|
Light is a form of energy
that enables us to see the world around us. From the time we are born, we
learn to see and identify the physical world around us because light energy reflected from the object reaches the retina of our eye. In the absence
of any light, we will be unable to see anything like in a dark room. Sun
emits light energy. This light energy is a product of nuclear fusion of
elements that is happening inside the sun. Light is emitted in the form of
small packets called photons. Newton propounded the corpuscular theory of
light. Light sometimes also behaves like a wave (electromagnetic wave). This
is known as duality of light. Light photons or waves always travel in a
straight line, unless they are acted upon by a medium. For example, light
photons may bend due to extremely heavy mass such as a galaxy.
What
we will study in this chapter
:
1. Reflection from a plane surface
2. Reflection from a spherical surface
3. Lenses
4. Dispersion of light
1.
Reflection from a plane surface
If
you stand in front of a plane mirror, you will see an image of yourself.
This happens because light passes from you to the mirror and bounces back to
your eye. Mirror glass is
coated at the back with silver or some polished material, which is able to
reflect light. In the ensuing discussions, we will see how this happens.
Let XY be a plane mirror. A light ray is coming from point A and is striking the plane mirror
at point O. Point O is called as the point of incidence.
Ray AO is called the incident ray.
Let
NO be a line perpendicular to XY. Line NO is called the normal (to the
mirror).
The
angle made by AO with NO is called the angle of incidence.
 AON
= angle of incidence =
i
The
ray AO will be reflected back from the plane mirror XY along the direction OB.
OB
is called the reflected ray and the angle made by OB
with NO is called the reflected angle.
NOB
= angle of reflection = r
The
following are the laws of reflection
1.
Angle of incidence is equal to the angle of reflection. i
= r
2.
A ray traveling along the normal NO is reflected back along NO
itself.
3.
The incident ray, the reflected ray and the normal lie in the same
plane (or are co-planar).
4.
Image is as far back from the
mirror as the object is in front of the mirror.
5.
If a plane mirror is rotated by angle 
then the for a given incident ray AO the reflected ray is rotated by 2.
The
first four laws of reflection are easy to prove.
Take a plane mirror, a sheet of paper and four pins. On the paper draw a line XY and place the mirror along it. Draw a perpendicular to this line as NO. XY represents the plane
mirror.
Now draw a
line AO which is making a reasonable angle of incidence
i.
Measurei.
Now
place two pins P1 and P2 along AO. Keep the plane
mirror edge along XY with its glass facing P1 and P2.
Keep your eye on the level of the paper and see the reflection of P1
and P2 in the mirror from the other side of the normal. Place pins P3 and P4
in a straight line where you see the reflection of P1 and P2
respectively. Remove the mirror and draw a line through P3 and P4.
Let this line be OB. Measure NOB.
This is the angle of reflection r.
You will observe that i =
r.
Do
the same activity by changing angle subtended by AO with the normal NO. You
will obtain the same result.
To
prove the second law of reflection, keep pins P1 and P2 along
NO and observe the result. You will notice that the reflection of the pins
is along the same line.
The
third law of reflection is easy to understand from the activity you have
just done. The incident ray, the normal and the reflected ray lie in the
same plane. Try placing your eye a little above the paper and see if you are
able to put pins P3 and P4 in any plane other than the
plane of the paper. You will not be able to do so. Thus the incident ray, the normal and the reflected rays
are co-planar.
To
verify the fourth law of reflection, measure the perpendicular distance
between P1 and XY. Measure the perpendicular distance between P3
and XY. You will observe that the distances are equal. This means that
the image is formed at the same distance behind the mirror.
To
understand the fifth law of reflection, see the figure given below.
Rotate
the mirror XY through .
The normal NO also is rotated through the same angle. Let the new normal be
denoted by N’O.
The incident ray AO was making an angle i
with respect to normal NO. After the mirror is rotated, the incident ray
makes an angle (i
+)
with respect to the new normal N’O.
Thus
the angle of incidence now is (i
+).
The reflected ray OB’ is making an angle (r
+)
with N’O.
Since
i
= r,
the angle made by OB’ with N’O is (i
+).
With the original normal NO, the reflected ray OB’ makes an (i
+)+
= (i
+2).
,
the reflected ray gets rotated by 2.
Points
to be noted about images formed by a plane mirror
:
1.
The image formed is always erect (in spherical mirrors that we will
study later, the images formed are sometimes inverted).
2.
The image size is the same as the size of the object (in spherical
mirrors that we will study later, the images formed are sometimes magnified
or reduced).
3.
The image is laterally inverted
(stand in front of the mirror and raise your right hand. Your image, it will
appear to you is raising its left hand!!). Right-left symmetry is inverted in
case of reflection from a plane mirror.
4
The distance of the image from the mirror is the same as the distance
of the object from the mirror.
5. The image is virtual (there are two types of reflected images : real and virtual. A real image is when
the object and the image is on the same side of the mirror. A real image can be captured on a screen. A virtual image is when the object and the mirror are on opposite side of the mirror. In case of spherical mirrors, depending on the situations, we can get either real or virtual image. For plane mirrors, the image is always virtual).
|
