pencil to put two crosses $X_3$ and $X_4$ on the emergent ray. • He uses a pencil to put two crosses $X_5$ and $X_6$ on the ray reflected from the face AB. • He removes the glass block and the ray box. Fig. 3.2 shows the student’s sheet of paper.