While tracing the path of a ray of light passing through a rectangular glass slab a student tabulated his observations as given below :

S. No.	∠i	∠r	∠e
I	60°	40°	61°
II	50°	36°	51°
III	40°	28°	39°
IV	30°	20°	31°

The correct observation is :

In a glass slab experiment, two conditions must be satisfied:

1. The angle of emergence ($\angle e$) should be approximately equal to the angle of incidence ($\angle i$) due to the parallel faces of the slab. In all four observations, $\angle e \approx \angle i$ (differing by only $1^\circ$, which is within experimental error limits).
2. The ratio of the sine of the angle of incidence to the sine of the angle of refraction ($\frac{\sin i}{\sin r}$) must equal the refractive index of glass ($n \approx 1.5$).

• For observation IV, $\frac{\sin 30^\circ}{\sin 20^\circ} \approx 1.46$, which is closest to $1.5$.
• For other observations, the ratio is significantly lower ($1.30$ to $1.37$), which deviates too much from the standard refractive index of glass.
• Therefore, observation IV is the correct one.