Rahim recorded the following sets of observations while tracing the path of a ray of light passing through a rectangular glass slab for different angles of incidence.

$$\begin{array}{|c|c|c|c|} \hline \text{S. No.} & \text{Angle of incidence} & \text{Angle of refraction} & \text{Angle of emergence} \\ \hline \text{I} & 45^\circ & 41^\circ & 45^\circ \\ \hline \text{II} & 40^\circ & 38^\circ & 38^\circ \\ \hline \text{III} & 45^\circ & 41^\circ & 40^\circ \\ \hline \text{IV} & 41^\circ & 45^\circ & 41^\circ \\ \hline \end{array}$$

The correct observation is recorded at serial number:

For a rectangular glass slab, the light ray enters from air (refractive index $n_{\text{air}} \approx 1.0$) to glass (refractive index $n_{\text{glass}} \approx 1.5$).

1. Bending towards the normal: Since glass is optically denser than air, the light ray bends towards the normal upon entry. This means the angle of refraction ($r$) must be strictly less than the angle of incidence ($i$):

$$r < i$$

1. Parallel Emergence: Since the opposite refracting faces of the rectangular glass slab are parallel, the emergent ray is parallel to the incident ray. This means the angle of emergence ($e$) must be equal to the angle of incidence ($i$):

$$e = i$$

Let us evaluate the given observations:

• Observation I: $i = 45^\circ$, $r = 41^\circ$, $e = 45^\circ$. Here, $r < i$ ($41^\circ < 45^\circ$) and $e = i$ ($45^\circ = 45^\circ$). This observation is correct.
• Observation II: $i = 40^\circ$, $r = 38^\circ$, $e = 38^\circ$. Here, $e \neq i$ ($38^\circ \neq 40^\circ$), which is incorrect.
• Observation III: $i = 45^\circ$, $r = 41^\circ$, $e = 40^\circ$. Here, $e \neq i$ ($40^\circ \neq 45^\circ$), which is incorrect.
• Observation IV: $i = 41^\circ$, $r = 45^\circ$, $e = 41^\circ$. Here, $r > i$ ($45^\circ > 41^\circ$), which implies the ray bent away from the normal when entering a denser medium. This is physically incorrect.