Video Transcript
The diagram shows three points — I,
II, and III — that a light ray might possibly pass through after being
reflected. Which of these points would the
light ray pass through?
To answer this question, we need to
predict the path of a ray of light after it’s been reflected from this surface. This arrow here represents the
initial incident ray of light. The light is traveling in the
direction shown by the arrow. Since there is nothing in the way
of the incident ray’s path, we know it will travel in a straight line and continue
on in this direction until it reaches the surface. It’ll be helpful to draw in an
extrapolated line, like this.
Now, we need to work out which
direction the light travels in once it’s been reflected by the surface. Let’s start by drawing the surface
normal at the point where the ray meets the surface. Recall that a “normal” is simply a
line that’s perpendicular to the surface that reflects the light. It’s especially useful for ray
diagrams like this, since we should be measuring angles with respect to the surface
normal.
In fact, let’s go ahead and define
the angle of incidence as the angle between the normal and the incident light
ray. We’ll label this angle as 𝜃 with a
subscript i to denote that this is the angle of incidence.
Now, to draw the next part of our
ray diagram, we need to recall the law of reflection. The law of reflection states that
the angle of incidence is equal to the angle of reflection. In other words, the angle between
the normal and the incident light ray is equal to the angle between the normal and
the reflected ray.
So on our diagram, we can draw in
the reflected ray, which we know should be traveling away from the surface at an
angle of 𝜃 sub r that is equal to 𝜃 sub i, as measured from the normal.
Now all we need to do to answer
this question is continue the path of the reflected ray and see which of the three
points it passes through. Once we do this, we see that the
light ray will pass through point III.
Note that it would be impossible
for this particular ray to pass through any of the other points. To pass through either point I or
point II, the angle between the ray and the normal would have to be smaller. But, this can’t happen; the law of
reflection states that the angle of reflection must be equal to the angle of
incidence.
Thus, we have determined that after
being reflected, the light ray would pass through point III.
