Describe an experiment to show refraction of light by transparent blocks of different shapes

3.2.2 Refraction of Light

Objective

To describe an experiment that demonstrates the refraction of light when it passes through transparent blocks of different shapes (e.g., rectangular slab, triangular prism).

Theory (brief)

When a light ray passes from one transparent medium to another, its speed changes, causing the ray to change direction. This phenomenon is called refraction. The relationship between the angles of incidence (\$i\$) and refraction (\$r\$) is given by Snell’s law:

\$n1 \sin i = n2 \sin r\$

where \$n1\$ and \$n2\$ are the refractive indices of the first and second media respectively.

Apparatus

Experimental Setup

1. Place a sheet of white paper on a flat table.
2. Position the ray box so that the emerging ray strikes the paper at a convenient point near the centre.
3. Place the transparent block (first the rectangular slab, then the triangular prism) on the paper in the path of the ray. Ensure the block can be rotated about a vertical axis to vary the angle of incidence.
4. Attach a protractor to the base of the block or use a rotating platform with degree markings to set the desired angle of incidence \$i\$.
5. Mark the incident ray, the refracted ray inside the block, and the emergent ray on the paper with a pencil.

Procedure

1. Set the incident ray to strike the block at normal incidence (\$i = 0^\circ\$). Observe that the ray continues in a straight line.
2. Increase the angle of incidence in steps (e.g., \$10^\circ\$, \$20^\circ\$, \$30^\circ\$) and for each step:

   • Record the angle of incidence \$i\$.
   • Mark the point where the ray enters the block.
   • Mark the point where the ray exits the block and draw the emergent ray.
   • Measure the angle of emergence \$e\$ (the angle between the emergent ray and the normal to the exit face).
   • For the rectangular slab, also measure the lateral displacement \$d\$ between the incident and emergent rays.

3. Repeat the above steps with the triangular prism, noting that the emergent ray is deviated from the original direction. Record the angle of deviation \$\delta\$ for each incidence angle.

Observations (example format)

Analysis

For the rectangular slab, the emergent ray is parallel to the incident ray, confirming that the angles of incidence and emergence are equal when the faces are parallel. The measured lateral shift \$d\$ can be related to the thickness \$t\$ of the slab and the refractive index \$n\$ by:

\$d = t \tan\left(i - r\right)\$

For the triangular prism, the ray is deviated. The deviation \$\delta\$ depends on both the prism angle \$A\$ and the refractive index \$n\$:

\$\delta = i + e - A\$

Using the recorded values, \$n\$ can be estimated by rearranging Snell’s law.

Conclusion

The experiment clearly demonstrates that light changes direction when it passes from one transparent medium to another. The amount of bending depends on the angle of incidence and the refractive indices of the media. Parallel‑faced blocks produce a lateral shift without changing the ray’s direction, whereas non‑parallel faces (prism) cause a measurable deviation.

Safety Precautions

• Never look directly into the ray box or laser source.
• Use low‑power laser pointers if a ray box is unavailable.
• Handle glass prisms with care to avoid breakage.
• Keep the work area free of clutter to prevent accidental trips.

Extension Activities

• Determine the refractive index of the material by plotting \$\sin i\$ against \$\sin r\$ and finding the gradient.
• Investigate how the colour (wavelength) of light affects refraction using a white‑light source and a diffraction grating.
• Explore total internal reflection by using a high‑index prism and increasing the angle of incidence beyond the critical angle.