Question Video: Determining the Path of Light Rays Undergoing Reflection | Nagwa Question Video: Determining the Path of Light Rays Undergoing Reflection | Nagwa

Question Video: Determining the Path of Light Rays Undergoing Reflection Physics • Second Year of Secondary School

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The diagram shows a point P that one of the three light rays A, B, and C might possibly pass through after being reflected. Which light ray would pass through the point?

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Video Transcript

The diagram shows a point P that one of the three light rays A, B, and C might possibly pass through after being reflected. Which light ray would pass through the point? (A) Ray A, (B) ray B, (C) ray C, (D) all these rays.

The question asks us to find out which of the three rays, A, B, or C, passes through point P after reflecting off the surface shown in the diagram. Let’s recall that in the absence of anything in the way, light rays travel in straight lines. But of course in this case, there is something in the way of these light rays. There’s this surface right here, which the light rays will reflect off.

We can see that the surface is flat, which means that these light rays will be undergoing specular reflection. The law of reflection tells us what happens to light rays reflecting off a surface. Considering a flat surface, we can draw in a line perpendicular to it, which is known as the normal line or the normal to the surface. Let’s suppose we have an incident light ray that makes an angle of 𝜃 sub 𝑖 to this normal line. This is the angle of incidence of the light ray.

The ray will be reflected from the surface according to the law of reflection, which says that the angle of incidence is equal to the angle of reflection, but on the opposite side of the normal. The angle of reflection, which we’ll call 𝜃 sub 𝑟, is the angle that the reflected light ray makes to the normal line, that is, this angle here. So the law of reflection says that 𝜃 sub 𝑖 is equal to 𝜃 sub 𝑟. Or the angle that the incident light ray makes to the normal line is equal to the angle that the reflected light ray makes to the normal line.

Using this information, we can extend the path of each of these three light rays from the question. Let’s begin with ray A. We know it travels in a straight line until it meets the surface. Then, we need to draw in the normal line at the point where the ray hits the surface. We know from the law of reflection that the angle between the normal and the reflected ray is equal to this angle here, the angle of incidence. Measuring this angle and drawing in the reflected ray at this same angle on the other side of the normal, we see that ray A does not pass through point P.

We can do the exact same thing for rays B and C. Extending the rays until they meet the surface, drawing in the normal line in each case, and applying the law of reflection, we have the path of the reflected rays for light rays B and C. We see that ray C does not pass through point P, but ray B does.

So the correct answer here is given in option (B). The only light ray that passes through the point marked P is ray B.

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