After tracing the path of a ray of light passing through a rectangular glass slab for four different values of the angle of incidence, a student reported his observations in tabular form as given below:

The best observation is

1. I

2. II

3. III

4. IV

The correct option is A

I

For best observation in the case of refraction through a glass slab, some conditions should be satisfied as follows:

i. Incident angle $\angle i~$Emergent angle $\angle e$

ii. Refracted angle $\angle r<$Incident angle $\angle i$

iii. $\frac{\sin \angle i}{\sin \angle r}=$Refractive index of glass $\left(1.52\right)$ as the ray is passing through the glass slab.

Correct answer explanation:

• Option I satisfies all the conditions as the incident angle ${30}^o~$emergent angle ${29}^o$.
• Refracted angle ${19}^o<$Incident angle ${30}^o$
• $\frac{\sin \angle i}{\sin \angle r}=\frac{\sin {30}^o}{\sin {19}^o}=1.53\left(~1.52\right)$

Incorrect answer explanation:

For option II,

• It satisfies condition 1 as the incident angle ${40}^o=$emergent angle ${40}^o$.
• Also satisfies condition 2 as refracted angle ${28}^o<$Incident angle ${40}^o$
• It does not satisfy condition 3 as $\frac{\sin \angle i}{\sin \angle r}=\frac{\sin {40}^o}{\sin {28}^o}=1.37\left(\ne 1.52\right)$

For option III,

• It satisfies condition 1 as the incident angle ${50}^o=$emergent angle ${50}^o$.
• Also satisfies condition 2 as refracted angle ${36}^o<$Incident angle ${50}^o$
• It does not satisfy condition 3 as $\frac{\sin \angle i}{\sin \angle r}=\frac{\sin {50}^o}{\sin {36}^o}=1.3\left(\ne 1.52\right)$

For option IV,

• It satisfies condition 1 as the incident angle ${60}^o~$emergent angle ${59}^o$
• Also satisfies condition 2 as refracted angle ${40}^o<$Incident angle ${60}^o$
• It does not satisfy condition 3 as $\frac{\sin \angle i}{\sin \angle r}=\frac{\sin {60}^o}{\sin {40}^o}=1.35\left(\ne 1.52\right)$
• Therefore, the best observation is no. I

Thus, the correct option is option A