Physics 5.9 - Refraction in glass core practical

Reading a ray through glass

A ray is a model: a straight line showing the direction in which a wave and its energy propagate. Its arrowhead points in the travel direction. A ray is not a narrow material track inside the light.

At the point where a ray meets a boundary, draw a normal at $90^\circ$ to the surface. The angle of incidence, $i$, is between the incident ray and the normal. The angle of refraction, $r$, is between the refracted ray inside the block and the normal. Neither angle is measured from the glass surface.

In the glass-block practical, observe this path:

1. At an oblique angle, the ray entering glass from air changes direction towards the normal, so $r$ is smaller than $i$.
2. At the second face, the ray leaving glass for air changes direction away from the normal.
3. Because the two faces of a rectangular block are parallel, the emergent ray is parallel to the incident ray but shifted sideways.
4. If the ray travels along the normal, $i = 0^\circ$ and its direction does not turn.

[DIAGRAM: asset_slug: phy_c_glass_tracing_verified; description: Rectangular glass block with incident and emergent rays parallel, bending towards the normal on entry and away on exit. Both normals and separated ray marks shown.]

The practical turns this geometry into evidence. Its purpose is to investigate how changing $i$ affects $r$, while also tracing the complete path through the rectangular block.

The glass-block core practical

Apparatus and arrangement

Use a ray box fitted with a single-slit grating and low-voltage power supply, a rectangular glass block, white paper, a sharp pencil, a ruler and a protractor. The single-slit grating produces a narrow beam whose centre can be traced more precisely than a broad patch of light.

Reproducible method

1. Fix a sheet of white paper flat on the bench. Place the rectangular glass block near its centre and trace its outline with a fine pencil.
2. Mark a point near the middle of one face. Use a ruler and protractor to draw a normal through this point at $90^\circ$ to the face.
3. Draw an incident guide line at $10^\circ$ to the normal. Replace the block exactly on its outline.
4. Fit the single-slit grating, switch on the ray box and align the centre of its narrow ray with the incident guide line. Keep the beam at bench level.
5. Mark two well-separated points along the incident ray, the points where the ray meets and leaves the block, and two well-separated points along the emergent ray. Do not move the block while marking.
6. Switch off the ray box and remove the block. Join the points with a ruler to reconstruct the incident, refracted and emergent ray paths. Draw a normal at the exit face too.
7. Put the protractor centre exactly at the entry point, align its zero line with the normal, and measure $i$ and $r$ to the nearest degree. View from directly above to reduce parallax.
8. Repeat for $i = 20, 30, 40, 50,\ 60^\circ$. This gives a useful range at equal intervals rather than changing several factors at once.
9. Make three independent traces for each incidence angle, using a fresh outline or fresh sheet each time. Calculate the mean $r$. Repeat an obviously inconsistent trace before excluding it; do not discard a result only because it misses the trend.