Video Transcript
Which of the following examples of
reflection of light rays from a surface show specular reflection? (A) I only, (B) I and II, (C) I and
III, (D) none of these examples, (E) all of these examples.
We have been given four diagrams in
which we can see light rays directed at various surfaces. We are asked to state which of
these diagrams shows specular reflection.
Let’s recall that we can identify
two different categories of reflection: specular reflection and diffuse
reflection. Specular reflection occurs when the
surface is flat. The normal line, that is, the line
perpendicular to the surface, has the same direction at all points on the
surface. Diffuse reflection occurs when the
surface is uneven. The normal line will have different
directions at different positions on the surface.
Now, when light reflects off a
surface, it does so according to the law of reflection. This law says that the angle of
incidence, that is, the angle between the incident ray and the normal, is equal to
the angle of reflection. That’s the angle between the
reflected ray and the normal. For specular reflection, no matter
where a light ray hits the surface, the normal has the same direction. So a light ray incident at a given
angle will be reflected at the same angle no matter which point on the surface it
hits.
However, for diffuse reflection,
light rays hitting the surface at different positions experience different surface
normals. As a result, the reflected rays get
scattered in different directions depending on where they hit the surface.
When considering whether reflection
is specular or diffuse, the direction of the incident rays themselves is
irrelevant. All that matters is whether the
surface is flat or not. We can see that diagrams I and II
show light rays directed at a flat surface. Therefore, these diagrams
illustrate specular reflection. Meanwhile, diagrams III and IV show
light rays directed at an uneven surface. That means that these diagrams
illustrate diffuse reflection.
The correct answer is therefore
option (B). The diagrams that show specular
reflection are I and II.
