Question

To determine the refractive index of glass, place a glass block PQRS on a white paper and draw the boundary of the glass slab. Fix a pin O touching the glass slab on the side PS. Looking through the opposite face QR, fix two pins P₁ and P₂ along the same line as the image of the pin O. Remove the pins P₁ and P₂ and mark their positions. Join P₁ and P₂ and produce this line to cut the normal OA at I. Then I is the apparent position of O as seen through the glass slab (Fig.).

\(O A=\) Real depth (thickness) of the slab

\(I A=\) Apparent depth (thickness) of the slab

\[ \begin{aligned} \text { Refractive index }(\mu) & =\frac{\text { Real depth }}{\text { Apparent depth }} \\ & =\frac{O A}{I A} \end{aligned} \]

Answer 1

Aim: To determine the refractive index of a glass slab using real depth and apparent depth.

Principle: When the pin O is viewed through the glass slab, refraction makes its image appear raised. The original pin lies on the normal OA, while the apparent image lies at I nearer the top surface. Therefore, OA gives the real depth and IA gives the apparent depth.

Procedure:

1. Place the glass slab PQRS on a white sheet and trace its outline.
2. Fix a pin O touching the side PS of the slab.
3. Looking through the opposite face QR, fix two pins P₁ and P₂ in line with the image of O.
4. Remove P₁ and P₂, mark their positions, join them, and produce the line to cut the normal OA at I.

Observation:

• OA = real depth or actual thickness of the glass slab
• IA = apparent depth or apparent thickness of the glass slab

Formula: \[\mu = \frac{\text{Real depth}}{\text{Apparent depth}} = \frac{OA}{IA}\]

Conclusion: The refractive index of glass is obtained by measuring OA and IA and taking their ratio. Repeating the experiment for different positions of the pin improves accuracy.