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Light
Incident ray :The light ray, which strikes any surface, is called the incident ray
Reflected ray: The ray that comes back from the surface after reflection is known as the
reflected ray.
Point of incidence: The point where the light ray strikes is called point of incidence
Normal: The line which is perpendicular to the surface at point of incidence is called the
normal to the surface at the point of incidence.
Angle of incidence(∟i) : The angle between the normal and the incidence ray is known as
the angle of reflection.
Angle of reflection(∟r): The angle between the normal and the reflected ray is known as
the angle of reflection.
Laws of reflection of light:

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.
Lateral inversion: An image formed by a mirror the left of the object appears on the right and the
right appears on the left. This is known as lateral inversion
Regular Reflection and Diffuse Reflection of Light
Regular Reflection:
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.
Diffuse reflection:
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.
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Characteristics of image formed by a plane mirror
 It always form virtual, erect image.
 Size of image is equal to size of object
 Image and object are equidistant from the mirror.
 Image is laterally inverted.
Uses of plane mirrors
We commonly use plane mirrors in our homes to look at our reflection. In beauty parlour, plane
mirrors kept at an angle are used to view the side of the head. Plane mirrors parallel to each
other ae used to view the back of the head.
Plane mirrors are used to reflect light on an object. For example, during outdoor shooting of film
metal sheets are used as plane mirrors to reflect sunlight on the actors.
They are also used in solar cookers to reflect light on the food being cooked.
They are used in periscopes. From a submarine under the sea, a sailor can see objects and
enemy ships on the surface of the sea by using a periscope.
 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
450 with the side of the tube.
 Some of the uses of periscopes are given below:
  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.
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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 , then the
number of images formed in them is given by the formula:
No. of images formed = (3600/ ) - 1
Kaleidoscope : The kaleidoscope is an instrument which produce multiple reflections of coloured
glass pieces {or coloured plastic pieces} and create beautiful patterns.
Construction: The Kaleidoscope consists of three long and narrow strips of
plane mirrors inclined at 600 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.
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.
A human eye: The human eye is like a camera. Its lens system forms an image on alight
sensitive screen called the retina. The eye ball is approx. spherical in shape with a diameter of 2.3cm.
The human eye has the following parts:(a) Cornea:-The transparent spherical membrane covering the front of the eye.
(b) Iris:-The coloured diaphragm between the cornea and lens.
(c) Pupil:-The small hole in the iris.
(d)Eye lens:-Its is a transparent lens made of jelly like material.
(e) Ciliary muscles:-These muscles hold the lens in position.
(f) Retina:-The back surface of the eye.
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(g)Blind spot:-The point at which the optic nerve leaves the eye. An image formed at this point is not
sent to the brain.
(h) Aqueous humour:-A clear liquid region between the cornea and the lens.
(i) Vitreous humour:-The space between eye lens and retina is is filled with another liquid called
Vitreous humour.
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 ability 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.
The visually- impaired:
There are an estimate 180 million visually impaired people in the world while the number in india is
around 7,00,000. Some of them are born with defects and others lose their eyesight because of
diseases such as cataract. Since visually impaired people depend a lot on the senses of felling and
hearing, they often develop a very keen sense of tough and hearing.
Use of Braille: The Braille system is a method of writing for the blind. It employs groups of raised
dots to represent printed letters and numbers. It was invented by Louis Braille of France in the early
1800s. It is not a separate language, but just another way to read and write English or any other
language.
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