Question Video: Identifying the Reflected Ray for a Given Incident Ray in 3D | Nagwa Question Video: Identifying the Reflected Ray for a Given Incident Ray in 3D | Nagwa

Question Video: Identifying the Reflected Ray for a Given Incident Ray in 3D Science • Second Year of Preparatory School

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A vertical mirror rests on a flat horizontal surface. The orange arrow shows the incident light ray on the mirror. Which of the following is true? [A] The direction of the reflected ray must be as shown by the red arrow. [B] The direction of the reflected ray could be as shown by either the red arrow or the blue arrow.

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Video Transcript

A vertical mirror rests on a flat horizontal surface. The orange arrow shows the incident light ray on the mirror. Which of the following is true? (A) The direction of the reflected ray must be as shown by the red arrow. Or (B) the direction of the reflected ray could be as shown by either the red arrow or the blue arrow.

To answer this question, we’ll need to understand what the difference is between the red arrow and the blue arrow. So let’s go over exactly what’s being shown in the diagram. We know that this mirror stands vertically and that this flat surface is horizontal. Notice this dashed line that’s perpendicular to the face of the mirror. This line represents the normal to the mirror, and it lies in the same horizontal plane as the flat surface. The orange arrow, which represents the incident ray, and the red arrow also lie in the horizontal plane. The blue arrow, however, looks a little different. The blue arrow starts here at the same point on the mirror as the red arrow, but it appears to rise up out of the horizontal plane. That’s why we see this dotted line. It’s there to show us that the blue arrow rises up above the red arrow.

To see this better, we can draw a side view of the diagram. This green line represents the horizontal surface or the plane where the incident ray and the normal line lie. This line represents the mirror, which stands vertically. We can see that the blue arrow starts here at the bottom of the mirror and points upwards by some amount. Therefore, the blue arrow does not lie in the horizontal plane.

Now, the key to answering this question is to simply recall that for light reflecting off a flat surface, the incident ray, the normal to the surface, and the reflected ray must all lie in the same plane. We know that the incident ray and the line normal to the mirror lie in the horizontal plane. So the reflected ray also has to lie in this plane. The red arrow lies in this plane, but the blue arrow does not. Therefore, we know that the blue arrow could not show the direction of the reflected ray. So answer option (A) is correct. The direction of the reflected ray must be as shown by the red arrow.

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