The optical alignment system at a small theatre uses a low-power laser to check that a movable mirror is set correctly before a light show. Fig. 1 is a diag...
Question 1 Report
The optical alignment system at a small theatre uses a low-power laser to check that a movable mirror is set correctly before a light show. Fig. 1 is a diagram of one check. A ray from the laser strikes the plane mirror at point P and is reflected towards a sensor. The dashed line is the normal. The angles shown are measured between each ray and the normal, not between a ray and the glass surface. The technician records the position of the sensor after the test so that the same reflected light path can be used next time.
(a) What is the name of the effect that changes the direction of the laser ray at P? [1]
(b) Calculate the angle of reflection in Fig. 1. [2]
(c) Sketch on Fig. 1 a ray that would reach a sensor placed directly above P. Use the normal to guide your construction. [2]
(d) Explain why a rough painted signal board would not send a narrow beam of light to one precise sensor position. [3]
(e) Suggest one safety precaution when the laser is used during the alignment test. [2]
Answer Details
(a) The direction change at P is reflection. [1]
(b) The angle of incidence is \(32^\circ\). By the law of reflection, angle of reflection equals angle of incidence, so the angle of reflection is \(32^\circ\). [2]
(c) A sensor directly above P is reached by a ray travelling vertically upwards along the normal. [2]
(d) A rough painted surface has differently angled normals at different points. Light is therefore reflected in many directions, which is diffuse reflection. The reflected beam spreads, so little or no light reaches one precise sensor position. [3]
(e) Do not look into the laser beam. Also keep it below eye level, wear suitable eye protection, or switch it off before adjusting the mirror. Any two precautions gain credit. [2]
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