After tracing the path of rays of light through a glass slab for three different angles of incidence, a student measured the corresponding values angle of refraction $r$ and angle of emergence $e$ and recorded them in the table given below. The correct observations are:

When a ray of light passes through a rectangular glass slab, it undergoes refraction at two parallel interfaces (air-to-glass and glass-to-air):

1. At the first interface (air to glass): Light travels from an optically rarer medium (air) to an optically denser medium (glass). Therefore, the refracted ray bends towards the normal. This means the angle of refraction ($r$) is always less than the angle of incidence ($i$), i.e., $r < i$.
2. At the second interface (glass to air): Light travels from an optically denser medium (glass) to an optically rarer medium (air). The ray bends away from the normal. Since the opposite refracting faces of the glass slab are parallel to each other, the extent of bending of the ray of light at the opposite parallel faces is equal and opposite. Thus, the emergent ray is parallel to the incident ray, which means the angle of emergence ($e$) is equal to the angle of incidence ($i$), i.e., $e = i$.

Let us analyze the given observations:

• Observation I: $i = 30^\circ$, $r = 19^\circ$, $e = 30^\circ$. Here, $e = i = 30^\circ$ and $r = 19^\circ < 30^\circ$. This is physically correct.
• Observation II: $i = 45^\circ$, $r = 28^\circ$, $e = 45^\circ$. Here, $e = i = 45^\circ$ and $r = 28^\circ < 45^\circ$. This is physically correct.
• Observation III: $i = 60^\circ$, $r = 35^\circ$, $e = 60^\circ$. Here, $e = i = 60^\circ$ and $r = 35^\circ < 60^\circ$. This is physically correct.

Since all three sets of observations satisfy the conditions $e = i$ and $r < i$, all of them are correct.

Why other options are incorrect:

• Option A (I and II only): Incorrect because Observation III is also physically accurate and follows the laws of refraction.
• Option B (II and III only): Incorrect because Observation I is also physically accurate.
• Option C (I and III only): Incorrect because Observation II is also physically accurate.