In this experiment, you will investigate the lateral displacement of a ray of light passing through a rectangular glass block. You traces the path of a ray ...
Question 1 Report
In this experiment, you will investigate the lateral displacement of a ray of light passing through a rectangular glass block. You traces the path of a ray through the block using the pin method. The completed ray-trace diagram is shown in Fig. 5.1. The block has a thickness t of 4.5 cm.
(a) Using Fig. 5.1, measure and record the angle of incidence i at surface AB. [1]
(b) Using Fig. 5.1, measure and record the lateral displacement s, the perpendicular distance between the incident ray extended and the emergent ray. [1]
(c) Describe and explain what would happen to the lateral displacement if the angle of incidence were increased. [2]
(d) Describe and explain what would happen to the lateral displacement if a thicker block of the same material were used with the same angle of incidence. [2]
(e) State what happens to the direction of the emergent ray compared to the incident ray. [1]
Answer Details
(a) The angle of incidence i is measured between the incident ray and the normal (the dashed vertical line) at surface AB. From Fig. 5.1:
\[ i = 45^\circ \] [1]
(Accept 44° to 46°.)
(b) The lateral displacement s is the perpendicular distance between the incident ray extended (shown as a dashed line) and the emergent ray. Measuring this distance on Fig. 5.1:
\[ s = 1.8 \text{ cm} \] [1]
(Accept 1.6 to 2.0 cm, consistent with the diagram.)
The lateral displacement is measured at right angles to the direction of the original ray, not along any surface of the block.
(c) If the angle of incidence were increased:
The lateral displacement would increase. [1]
A larger angle of incidence causes the ray to refract more sharply at the first surface, making a larger angle with the normal inside the glass. The refracted ray therefore travels a longer diagonal path through the block before emerging at the second surface. Since the emergent ray is displaced further from the original incident ray path, the lateral displacement increases. [1]
(d) If a thicker block of the same material were used with the same angle of incidence:
The lateral displacement would increase. [1]
Although the angle of refraction inside the block would be the same (same material, same angle of incidence, same refractive index), the refracted ray must travel through a greater thickness of glass. This longer path through the glass means the ray emerges further from the extended incident ray, giving a larger lateral displacement. [1]
(e) The emergent ray is parallel to the incident ray. [1]
At the first surface (AB), the ray bends towards the normal as it enters the denser glass. At the second parallel surface (CD), it bends away from the normal by the same amount as it re-enters the air. The two refractions are equal and opposite in terms of angular change, so the emergent ray has the same direction as the incident ray, just shifted sideways.
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