RECTILINEAR PROPAGATION OF LIGHT
RECTILINEAR
PROPAGATION OF LIGHT
Sources of light
Light
is a form of energy called luminous energy. This energy cause sensation of
vision, enabling us to see. There are various sources of light, for example the
sun and the stars are the natural sources of light. Artificial sources of light
are the candle, electric torch, the electric lamp, incandescent and are lights
and fluorescent light.
Luminous And Non-Luminous Object.
Self-luminous
and Luminous sources of light are those that generate and emit light by
themselves. Examples are the sun, stars, fire-flies and some deep-sea fishes,
and the artificial light sources.
Non-luminous
bodies depend on natural and artificial sources to illuminate them. They are
seen when only they reflect the light from a luminous body. For example, light
from a car headlamp falling on a road sign in the night, causes the sign to
throw back part of this light into the eyes of the car-driver thereby enabling
the road sign to be seen.
Transparent, Translucent And
Opaque Objects.
Light
is an electromagnetic wave. It can pass through a vacuum and through a material
medium. If light shines on a body, part of the light is transmitted through the
body, the rest is reflected. The amount of light passing through a body depends
on the nature of the body. If a large percentage of light falling o a body
passes through it, the body is said to be transparent. Example of such bodies
are glass and water. Because light is easily transmitted through these
transparent bodies, we can see objects through them easily.
Some
objects like frosted glass and tissue paper allow some small amount of light to
pass through them. Such objects are called translucent objects. Because the
amount of light passing through translucent bodies in small, objects cannot be
seen clearly through them.
Some
other bodies do not allow any light to pass through them. These are called
opaque objects. Examples of opaque objects are wood, bricks, walls and metal sheets.
Types Of Beams: Convergences Divergence And
Parallel Beams.
There
are three types of beams
i.
A parallel beam: is one in which the
light rays are parallel to one another. Search lights give off parallel beam of
light.
ii.
A convergent beam: is one in which the
rays converge or meet at a point. A hand lens can be used to produce such a
beam.
iii.
A divergent beam: is one in which the
light rays all come from a point and spread out or diverge from the source.
Lamps produce a divergent beam of light.
Ray Box
Rays
of light are produced in the laboratory by means of a ray box. A simple
laboratory ray box consists of a box made of wood or cardboard inside which is
a source of light, e.g a candle or an electric lamp. On one face of the box is
a hole or a slit or several slits through which light rays can travel out of
the box.
Natural Effects Of Rectilinear Propagation Of
Light.
The
phenomenon of light travelling in straight lines is known as rectilinear
propagation of light. Two natural effects that result from the rectilinear
propagation of light are the formation of shadows and eclipses. The principle
of operation of the pin-hole also depend on the fact that light travels in a
straight lines.
Shadow
is an area in which light rays from a source cannot reach. A shadow is produced
by the obstruction of light by an opaque object. If the light source is large,
or extended the shadow formed consist of two parts, a complete dark area known
as the umbra and an outer light or grey area known as the Penumbra or partial
shadow. In the umbra region, the light from the source is completely blocked by
the opaque body. In the penumbra region the light is partially blocked by the
opaque object.
Eclipse Of Sun, Eclipse Of The Moon And
Annular Eclipse
An
eclipse is a result of a shadow cast by one heavenly body on another. An eclipse of the sun occurs when the Sun casts
the shadow of the Moon on Earth’s surface. An eclipse of the Moon occurs when
the Moon lies in the shadow of the Earth. Annular eclipse is when the Earth and
the Moon be in positions where the extreme rays and the Moon’s edge intersect
before reaching the Earth.
Pin –Hole Camera Magnification
Magnification
is defined as the ratio of the size (or height) of the image to the size (or
height) of the object.
Magnification,
M=Height of image/ height of object
M = lenght of camera / distance of object from pin-hole = size of image / size of object
Example
A
pin-hole camera 26cm long is used to photograph a building 11m high located 32m
from the camera. Calculate the height of the image on the film.
Solution
Height
of image=?, Height of object= 11m, object distance from camera = 32m, length of
camera= 26cm.
M= hi
Assignment
1.
An object is 6.5cm tall, and is 17.5cm in
front of a pin-hole camera that is 6.0 cm long. Calculate the height of the
image and the magnification [ans- 15 cm and 0.21]
2.
A pin-hole camera that is 5cm long
produces an images that is 3 times larger than the object height. Determine the
location of the object. [ans- 1.6 cm]
3.
If an object is 7.0cm tall, and it is 15.0
cm in front of a pinhole camera whose image distance is 6.0 cm, calculate the
height of the image. [ans- 2.8 cm]
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