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.
