PAG8: Interactions of Waves with Matter

Use an approved ray source to produce a narrow visible ray. Outline the reflecting surface or transparent block on paper, mark several points along each visible ray and keep the apparatus in the same position. After removing the apparatus, join marked points with a ruler. A single broad mark gives poor direction evidence; two separated points define a path more clearly.

At each boundary point, draw a normal perpendicular to the surface. The angle of incidence and the reflected or refracted angle are measured from that normal, not from the surface. Use different incident angles and retain the raw measurements. Avoid looking into the source or using an unapproved laser arrangement.

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For a plane reflecting surface, the reflected and incident rays make equal angles with the local normal. A rough surface produces diffuse reflection because its small surface regions have different normals; the local law is not abolished. Compare a smooth surface with an appropriate rougher surface under similar illumination, distinguishing a spread of reflected directions from an absence of reflection.

For oblique incidence from air into common transparent glass or plastic, light slows and the ray bends toward the normal. On emerging into air it bends away from the local normal. A parallel-sided block in the same outside medium gives an emerging ray parallel to the incident ray but laterally displaced in the ideal geometry. At normal incidence there is no directional bending even though speed changes.

Compare repeated angle readings and the quality of the ray marks. A block moved after its outline was drawn gives a systematic path error. A thick ray, uncertain normal or protractor resolution affects angular precision. Improve a named limitation, such as using longer marked ray segments or securing the block, rather than saying only “use more accurate equipment”.

Comparing substances requires controlled geometry and the same incident-angle convention. Do not compare one refracted angle at 20° incidence with another at 60° and attribute the difference only to material. Wave reflection/refraction can also be observed at suitable water-depth boundaries; the medium conditions change wave speed and wavelength while the source frequency stays the same. The centre selects additional wave/matter contexts where required.

Plane-mirror reflection and refraction through a suitable prism or transparent block can contribute to the electromagnetic-wave interaction requirements. Use the approved geometry and local normals, and explain how the observations support a reflection or refraction conclusion.

The group also concerns observations of wave interactions in fluids and solids. A suitable ripple-tank activity can compare reflection at a barrier and refraction at a depth boundary, using clear wavefronts and controlled source frequency. The centre’s chosen activities must collectively cover the stated apparatus and techniques; a single glass-block ray trace should not be presented as proof of every requirement.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.