BAL BHARATI PUBLIC SCHOOL, PITAMPURA, DELHI
CLASS 8
LIGHT
NOTES
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TERM –II (2013-2014)
The ray of light which falls on the mirror surface is called incident ray.
The point at which the incident ray strikes the mirror is called the point of incidence.
The ray of light which is sent back by the mirror is called the reflected ray.
The ‘normal’ is a line drawn at right angles to the mirror surface at the point of
incidence.
The angle between incident ray and normal is called the angle of incidence.
The angle between reflected ray and normal is called the angle of reflection.
Laws of reflection of light:
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The incident ray, the reflected ray, and the normal {at the point of incidence}, all lie in
the same plane.
The angle of reflection is always equal to the angle of incidence.
Regular Reflection and Diffuse Reflection of Light
Regular Reflection
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In regular reflection, a parallel beam of incident light is reflected as a parallel beam
in one direction.
Regular reflection of light occurs from smooth surfaces like that of a plain mirror.
Images are formed by regular reflection of light.
Diffused Reflection
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In Diffuse reflection, a parallel beam of incident light is reflected in different
directions.
The diffuse reflection of light takes place from rough surfaces
A sheet of paper produces diffuse reflection of light. No image is formed in diffuse
reflection of light.
Characteristics of image formed by a plane mirror
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The image formed by a plane mirror is virtual & Erect.
The image formed in a plane mirror is the same distance behind the mirror as the object
is in front of it.
The image formed in a plane mirror is of the same size as the object.
The image in a plane mirror is laterally inverted.
Periscope
A Periscope is a device through which a person can se the objects that are out of the direct
line of sight. For example, by using a periscope, we can see the objects on the other side of a
high wall which cannot be seen by us directly. Periscope has two plane mirrors arranged
parallel to one another. Each plane mirror, however, makes an angle of 45 0 with the side of
the tube.
Some of the uses of periscopes are given below:
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Periscope is used to see over the heads of a crowd
A Periscope is used by soldiers sitting in bunker to observe the enemy
activities outside {over the ground}.
A Periscope is used by a navy officer sitting in a submarine to see ships over
the surface of water in the sea.
Multiple Images
When two plane mirrors are kept inclined at an angle, they can form
multiple images of an object. If two plane mirrors are inclined at an angle
x, then the number of images formed in them is given by the formula:
No. of images formed = (360 0 / x ) - 1
Kaleidoscope
The kaleidoscope is an instrument which produces multiple reflections of
coloured glass pieces {or coloured plastic pieces} and creates beautiful
patterns.
Construction
The Kaleidoscope consists of three long and narrow strips of plane mirrors inclined at 60 0 to
one another forming a hollow prism, and fitted into a cardboard tube. One end of the
cardboard tube is closed by an opaque disc {cardboard disc} having a small hole at its centre.
The other end of cardboard tube is closed with circular discs of glass. The inner disc being of
transparent glass {clear glass } and the outer disc of ground glass {translucent glass}. A
number of small pieces of different coloured glass {or plastic } and having different shapes
are kept between the two glass discs {which can move around freely in the space between
the two glass discs}.
When we hold the kaleidoscope tube towards light and look inside it through the small hole,
we see beautiful patterns of coloured glass.
Patterns formed by Kaleidoscope:
Dispersion of light
The splitting up of white light into seven colours on passing through a transparent medium
like a glass prism is called dispersion of light. The seven colours of the spectrum of white
light are: violet, indigo, blue, green, yellow, orange and Red.
The Human Eye
The main parts of the human eye are : Cornea, Iris, Pupil, Ciliary muscles, Eye lens
{which is a flexible convex lens}, Retina and Optic nerve.
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Cornea is made of a transparent substance
The light coming from an object enters into the eye through cornea. Function of
cornea is to protect the eye.
Iris is the colored part of the eye. The iris has a hole at its centre which is called
pupil. Pupil appears like a dark spot in the centre of iris because no light is reflected
from it. Iris controls the amount of light entering the eye by adjusting the size of
pupil.
The eye-lens is a convex lens made of a transparent and flexible material.
The eye-lens is held in position by ciliary muscles. The function of ciliary muscle is to
hold the lens & changes its size according to the need.
The retina is a screen on which the image is formed in the eye.
The optic nerve carries the image formed on retina to the brain in the form of
electrical signals.
Rods and cones
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Rods are the rod-shaped cells present in the retina of an eye which are sensitive
to dim light.
Cones are the cone-shaped cells present in the retina of an eye which are
sensitive to bright light. Cones also cause the sensation of colour of objects in our
eyes.
Blind spot
At the junction of optic nerve and retina in the eye, there are no light sensitive cells {no rods or
cones }due to which no vision is possible at spot. This is called blind spot.
Persistence of vision
The image of an object seen by our eyes persists {or remains}on the retina for about 1/16th of a
second even after the object has disappeared from our view. The abilily of an eye to continue to
see the image of an object for a very shot duration even after the object has disappeared from
view, is called persistence of vision.
Defects of the Eye:
a) Myopia (short-sightedness) : In this defect the person is able to see the near by objects but
not able to see the far-off objects.
This defect can be corrected by using spectacles containing Concave lens.
b) Hypermetropia (Far-sightedness) : In this defect the person is able to see the far-off objects
but not able to see the near by objects.
This defect can be corrected by using spectacles containing Convex lens.
c) Cataract : Cataract develops when the eye lens of person becomes cloudy (or even opaque)
due to the formation of membrane over it. Cataract decreases the vision of the eye gradually. It
can even lead to total loss of vision of the eye.
The opaque lens is removed from the eye of the person by surgical operation & a new artificial
lens is inserted in its place.
Points to remember
1.)
There can be only one reflected ray for a given single incident ray falling on a plane
mirror.
2.)
A ray of light which is incident normally on a mirror is reflected back along the same
path because the angle of incidence for such a ray of light is 0o the angle of reflection is also 0o.
3.)
Diffuse reflection is caused by the roughness in the reflecting surface of an object.
4.)
Laws of reflection are valid for both regular reflection & irregular reflection.
5.)
If a person is standing at a distance of 1 metre in front of a plane mirror, then distance
between the person & his image will be 2 metres.
6.)
In an image formed by plane mirror, the left side of the object appears on the right side
in the image whereas the right side of the object appears on the left side in the image. This
change of sides of an object & its mirror image is called Lateral inversion.
7.)
As the angle between the two plane mirrors decreases, the number of images formed
increases.
When the angle between the two plane mirrors becomes 0o ie. When the two mirrors are
parallel to each other then infinite number of images is formed.
8.)
An interesting feature of a kaleidoscope is that we can never see the same patterns
again. Every time a new pattern is formed. Kaleidoscope are used by designers of wall papers &
fabrics, as well as by artists to get ideas for new patterns.
9.) Rainbow in the sky is natural phenomenon showing the dispersion of sunlight. Rainbow is
produced by the dispersion of sunlight by the tiny rain drops suspended in the atmosphere
(which act as tiny prisms made of water).
10.)
The adjustment of the size of pupil takes some time. That’ when we go from a bright
light to a darkened cinema hall, at first we cannot see our surroundings clearly.
11.)
The range of vision of a normal human eye is from infinity to about 25 centimetres.
aving a small hole at its ce
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