GEOMETRICAL
OPTICS
|
in this
article ·
Introduction. ·
Reflection of
light and its types. ·
Spherical
mirrors. ·
Refraction of
light and its laws. |
Introduction
Light is the main focus of this unit.
We shall describe phenomena of light such as reflection, refraction and total
internal refraction. We will learn hoe images are formed by mirrors and lenses
and will discuss working principle of compound microscope and telescope.
Reflection
of light
Reflection of light is illustrated
in this topic. When a light ray from air along the path .the coming ray is
called incident ray. The ray formed between the normal and incident ray is
called angle of reflected angle and the angle between the reflected ray and
normal is called reflected ray or angle of reflection.
When light travel in a certain medium
falls on the surface of another medium, a part of it turns back in the same
medium. This is called reflection of light.
Laws of reflection of light
·
The incident ray, the normal, and the
reflected ray at the same point of incident all lies at the same plane.
·
The angle of incident is equal to the
angle of reflection i.e. I=r.
Types of reflection
The nature of reflection depends on the
smoothness of the surface. For example, a smooth surface of silver reflects
parallel rays of light in one direction only. The reflection by this smooth
surface is called regular reflection. Most of the object in every life is not
smooth on microscopic level. The rough surface of these object reflect the rays
of light in many directions. Such type of reflection is called irregular
reflection.
Spherical mirrors
A mirror whose polished, reflecting
surface is a part of a hollow sphere of plastic is called a spherical mirror.
In a spherical mirror, one of the two curved surfaces is coated with a thin
layer of silver followed by a coating of red lead oxide paint. Thus, one side
of the spherical mirror is opaque and the other side is a highly polished
reflecting surface. Depending upon the nature of reflecting surface, there are
two types of spherical mirror as shown below.
· Concave
mirror: a spherical mirror whose inner curved surface is reflecting is called
concave mirror. In concave mirror the size of object depend on the position
real image. Both virtual and real images can be formed by concave mirror.
· Convex
mirror: a spherical mirror whose outer curved surface is reflecting is called
convex mirror. In convex mirror the size of the image is always smaller then
they placed object. Only virtual image is formed by the convex mirror.
· Pole:
it is the midpoint of curved surface of spherical mirror. It is also called
vertex.
· Centre
of curvature: a spherical mirror is a part of a sphere. The centre of this
sphere is called centre of curvature.
· Radius
of curvature: it is the radius of the sphere of which spherical mirror is the
part.
· Principle
axis: it is the line joining centre of curvature and pole of the spherical
mirror.
· Focal
length: it is the distance from the pole to the principle focus measured along
the principle axis. This mean that as the radius of curvature is reduced, so
too is the focal length of the reflecting surface.
Refraction of light
The bending of light when it passes from
denser to rarer medium is called refraction of light.
Law of reflection of light.
· The
incident ray, the refracted ray, and the normal at the point of incidence all
lies in the same plane.
· The
ratio of the sine of the angle of incident to the sine of angle of reflection
is always equal to a constant. I.e. sin/sin r=constant=n.
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