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How an image is formed in a plane mirror??

Rays of light from a source travel outwards in all directions. Several of
these rays may enter our eyes. If we know the direction of these rays
then we can trace two or more rays backwards until they meet at the

source.


If P is the point of the pencil, then rays of light reflected from P will

spread out in all directions from this point. Some of these rays form two
narrow cones of light that enter our eyes, as shown in Figure 2.8. When
we look at an object our eyes automatically focus to produce an image
of the point. If the object is moved further away, the angle between the
two rays forming a cone entering our eye is smaller and so the object
will appear smaller.

This focussing process is an inbuilt method of tracing the rays that

enter our eyes back until they meet. We can estimate distances more
accurately using both eyes than using only one eye. You can test this
very easily by closing one eye and trying to thread a needle, and then



The image in a plane mirror is virtual.
Carefully examine the printing on the bottom half of the
carton of milk and on its image.
doing it with both eyes open.

Tracing rays back until they meet not only helps in locating objects

but it is also the method by which we locate the position of an image in
a plane mirror.

When an object is placed in front of a mirror you can see an image of

that object in the mirror. To understand how this image is formed consider
Figure.

O is an object placed at a given distance in front of a plane mirror

MN. Two rays of light are drawn from O and these rays strike the mirror
at A and B respectively. By applying the law of reflection that i = r,
the two reflected rays can be drawn. The normals at A and B are shown.
For ray OA, i = r and for ray OB, i′ = r′.

These two reflected rays are shown entering an observer’s eye. To the

observer these two rays appear to have come from the point I, which is
behind the mirror. In viewing this situation, the observer has automatically
traced the two rays back to I. The point I is said to be the image
of O. For the situation described, the line OI can be drawn. The line OI,
and ray OA and its reflection are shown in Figure.

By considering the angles of the two triangles OMA and IMA it can

be seen that OM = IM. This means that the image is as far behind the
mirror as the object is in front of the mirror. We say that the object
distance (OM) is equal to the image distance (IM).

Figure also illustrates that the line joining the object and the

image is perpendicular to the mirror surface. Now consider the case of
the image of an extended object instead of a point object.
Figure shows an extended object, PQ, placed in front of a plane
mirror, MN.

Two rays from each end of the object PQ have been drawn and these

have been reflected by the mirror MN into an observer’s eye. To the
observer, the rays appear to come from P′Q′, so that P′Q′ is the image
of PQ.

Note that P′ is as far behind the mirror as P is in front of the mirror.

The same applies to Q and Q′.
This diagram illustrates another property of the image in a plane
mirror. The image is the same size as the object. This is shown by the
fact that PQ = P′Q′.


Real and virtual images



There is a basic difference between the image formed by a plane mirror

and that formed on a screen by a projector. The rays of light in Figure
 appear to diverge from point I. These rays of light do not actually
pass through the image point. Such an image is said to be virtual.
A virtual image is defined as one through which the rays of
light do not actually pass.

Compare this with the image formed on a screen by a projector. The

rays of light actually converge on the image. Such an image is said to be
real.

A real image is defined as one through which the rays of light

do actually pass. Real images will be further discussed in another post.
In Figure, if a screen is placed at I, behind the mirror, the image
will not appear on the screen. Virtual images cannot be projected onto
a screen.

The image of a real object in a plane mirror is always virtual and the

same size as the object. Curved mirrors and lenses can produce either
real images or virtual images depending on the conditions.

If a printed page is viewed in a plane mirror the letters will appear

reversed. This can be seen if you place a mirror vertically on a page of
this book. The image appears to be inverted left to right, and vice versa.
A person with a spot on his or her right cheek will see an image with
a spot on the left cheek. The real reversal is front to back, and the image
is turned through itself. This gives the appearance of reversal left to
right and is called lateral inversion (incorrectly) in some references. If
you want to see yourself as others see you, obtain a second flat mirror
and place it at right angles to the first. You should see three images and
the centre one is you as others see you. Check the nature of the other
two images. Can you explain how this is formed?

It is for this reason that some signs on motor vehicles, such as on the

front of fire engines, are inverted – so that they can be read correctly
when viewed through a rear vision mirror by the driver of a motor car.

Figure 1

The word on the fire engine appears correct when viewed by a driver through the
rear vision mirror of a motor car.

Figure 2

The image in a plane mirror is virtual.
Carefully examine the printing on the bottom half of the
carton of milk and on its image.


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