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
Describe and apply the law of reflection and explain the effect of reflection from a plane
surface on waves
When a wave is reflected from a flat-surface it will obey the Law of Reflection. The Law of Reflection states
that the angle of the incoming (or incident) wave, in relation to a line perpendicular to the reflecting surface
(normal) at the point where the incident wave hits the surface, is equal to the angle the reflected wave will
make with the normal.
The Law of Reflection states that the angle of incidence equals the angle of reflection from a plane surface and
the incident ray, the reflected wave and the normal to the reflecting surface all lie in the same plane.
Electromagnetic waves can be reflected from smooth, flat, mirror like surfaces or from irregular or bumpy
surfaces. When the waves are reflected from a flat surface, the reflection is regular. When the reflection is
from a bumpy or irregular surface, it is diffuse.
The Law of Reflection
properties include:
The
The
The
The
allows for the explanation of properties of the image formed by the reflection. These
size of the image
nature of the image
apparent location of the image
orientation of the image compared to the original object.
Images can be classified as real or virtual. A real image is one that can be projected onto a screen. A
virtual image can be seen by cannot be projected onto a screen.
Reflection and Wave phase change
When a wave is reflected from the surface of a material having a higher index of refraction, the wave
undergoes a phase change of 180 degrees. The reflected wave is out of phase by half a cycle compared with
the incident wave. This is the case when light travelling through air is reflected from a glass mirror.
When light is reflected from the surface of a material with a lower index of refraction, there is no phase shift.
Note: The frequency of the reflected waves remains unchanged
Reflection from curved mirrors.
Curved mirrors concentrate the initially parallel light rays to a point known as the focus and are hence called
converging mirrors. If the light rays originate from a point source at the focus, they will be parallel after
being reflected. These curved mirrors are silvered on the concave side and are known as concave mirrors.
Other curved mirrors are silvered on the convex side. They are commonly used too give a wider field view.
These mirrors cause the parallel rays incident on their surface to be reflected as through they diverge from a
focus behind the mirror. Hence the are known as convex or diverging mirrors.
Ray tracing diagrams can be used to show the nature of an image formed by a mirror. It is determined by
constructing any two of the following rays:
1) The first ray travels in a direction that is parallel to the principal axis and is from the top of the object to
the mirror surface. After this reflection, this ray passes through the focus, or should be drawn as if it
appears to come from the focus if the mirror is convex.
2) The second ray should be drawn from the top of the object, through the focus and on reflection becomes
parallel to the principal axis.
3) The third ray is drawn toward the pole of the mirror and is reflected such that the angle it makes with
the principal axis is identical to the angle of incidence of the ray.
4) The fourth ray travels from the top of the object through the centre of the curvature. This will fix the top
of the image on itself because it is incident normally on the mirror’s reflecting surface.
The bottom of the object lies on the principal axis. The intersection of these four rays defines the position and
the nature of the image formed by an object on reflection form the mirror. The nature of the image can be
predicted based on the position of the object and the mirror type.
For concave mirrors:
-
Inside the focal length of the mirror – Image is virtual, upright and enlarged
Between the focal length and the centre of the curvature – Real, inverted and enlarged
Beyond the curvature – Real, inverted and diminished.
Special Case: If the object is at the focal length, the image will actually form at infinity. If the
object is at the centre of the curvature the image that forms is real, identical in size but inverted.

Describe one application of reflection for each of the following: plane surfaces and concave
surfaces
Plane Surfaces:
Telescopes
A Newtonian reflecting telescope. The main light-collecting mirror is a concave (spherical or parabolic) mirror.
The parabolic mirror has the advantage that it tends to focus the light collected by the mirror to a point more
accurately than a spherical mirror. The secondary mirror is a plane mirror.
The concave mirror with a larger diameter is advantageous as it has a greater light gathering ability. This
enables the observation of fainter astronomical object.
The plane mirror is used to reflect the light rays to the eye piece, where a secondary plane mirror focuses the
rays so that they are parallel, thus increasing the clarity of the image.
Concave Surfaces:
Torches + Driving Lights
Large spherical mirrors focus the parallel rays that hit near the pole of the mirror at a slightly different place
than the parallel rays at the edge of the mirror. This is known as spherical aberration. Parabolic mirrors do
not show spherical aberration and focus all rays at the focus.
Torches and driving lights are advertised as having parabolic, concave mirror reflectors. In some torches the
ability to move the filament from the focus means the torches are able to produce a spot or a flood beam.
When the beam is adjusted to spot, the filament is at the focus. When the beam is set to flood, the filament is
either beyond or before the focus of the reflector mirror.
Satellites
Satellite dishes receive weak radio signals from satellites in space or detect naturally emitted radio
frequencies from astronomical objects. These signals are received by the dish as parallel rays that hit the dish
surface. A large, concave, parabolic reflecting dish collects the weak-intensity signals, reflects them
and focuses the signals to a receiver aerial at the focus of the satellite dish.`