Light
Name: Mohammed Ashfan Ahmed Mallick
Class: VII
Section: A
Subject Teacher: Anita Singh

Light And How Does It Travels In A Straight Line
• Light is a naturally occurring substance that enhances vision and makes objects visible. Sun is the natural source of light.
Travelling Of Light In A Straight Line:
 The path of light gets blocked when an object covers the source of light.
 This is illustrated by trying to see the flame of a candle using a straight pipe and then with a bent pipe. This proves that
light travels in a straight line. Travelling of light in a straight line, is also known as Rectilinear Propagation Of Light
Based On Their Interaction With Light, Objects Can Be Classified As:
• Transparent objects allow light to pass through them completely
Example: Glass
• Translucent objects allow light to pass through them partially.
Example: Butter Paper
• Opaque objects do not allow any light to pass through them.
Example: Book

Reflection And
Its Laws
• When light is incident on a certain surface, it either gets reflected
or bounces back. Such surfaces are well polished and act like a mirror.
The phenomenon of light bouncing off surfaces is called reflection.
Laws
• The ray of light that strikes a reflecting surface is called as the incident
ray and the ray that gets reflected back is called the reflected ray
• The imaginary line that is perpendicular to the reflecting surface at the
point of incidence is called the normal
• Angle of incidence ∠i is the angle between the incident ray and the
normal. Angle of reflection ∠r is the angle between the reflected ray and
the normal.
• Law of Reflection states that the Angle Of Incidence {∠i }, is always equal
to the Angle Of Reflection {∠r }(∠i = ∠r).
• The Angle Of Incidence {∠i } and Angle Of Reflection the normal all lie on
the same plane.

Activity { For Reflection } And Images
• ∠AOX is called the angle of
incidence.
• ∠BOX is called the angle of
reflection.
• ∠MON is called the normal.
We Can Say That The
Reflected Ray Leaves The
Mirror At The Same Angle At
Which Incident Ray Strikes It.
• Rays of light from an object
after reflection by a mirror
tend to converge or appear
to diverge from a certain
point in front of the mirror.
This copy of the object is
called an image.

Real And Virtue Image
• A real image is formed by the actual
crossing of light rays after reflection. Real
images can be viewed on a screen.
• A virtual image is the plain crossing of
diverging light rays after reflection. Virtual
images cannot be viewed on a screen.
• The image formed by a plane mirror is
erect, virtual and is of the same size as the
object. The image is at the same distance
behind the mirror as the object is in front
of it.

How We See Objects? – Case I
As light from an object enters our eyes, its real image is formed on the screen of eye or The Retina.
This image {Figure A } is smaller than the object and is upside down!
The brain studies the image. It knows that light travels in straight lines, and is, therefore, able to
examine the direction., calculate the distance and the size of the object.
We, therefore, see the object in its right direction, correct size and right distance and the right side
up.

How We See
Objects? – Case II
• The ray of light from an Object A
is somehow bent at Point O and
then made to fall on our eyes.
• The brain cannot detect the that
the light has been bent or not,
Since, it knows that light travels
in a straight lines it thinks that
the light is coming from B.
• We therefore see the object at B,
although there is nothing really
at B. B is the virtual image of A.
• It always seems that the light is
coming from the virtual image.

Properties Of Image Formed By A Plane Mirror
• Properties of image formed by a plane mirror are as follows:
1. They are virtual.
2. They are erect.
3. They are of the same size as the object.
4. They appear to be as much behind the mirror as the object is in the
front of it.
5. They are laterally inverted.

Lateral
Inversion
• An image formed by a plane
mirror undergoes lateral
inversion, i.e., the right side of
the object appears as the left
side in the image.
• That is why the word
AMBULANCE is written
backwards on ambulance vans
so that it appears in the correct
order when viewed on a mirror.

