Required Practical: Investigating Reflection & Refraction

Setup for Reflection Investigation

To investigate reflection, you need a plane mirror, a ray box or laser pointer, a sheet of plain paper, and a protractor.

• Place the plane mirror vertically on the paper and fix it in place.
• Use the ray box to shine a narrow beam of light (incident ray) at the mirror.
• Mark the point where the incident ray strikes the mirror.
• Mark the path of the incident ray and the reflected ray on the paper using a pencil.
• Use a protractor to draw a normal line (a dotted line perpendicular to the mirror surface) at the point of incidence.
• Measure the angle of incidence (between the incident ray and the normal) and the angle of reflection (between the reflected ray and the normal).

Make sure the ray paths are clearly marked and labelled for accurate measurement.

Measuring Reflection

To verify the law of reflection:

• Vary the angle of incidence by changing the direction of the incident ray.
• For each angle of incidence, measure the corresponding angle of reflection.
• Record your results in a table, including the angle of incidence and the angle of reflection.
• Check that the angle of incidence equals the angle of reflection for all measurements.

Accurate data recording is essential to confirm the law of reflection.

For instance, if the angle of incidence is measured as 306, the angle of reflection should also be 306.

Setup for Refraction Investigation

To investigate refraction, you need a rectangular glass block, a ray box or laser pointer, a sheet of paper, and a protractor.

• Place the rectangular glass block on the paper and trace its outline.
• Shine a narrow beam of light at one side of the block at a known angle of incidence.
• Mark the incident ray on the paper before it enters the block.
• Mark the refracted ray inside the block and the emergent ray as it leaves the block.
• Draw the normal at the point where the light enters the block (perpendicular to the surface).
• Measure the angle of incidence (between incident ray and normal) and the angle of refraction (between refracted ray and normal).

Ensure all ray paths and angles are clearly marked for precise measurement.

Measuring Refraction

To investigate how light bends when passing between air and glass:

• Change the angle of incidence by adjusting the ray box.
• For each incident angle, measure the corresponding angle of refraction inside the glass block.
• Record your data carefully in a table.
• Use Snell's Law to calculate the refractive index $n$ of the glass:

$$n = \frac{\sin i}{\sin r}$$

where $i$ is the angle of incidence and $r$ is the angle of refraction.

For example, if the angle of incidence is 406 and the angle of refraction is 256, the refractive index is:

$$n = \frac{\sin 40^\circ}{\sin 25^\circ} = \frac{0.6428}{0.4226} \approx 1.52$$

Analysis and Conclusion

After collecting your data:

• Compare the measured angles of incidence and reflection to confirm the law of reflection: angle of incidence = angle of reflection.
• Compare the angles of incidence and refraction to verify Snell's Law and calculate the refractive index of the glass.
• Discuss any discrepancies or errors in your measurements, such as:
• Difficulty in drawing precise ray paths or normals.
• Parallax errors when measuring angles with the protractor.
• Imperfect alignment of the ray box or laser.
• Possible surface imperfections in the mirror or glass block affecting accuracy.
• Suggest improvements to reduce errors, for example:
• Use a sharper, narrower ray of light for clearer rays.
• Use a more accurate protractor or digital angle measurer.
• Repeat measurements multiple times and take averages.