|
Common Misconception |
Fact |
|
Light travels
along any path. |
Light travels in a straight line. |
|
We cannot see
reflections of objects in water. |
The surface of the water (when still) can act like a mirror and change the path of light like a
reflecting surface. |
|
A mirror is
transparent. |
A mirror is
opaque. |
|
A wet surface
reflects more light than a dry surface. |
A wet surface
absorbs more light than a dry surface and hence looks darker. |
Light can be defined as a form of energy. It helps us to see
things. Photosynthesis is one of the processes by which plants prepare their own
food for their growth in the presence of sunlight. Light energy is an invisible
form of energy that causes the sensation of vision.
Source of light
An object that emits light is called a source of light. The
sun, the stars and the glowworm are some of the natural sources of light,
whereas, oil lamps, candles, and electric lights are examples of artificial sources
of light.
Objects that give off their own light are called luminous objects
and the objects that do not give off their own light are called the
non-luminous objects. The non-luminous objects can be seen due to the light
reflected by luminous objects.
Optical Medium
The term medium is used for any substance through which light
might pass.
Example Glass, water, air etc.,
Transparent medium
Substances that allow most of the
light to pass through them are called transparent substances. We can see through
them clearly. A transparent substance is called an optical medium.
Example
Glass, water, air etc.,
Translucent medium
Substances which allow only a part
of the light to pass through it and through which objects cannot be distinctly
seen.
Example
Butter paper, Paraffin wax, greased
paper, ground glass etc.,
Opaque medium
The opaque object does not allow any light to pass through it. They reflect or absorb all the light that
falls on them. We cannot see through them.
Example
Wood, stone, rubber, walls, books
etc.,
An optical medium that has different
compositions at different points is called a heterogeneous medium.
Example
Air, muddy water, fog, mist, smoke of
clouds etc.,
A point source of light
If a screen with a pinhole is placed in front of a lighted
bulb then a point source of light is obtained. From the point source, light
propagates uniformly in all directions.
A ray
Light from a point source spreads out in all directions. The directions
of a very narrow path of light are known as a ray of light.
A ray of light is represented by a straight line with an
arrowhead. The arrowhead on it gives the direction of light.
Beam of Light
A collection of rays of light is called a beam of light. A
beam of light is classified into three kinds viz. parallel beam of light,
convergent beam of light, and divergent beam of light.
Parallel beam of light
These rays of light are parallel to each other such
that the distance between the rays remains the same. Light from distant objects
gives a parallel beam of light.
A convergent beam of light
In a convergent beam of light, the light rays meet at
a point.
Divergent beam of light
In a divergent beam of light, the light rays spread
out from a point.
Reflection
The phenomenon by which incident light energy is
partly or completely sent back into the same medium from which it is coming
after being obstructed by a surface is called reflection.
Some terms related to
reflection
Point of incidence
The point at which the incident ray strikes the
reflecting surface is called the point of incidence.
Incident ray
A ray of light that falls on the reflecting surface
is called an incident ray.
Reflected ray
The light ray obtained after reflection from the
surface in the same medium in which the incident ray is travelling is called
the reflected ray.
Normal
The perpendicular drawn to the surface at the point of
incidence is called the normal.
Angle of incidence
The angle that the incident ray makes with the normal
at the point of incidence is called the angle of reflection. It is denoted by the letter i.
Angle of reflection
The angle at which the reflected ray makes the normal at
the point of incidence is called the angle of reflection. It is denoted by the letter r.
Glancing angle of incidence
The angle between the incident ray and the reflecting
surface is called the ‘glancing angle of incidence’. It is denoted by the letter gi.
Glancing angle of reflection
The angle between the reflecting surface and the
reflected ray is called the ‘glancing angle of reflection’. It is denoted by gr.
Plane of incidence
The plane containing the incident ray and the normal
is called the plane of incidence.
Plane of reflection
The plane containing the reflected
ray the normal is called the plane of reflection.
Laws of Reflection
a)
The
angle of incidence ‘I’ is equal to the equal of reflection ‘r'.
b)
The
incident ray, the reflected ray and the normal to the surface at the point of incidence all lie on the same plane.
Types of Reflection
There are two types of reflection: Regular (or specular) and
Diffuse (or irregular) reflections. In regular reflection, the light is
reflected in one direction only. This occurs on smooth surfaces such as
mirrors. In irregular reflection, light is reflected through a range of
different angles. This occurs on rough surfaces such as paper, corrugated glass
or a painted wall. The surface of an ordinary paper may appear smooth, but if
seen under a microscope, it will appear rough with many small protrusions.
Characteristics of the
image formed in a plane mirror
a)
The
image is virtual, that is, it cannot be captured on a screen.
b)
The
image is as far behind the mirror as the object is in front of the mirror.
c)
The
image is upright.
d)
The
image is of the same size as the object.
e)
The
image is laterally inverted i.e., left to right inversion.
Applications of Mirrors
a)
Two
plane mirrors inclined at 450 are used in Periscopes, which enable a
person to look over high walls and other obstacles.
b)
Plane
mirrors are sometimes used in optical testing, where a person can read the
inverted letters by seeing their images in a mirror. This helps in economizing
the space in the testing room.
c)
Used
in blind corners of the roads to help drivers see around these corners before
making a turn.
d)
Used
in certain meters below the pointer helps us take proper readings without
parallax error.
e)
Plane
mirrors are also used in overhead projectors and telescopes.
Number of images formed on the inclination of two plane mirrors
The number of images of a point
object placed between two plane mirrors inclined at an angle θ,
N = (3600/θ) – 1
If 3600/θ is an even integer;
N = (3600/θ) – 1,
If 3600/θ is an odd integer and the object lies on the
bisector of the angle θ then n = 3600/θ
When two plane mirrors are kept
facing each other at an angle θ and an object is placed between
them, multiple images of the object are formed as a result of multiple
successive reflections.
When the two plane mirrors are placed
parallel to each other, then
N = (360/zero) = (infinite number of images)
Reviewed by knowledge people creators
on
November 21, 2021
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