The student draws a mirror line $MR$ and a normal line $NL$ perpendicular to it at point $B$. She draws an incident ray line $AB$ at an angle of incidence $i=35^{\circ }$ to the normal. She places two pins, $P_1$ and $P_2$, on the line $AB$ to define the incident ray.

(i) State a practical guideline for the minimum separation distance between pin $P_1$ and pin $P_2$ to ensure an accurate ray trace, and explain why a larger separation is preferred.

The student views the images of pins $P_1$ and $P_2$ in the mirror. She places two pins, $P_3$ and $P_4$, on the other side of the normal so that they align with the images of $P_1$ and $P_2$. She then repeats this with a different angle of incidence $i=45^{\circ }$, using pins $P_5$ and $P_6$ to trace the second reflected ray.

(i) Describe how the student uses the lines drawn through $P_3,P_4$ and $P_5,P_6$ to locate the position of the virtual image $Y$ of the object.

The student measures the perpendicular distance $a$ from the object pin $P_1$ to the mirror line $MR$ as $a=7.2cm$. She then measures the perpendicular distance $b$ from the virtual image point $Y$ to the mirror line $MR$ as $b=7.4cm$.

(i) State whether these measurements support the theory that the image formed in a plane mirror is at the same distance behind the mirror as the object is in front of it. Justify your statement, taking typical experimental uncertainty into account.

Suggest two physical sources of inaccuracy in this pin-tracing experiment, other than human error in reading a standard ruler.