Core Practical: Investigating Refraction

Refraction occurs because light travels at different speeds in different materials; it slows down in more optically dense materials (like glass or Perspex) and speeds up in less dense materials (like air).

When light enters a denser medium from a less dense medium, the ray slows down and bends towards the normal, resulting in an angle of refraction that is smaller than the angle of incidence ($r < i$).

Conversely, when light exits a denser medium into a less dense medium, it speeds up and bends away from the normal, making the angle of refraction larger than the angle of incidence ($r > i$).

If a light ray enters a medium exactly along the normal (at an angle of incidence of $0^{\circ}$), it changes speed but does not change direction.

Accuracy in Drawing: Always use a sharp pencil and a ruler for ray diagrams. Marks are often lost for 'thick' lines or rays that do not meet exactly at the boundary.

The Normal Rule: Ensure the normal line is exactly $90^{\circ}$ to the surface. If the surface is curved (like a semi-circular block), the normal at any point passes through the center of the circle.

Angle Measurement: Always measure angles from the normal, never from the surface of the block. This is the most common mistake in practical exams.

Consistency: When comparing different shapes, keep the light source at a constant distance and use the same color (wavelength) of light, as different colors refract by different amounts.

Misidentifying Angles: Students often measure the angle between the ray and the glass surface rather than the ray and the normal.

Ignoring the Internal Ray: Some forget to draw the ray inside the block, only showing the light entering and leaving. The internal path is essential for calculating the refractive index.

Parallax Error: When marking the path of the light beam, looking at the beam from an angle can lead to inaccurate dot placement. Always look vertically down over the paper.