Reflection Of Light by Plane Mirror
Let us do a quick activity before we move ahead with the lesson.
Close your eyes for 10 seconds and count to 10; now when you open
your eyes again, what do you observe? Do you observe all the things
around you? What are the phenomena that allow you to see things
around you? Does the phenomena Reflection of Light have anything
to do with this? Let us know more about it.
What is Reflection of Light?
When a ray of light falls on any object (polished, smooth, shiny
object), light from the object bounces back those rays of light to our
eyes and this phenomenon is known as “Reflection” or “Reflection of
This phenomenon is what enables us to look at the world around us
and is based on the property that light travels in a straight line. For
example, twinkling of stars or light reflected by a mirror.
Laws of Reflection
In the diagram given above, the ray of light that approaches the mirror
is known as “Incident Ray”. The ray that leaves the mirror is known as
At the point of incidence where the incident ray strikes the mirror, a
perpendicular line is drawn known as the “Normal”. This normal is
what divides the incident ray and the reflected ray equally and gives
us the “Angle of Incidence” (Qi) and “Angle of Reflection” (Qr).
Hence the above information gives us the “Laws of Reflection of
Light” which state that :
a. The angle of incidence is equal to the angle of reflection.
b. The incident ray, the normal and the reflected ray, all lie in the same
Learn more about Reflection of Light by Spherical Mirrors.
Types of Reflection :
There are majorly two types of reflection :
a. Specular/ Regular reflection
b. Diffused/ Irregular reflection
a. Specular/Regular reflection :
Specular/Regular reflection is a mirror-like reflection of rays of light.
Here the rays of light which are reflected from a smooth and shiny
object such as a mirror, are reflected at a definitive angle and each
incident ray which is reflected along with the reflected ray has the
same angle to the normal as the incident ray. Thus, this type of
phenomena causes the formation of an image.
Learn how is Image formed in Spherical Mirror here.
b. Diffused/Irregular reflection:
Diffused/Irregular reflection is a non-mirror-like reflection of light. In
this type of reflection rays of light that hit an irregular object with a
rough surface, are reflected back in all directions. Here, the incident
ray which is reflected along with reflected ray doesn’t have the same
angle to the normal as the incident ray.
Thus, this type of reflection doesn’t form an image.
Image Formation by a Plane Mirror
Let us take a mirror MM’ as shown in the diagram given above. Let us
suppose an object AB of size ‘h’ on the left-hand side of the mirror at
a distance ‘u’.
An incident ray of light AP from point A of the object AB falls on the
mirror MM’ at point P. This incident ray AP is reflected back in the
same path PA. Another ray OC falls on the mirror MM’ at a point O
and is reflected along the path OC. Now, since reflected rays PA and
OC are diverging and therefore cannot meet each other in front of the
mirror, hence we extend these rays PA and OC behind the mirror by
dotted lines. On extending these rays behind the mirror, we see that
these rays meet at point A’ at a distance V’. Therefore A’ is the virtual
image of point A of the object AB.
Similarly, a virtual image of point B will be formed behind the mirror
as B’ from the incident rays BO and BE.
Now, to get a complete image of the object AB, we join the point A
and B to point A’ and B’ by a dotted line. In doing so, we find that the
image A’B’ being formed is virtual, erect and of same shape and size
as the object AB; thereby giving us the characteristics of images
formed by the plane mirror.
Learn more about the Laws of Reflection of Light in detail.
Characteristics of Images formed by Plane Mirror
a. Images formed by a plane mirror are “Always Virtual”.
b. Images formed by a plane mirror are “Erect/Upright”.
c. Images formed by a plane mirror are of “same shape and size” as
that of an object.
Solved Example for You
Q. A ray of light strikes a reflective plane surface at an angle of 42o
with the surface.
a. Find the angle of incidence.
b. Find the angle of reflection.
c. Find the angle made by the reflected ray and the surface.
d. Find the angle made by the incident and reflected rays.
Sol: We’ll use a diagram given below to answer the question:
a. Angle of Incidence (Qi)= 900-420= 480
b. Angle of Reflection (Qr)= (Qi)= 480
c. x = 900-Qr = 900-480 = 420
d. Qi + Qr = 480 + 480 = 960
Terms Related to Spherical Mirrors
“Mirror, mirror on the wall, who’s the fairest of them all?” We’ve all
heard this dialogue from the story of Snow white and the seven
dwarfs. But what kind of mirror was it? What were the properties that
enabled these spherical mirrors to be so special? Well, let us find
answers to it in this section.
There’re few basic and important terms one needs to know while
studying spherical mirrors inside the chapter “Reflection and
a. Center of Curvature (c)
b. Radius of Curvature (r)
c. Pole (p)
d. Principal axis
f. Principal Focus or Focal Length
a. Center of Curvature (c) :
The Center of Curvature of a spherical mirror is the point in the centre
of the mirror which passes through the curve of the mirror and has the
same tangent and curvature at that point. It is denoted by the letter ‘c’.
b. Radius of Curvature (r):
It’s the linear distance between Pole and the Center of curvature.
c. Pole (p) :
It’s the midpoint of the spherical mirror.
d. Principal axis:
It’s an imaginary line passing through the optical centre and the centre
of curvature of any lens or a spherical mirror.
An aperture of a mirror or lens is a point from which the reflection of
light actually happens. It also gives the size of the mirror.
f. Principal Focus :
Principal Focus can also be called as Focal Point. It’s on the axis of a mirror or lens wherein
rays of light parallel to the axis converge or appear to converge after reflection or refraction.
Principal Focus is also what determines the Focal Length of the mirror.
It’s any given point, where light rays parallel to the principal axis, will
converge after getting reflected from the mirror.
What is an Image?
When an object is placed in front of a mirror, we see an image of the
object placed. This image appears to be behind the mirror, and is
called “Image”. The object is the source from which incident ray
occurs and the image that is formed is because of the reflected rays.
The image formed maybe “Real” or “Virtual”.
Wherein the “real” image is formed when the light rays from the
object actually intersect each other after reflection. Real images are
formed inverted and can be projected on to a screen. On the other
hand, a “virtual” image is formed when the light rays from the object
don’t actually intersect each other after reflection. Although they
“appear” to do so when they’re produced behind the mirror. Virtual
images are “always” erect and cannot be projected on a screen.
Concave and Convex Mirrors. (Source: Wikipedia)
See how Image is formed by Spherical Mirrors here
Types of Spherical Mirrors
Spherical mirrors are of two types:
a. Concave Mirror
b. Convex Mirror
Learn more about Reflection of Light by Spherical Mirrors in more
a. Concave Mirror