Image formed by a plane mirror is
always virtual and erect. The size of the image is equal to that of the object.
The image formed is as far behind the mirror as the object is in front of it
We can see
objects Only because object reflects light that falls on it.
This reflected
light, when received by our eyes, enables us to see things.
We are able to
see through a transparent medium as light is transmitted
through it.
Because of
Light there occurs image formation by mirrors, the twinkling
of stars, the beautiful colours of a
rainbow, bending of light by a medium etc.
light seems to travel in straight
lines as it gets stopped by opaque objects. That’s the reason behind formation
of shadows.
opaque object
on the path of light becomes very small, light has a tendency to
bend around it
and not walk in a straight line – an effect known as the diffraction of
light. light often behaves somewhat
like a stream of particles
New
Quantum theory reconciles the particle properties of light with the wave
nature. quantum is latin word which means Quantity
is
the minimum amount of any physical entity (physical property) involved in an
interaction.
REFLECTION OF LIGHT
highly polished
surface, such as a mirror, reflects most of the light
falling on it.
Laws of Reflection
(i) The angle
of incidence is equal to the angle of reflection, and
(ii) The
incident ray, the normal to the mirror at the point of incidence
and the reflected ray, all lie in
the same plane.
Activity
10.1
􀂄 Take a large shining
spoon. Try to view your face in its curved
surface.
􀂄 Do you get the image? Is
it smaller or larger?
􀂄 Move the spoon slowly
away from your face. Observe the image.
How
does it change?
􀂄 Reverse the spoon and
repeat the Activity. How does the image
look
like now?
􀂄 Compare
the characteristics of the image on the two surfaces.
SPHERICAL MIRRORS
spherical mirror, whose reflecting
surface is curved inwards is called a concave mirror (cave/cavity like shape).
spherical mirror whose reflecting surface is curved outwards, is called a convex mirror
centre of the reflecting surface of a spherical mirror is a point called the pole. It lies on the surface of the mirror
This sphere has a center called
center of curvature C.
centre of
curvature is not a part of the mirror. It lies outside its reflecting
surface. The
centre of curvature of a concave mirror lies in front of it.
However, it lies behind the mirror
in case of a convex mirror.
radius of the sphere of which the reflecting surface of a spherical mirror forms a part, is called the radius of curvature of the mirror R.
light from the Sun is converged at a point, as a sharp, bright spot by the mirror. In fact, this spot of light is the image of the Sun on the sheet of paper. This point is the focus of the concave mirror. The heat produced due to the concentration of sunlight ignites the paper.
Distance of image from the position
of the mirror gives the approximate value of focal length of the mirror. principal
focus of a spherical mirror lies midway between the pole and centre of
curvature R = 2f
intersection of at least two
reflected rays give the position of image, Any two of the following rays can be
considered for locating the image.
1.
A
ray parallel to the principal axis, after reflection, will pass through the
principal focus in case ofa concave mirror or appear to diverge from the
principal focus in case of a convex mirror
2.
A
ray passing through the principal focus of a concave mirror or a ray which is directed
towards the principal focus of a convex mirror, after reflection, will emerge
parallel to the principal axis.
3.
A
ray passing through the centre of curvature of a concave mirror or directed in
the direction of the centre of curvature of a convex mirror, after reflection,
is reflected back along the same path.
4.
A
ray incident obliquely to the principal axis, towards a point P (pole of the
mirror), on the concave mirror or a convex mirror, is reflected obliquely. The incident
and reflected rays follow the laws of reflection at the point of incidence (point
P), making equal angles with the principal axis.
Concave mirrors are commonly used in
torches, search-lights and vehicles headlights to get powerful parallel beams
of light. They are often used as shaving mirrors to see a larger image of the
face. The dentists use concave mirrors to see large images of the teeth of
patients. Large concave mirrors are used to concentrate sunlight to produce heat
in solar furnaces.
We can see a full-length image of a
tall building/tree in a small convex mirror. used as rear-view (wing), fitted
on the sides of the vehicle Convex mirrors are preferred because they always
give an erect, though
Questions
1. Define the principal focus of a
concave mirror.
Answer-
Light rays that are parallel to the
principal axis of a concave mirror converge at a specific point on its
principal
axis after reflecting from the
mirror. This point is called the principal focus of the concave mirror.
2. The radius of curvature of a
spherical mirror is 20 cm. What is its focal length?
Answer-
Radius of curvature (R) = 20 cm
Radius of curvature of the spherical
mirror = 2 × Focal length (f) R = 2f
f= R/2 = 20 / 2 = 10
Therefore, the focal length of the
spherical mirror is 10 cm.
3. Name the mirror that can give an
erect and enlarged image of an object.
Answer-
The mirror that can give an erect
and enlarged image of an object is Concave Mirror.
4. Why do we prefer a convex mirror
as a rear-view mirror in vehicles?
Answer-
Convex mirror is preferred as a
rear-view mirror in cars and vehicles as it gives a wider field of view, which
helps the driver to see most of the traffic behind him. Convex mirrors always
form an erect, virtual, and diminished image of the objects placed in front of
it.
The conventions are as follows –
(i) The object is always placed to
the left of the mirror. This implies
that the light from the object falls
on the mirror from the left-hand
side.
(ii) All distances parallel to the
principal axis are measured from the
pole of the mirror.
(iii) All the distances measured to
the right of the origin (along
+ x-axis) are taken as positive
while those measured to the left of
the origin (along – x-axis) are
taken as negative.
(iv) Distances measured
perpendicular to and above the principal axis
(along + y-axis) are taken as
positive.
(v) Distances measured perpendicular
to and below the principal axis
(along –y-axis) are taken as
negative.
Mirror Formula
distance of the object from its pole is called the object distance (u)
distance of the image from the pole
of the mirror is called the image distance (v)
distance of the principal focus from
the pole is called the focal length (f).
Magnification
h is the height of the object and h′
is the height of the image object distance (u) and
image distance (v)
A negative sign in the value of the magnification indicates that the image is real. A positive sign in the value of the magnification indicates that the image is virtual
QUESTION
1. Find the focal length of a convex
mirror whose radius of curvature is 32 cm.
Answer-
Radius of curvature (R) = 32 cm
Radius of curvature = 2 × Focal
length (f)
R= 2ff = R/2 = 32/2 = 16
Therefore, the focal length of the
given convex mirror is 16 cm.
2. A concave mirror produces three
times magnified (enlarged) real image of object placed at 10 cm in
front of it. Where is the image
located?
Answer-
Magnification produced by a
spherical mirror:
M=height of the image/ height of the
object = - image distance/ object
distance
M= h1/ h0 = - u/v
Let the height of the object h0= h
Then height of the image h1= -3h(image
formed in real)
-3h/h=-v/u =3
Object distance (u) = - 10 cm
v = 3 × (- 10) = - 30 cm
Therefore, the negative sign
indicates that an inverted image is formed in front of the given concave mirror
at a
distance of 30 cm.
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