Spherical
Mirror
• A spherical mirror (or curved
mirror) is a mirror which has the
shape of a piece cut out of a spherical
surface. They are of two types -
1. Concave Mirrors
2. Convex Mirrors
Concave Convex

Concave And
Convex Mirror
Concave Mirrors: If the outer
surface of the curved mirror is
painted and its inner surface
is a reflecting surface, then
this type of spherical mirror is
a concave mirror.
Convex Mirrors: If the inner
surface of the curved mirror is
painted and its outer surface
is a reflecting surface, then
this type of spherical mirror is
a concave mirror.
Concave Convex

Terms Related
To Concave And
Convex Mirrors
• Pole{P}: The midpoint of the spherical
mirror.
• Center of Curvature{C}: The point in the
center of the mirror that passes through
the curve of the mirror and has the same
tangent and curvature at that point.
• Radius of Curvature{r}: It’s the linear
distance between pole and the center of
curvature.
• Principal Axis: The imaginary line
passing through the optical center and
the center of curvature of any lens or a
spherical mirror.
• Principal Focus{F}: Principal Focus can
also be called Focal Point. It’s on the axis
of a mirror or lens wherein rays of light
parallel to the axis converge or appear to
converge after reflection

Difference Between Plane, Concave And Convex Mirror
Nature Of Image Plain Mirror Concave Mirror Convex Mirror
Real Or Virtual Always Virtual a) Virtual When The Object Is
Between Pole And
Principal Focus.
b) Real For All Other Positions
Always Virtual
Erect Or Inverted Always Erect a) Erect When The Object Is
Between Pole And
Principal Focus.
b) Inverted For All Other
Positions.
Always Erect
Size Always Same As Object a) Magnified And Virtual
When The Object Between
Pole And Principal Focus.
b) Magnified And Real When
The Object Is Between Pole
And Center Of Curvature.
c) Diminished When Object Is
Beyond Center Of
Curvature.
Always Diminished
Distance From Mirror Always Same As Object Distance Of Real Image From
Mirror Reduces As The Object
Moves Further From Mirror
Always Between Principal Focus
And Pole.

Uses Of Concave Mirrors
Some concave mirror uses are listed in the points below.
• Shaving mirrors
• Head mirrors
• Doctors Ophthalmoscope – Used to see the magnified image
of the teeth.
• Astronomical telescopes
• Headlights
• Solar furnaces
• Concave mirrors reflect light inward to one focal light.
• Therefore, they are mostly used to focus light.
• A concave mirror shows different image types depending on
the distance between the mirror and the object. Concave
mirrors are called converging mirrors because as light falls
on the mirror, it collects the light and refocuses the parallel
incoming rays

Uses Of Convex
Mirrors
• Street Light Reflectors
• Side-view Mirrors
• Sunglass Lenses
• Car Repair

Lenses
• A transparent medium
bounded by two surfaces of
which at least one is
spherical is called a Lens.
• They are of two types:
1. Converging or Convex
Lenses.
2. Diverging or Concave
Lenses.

Diverging Or
Concave Lenses
• A concave lens is a type of lens with
at least one side curved inwards.
• A concave lens with both sides
curved inward is known as a
biconcave lens. Concave lenses are
diverging lenses, that is, they
spread out light rays that have been
refracted through it.
• They could diverge a parallel beam
of light.
• For a concave lens, the edges are
wider than the center or the center
is thinner than the edges.
• Concave lenses are used in
spectacles in order to overcome
myopia or short-sightedness.

Terms Related To Lenses
Principal Axis: The principal axis is an imaginary line
passing through the centers of curvature and the pole.
Optical Center: The point on the principal axis at the
center of the lens is called Optical center.
Focal Length: The focal length is the distance between the
optical center and the focal point or focus of the lens.

Images Formed By Lenses
Convex Lens forms a real, inverted and
diminished image. When the object is placed
very close the lens the image formed is
erect, virtual and magnified. They are also
called as converging lens. This type of lens is
also used as a magnifying glass.
Concave Lens always forms a virtual, erect
and diminished image. They are also called
as diverging lens. They are used as camera
lenses and binoculars.

The Spectrum
• The bang of colours seen when
white light is splitted into its
different colours, The pattern
formed by these colours is
known Spectrum.
• Isaac Newton was the first
scientist in the 17th century, who
showed that ordinary white light
is made up of seven colours.
• Violet, Indigo, Blue, Green,
Yellow , Orange ,and Red, And
Also Known As Vibgyor.

Dispersion Of White Light Through Prism
• Sunlight may appear as white but in fact,
is made up of seven component colours.
This can be observed when a beam of
sunlight is passed through a triangular
prism or when light falls on a compact
disk (CD).
• The seven colours that appear are violet,
indigo, blue, green, yellow, orange and
red. These are also the colours of a
rainbow we see after a rainfall.
• The splitting of white light into its
component colours is called dispersion.