I guess you have seen yourself in a mirror. But have you
ever wondered how it works? In this section I am going to explain to you
what is the phenomenon called reflection of light. Surface which reflects almost all of light which
falls on it is called specular reflector.
Polished metals, liquid surfaces, and mirrors are such specular reflectors.
An object with flat, smooth reflecting surface is called plain mirror.
Now, let's carry out an experiment on reflection of light by a plain mirror.
To do this we place a sheet of white cardboard perpendicular to the reflecting
surface of plain mirror, and we send a pencil of light to the surface of the mirror
so that it just skims along the surface of cardboard. Pencil which strikes
the surface of the mirror is called the incident pencil and the pencil which
is reflected by the surface is called the reflected pencil. Results of the experiment are
shown in the Figure 1.
Figure 1 Specular reflection.
This experiment leads us to the conclusion that when light is reflected
from a plane specular surface, the incident ray, the normal (the perpendicular
to the surface at the point of contact), and the reflected ray
all lie in the same plane. This statement is called the first law of reflection.
Now we can measure the angles between the normal
and two rays by placing a white plastic protractor instead of the cardboard.
We can repeat measurements for different angles of incidence but conclusion
will be always the same: the angle of reflection (r) is equal to the angle
of incidence (i). This statement is called the second law of reflection..
It is hard to belive, but you have to know that all objects reflect rays,
but those which surface is not flat give diffuse reflection and become
indirect-lighting device. Yes, your hand also reflects and diffuse much
of the light it receives. Its surface is rough, so rays have different angles
of incidence and different angles of reflection and are diffused. This phenomenon allows
us to see objects which are not the sources of light.
Figure 2 Diffuse reflection
Light diffused by the object is reflected by the mirror and is received by
our eyes. As far as eye is concerned, then, light appears to be emitted
by the object behind the mirror. The object behind the mirror is called
the image of the object. Because the mirror causes the rays to appear as if they come
from the image, although they actually do not, we say that a virtual
image is formed.
Figure 3 S1 is the virtual image of S in the plain mirror.
Point S sends light in all directions. Let's mark some of rays that fall
on the reflecting surface of the mirror (O) as SA1,
S2, S3, normals as p1, p2,
p3 and reflected rays as A1B1,
A2B2, A3B3. Extensions behind
the mirror of the reflected rays intersect at one point (S1).
All three reflected rays act as though they were diverging from the point S1,
so at that point is the image of the point S.
Image of an object consists of images of its all points.
Figure 4 Rays reflected from the surface of the mirror reach eyes as if they came from the arrow A1B1
(the virtual image of the arrow AB)
On the top of the Pic du Midi in the French Pyrenees research workers
curious about the effects of ultrahigh temperatures have been able to
use mirrors to concentrate enough sunlight to melt steel. In India,
using inexpensive "mirror ovens", housewives cook dinner with concentrated
sunlight. What shape mirror has this ability to concentrate light?
We cannot concentrate light with a plane mirror, for the light always
diverges, appearing to come from the virtual image behind the mirror.
A Parabolic mirror has the shape we are looking for. A parabolic mirror
is a spherical cap with a polished, well-reflecting surface. Widely
used are parabolic mirrors which are small part of a sphere. Line
connecting the middle of the sphere with the middle of the mirror cap
is called the axis of revolution of the paraboloid. If we send
some rays parallel to the axis of revolution to the surface of the mirror,
all reflected rays intersect at one point on the axis of revolution. This
point is called the principal focus (F).
Figure 5 F - principal focus of the parabolic mirror,
FO - axis of revolution of the parabolic mirror
Figure 6 A flashlight. More than half of the light from a flashlight bulb
located at the focal point F of the paraboloid is reflected as a parallel beam.
Images formed by parabolic mirrors
Parabolic mirrors form two types of images of objects: real and virtual images.
If the object is placed on the axis of revolution and farther from
the surface of the mirror than the focal point, image formed are the real image.
Real images can be made visible by placing a screen on the axis of revolution.
To see the object clearly you have to move the screen slowly from the surface of the mirror
until sharp image is seen. Real images are always upside down.
Figure 7 The real image, upside down, smaller than the object.
Figure 8 The real image, upside down, in original size.
Figure 9 The real image, upside down, bigger than the object.
If the object
is between the mirror and the focal point virtual image is formed. It
can be seen by looking at the mirror. Virtual image is magnified and is
not upside down. If the object is placed at the focal point no image
Figure 10 The virtual image, straight, bigger than the object.
Surface of water is a plain mirror ...