Video Transcript
The diagram shows how a
protractor is used to measure the angles of incidence of two rays. Which color arrow shows the ray
with the greater angle of incidence?
Alright, so we have two
different rays incident on a flat mirror. The mirror is represented by
this solid vertical line. This question is asking about
angles of incidence. So let’s recall that the angle
of incidence is the angle between an incident ray and the line normal to the
face of the mirror, where normal just means perpendicular. This horizontal dashed line
then is normal to the mirror. So the angles of incidence are
the angles that the arrows make with this line. We can use the protractor to
measure these angles. And first thing, we should
notice that the normal line is at the zero-degree mark on the outer scale of the
protractor. This means that any angle we
measure with respect to this zero mark will be measured with respect to the
normal line, which makes our job fairly easy.
Let’s take a closer look at
where the arrows meet this scale on the protractor. We’ll start with the red
arrow. The red arrow is just slightly
beyond this medium-sized line that’s directly between the marks of 30 degrees
and 40 degrees. This medium-sized line then
marks 35 degrees. Looking a little closer at the
red arrow, we can see that it’s aligned with the first small line past the
35-degree mark. Each one of these small lines
corresponds to one degree. And therefore, we know that the
angle of incidence shown by the red arrow is 35 degrees plus one degree or 36
degrees.
Next, let’s look at the purple
arrow. This arrow is just a little bit
before this middle-sized line between the marks of 50 and 60 degrees. So it’s just before the
55-degree mark. More specifically, the purple
arrow is one small line before the 55-degree mark. So we know that it shows an
angle of incidence of 54 degrees. 54 degrees is greater than 36
degrees, so we’ve found that the purple arrow shows the ray with the greater
angle of incidence.
