Reflection of Light
• The phenomenon of bouncing back of light rays which fall on the surface of
an object into the same medium is called reflection of light.
• Laws of Reflection of light
1. The angle of incidence is equal to the angle of reflection.
2. The incident ray, the reflected ray and the normal to the mirror at
the point of incidence all lie in the same plane. (fig. refer textbook)
• These laws of reflection are applicable to all types of reflecting surfaces
including spherical surfaces
Real and Virtual images
• An image is formed when the light rays coming from an object meet at a
point or appear to meet after reflection or refraction.
• The images are of two types
REAL IMAGE VIRTUAL IMAGE
Real images can be obtained on a Virtual images can not be obtained
screen. on a screen.
Real images are formed by actual Virtual images are formed when
intersection of light rays. rays do not actually meet but
appear to meet.
Real images are inverted. Virtual images are erect.
Characteristics of images formed by Plane mirrors: -
• (i) Images formed by plane mirrors are always virtual and erect.
(ii) Size of the image is always equal to the size of the object.
(iii) The images formed by the plane mirror are as far behind the mirror as
the object is in front of the mirror.
(iv) The image is laterally inverted.
• Lateral inversion- If an object is placed in front of the mirror, then the right
side of the object appears to be the left side of the image and left side of
the object appears to be the right side of this image. This left right inversion
(change of sides) of an object and its mirror image is called lateral
inversion.
• Spherical mirrors
• Important terms
1. Centre of curvature - The reflecting surface of a spherical mirror
forms a part of a sphere. This sphere has a centre. This point is called
the centre of curvature of the spherical mirror. It is represented by
the letter C.
The centre of curvature is not a part of the mirror. It lies outside its
reflecting surface.
The center of curvature of a concave mirror lies in front of it whereas
it lies behind the mirror in case of a convex mirror.
2. Radius of curvature - The 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.
It is represented by the letter R.
3. Pole - The center of the reflecting surface of a spherical mirror is
called its pole. It is represented by letter P.
4. Principle axis - Straight line passing through the pole and the centre
of curvature of a spherical mirror is called principal axis of the mirror.
Principal axis is normal to the spherical mirror at its pole.
5. Aperture of the mirror - The effective diameter of the reflecting
surface of a spherical mirror is called aperture of the mirror.
Principle focus and focal length of a Spherical Mirror
• Rays parallel to the principal axis after reflection from a concave mirror
intersect at a point on the principal axis of the mirror. This point is called
the principal focus of the concave mirror.
• In case of a convex mirror rays parallel to the principal axis get reflected at
the reflecting surface of the mirror and these reflected rays appear to
diverge from a point on the principal axis. This point is called principle
focus of the convex mirror.
Principal focus of a spherical mirror is represented by F.
• Focal length - The distance between the pole and the principal focus of a
spherical mirror is called the focal length. It is represented by the letter f.
R = 2f
Rules for obtaining images formed by spherical mirrors
Rule 1
A ray of parallel to the principal axis, after reflection will pass through the
principle focus in case of a concave mirror or appear to diverge from the principle
focus in case of a convex mirror.
Rule 2
A ray passing through the principle 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.
Rule 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
This happens because the incident rays fall on the mirror along the normal to the
reflecting surface.
Rule 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.
Image formation by spherical mirrors
Image formation by concave mirror
• The nature, position and size of the image formed by a concave mirror
depend on the position of the object in relation to points P, F and C.
(Refer textbook for ray diagrams)
Uses of Concave mirrors
• Concave mirrors are used in torches, search lights and vehicle headlights to
get powerful parallel beams of light.
• Concave mirrors are 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
.
Image formation by a Convex mirror
• For all positions of object, a convex mirror always forms virtual, erect and
diminished image behind the mirror.
(refer textbook for ray diagrams)
Uses of Convex Mirrors
• Convex mirrors are used as rear-view mirrors in automobiles because
(i) they give an erect and diminished image for all object positions.
(ii) they provide a wider field view of traffic behind as they are curved
outwards.
• Convex mirrors are used in shopping malls to prevent shoplifting.
• These mirrors are used as reflectors in streetlights to illuminate a larger
area.
• Used in parking areas and near sharp/blind turns to see the traffic coming
from other side.
Sign convention for reflection by spherical mirrors
• 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.
• All distances parallel to the principal axis are measured from the pole of the
mirror.
• 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.
• Distances measured perpendicular to and above the principal axis (along +
y-axis) are taken as positive.
• Distances measured perpendicular to and below the principal axis (along -y-
axis) are taken as negative.
These new Cartesian sign convention for spherical mirrors are shown below in the
figure
Mirror formula
It gives the relationship between image distance (v) , object distance (u) and the
focal length (f) of the mirror
Magnification
Magnification produced by a spherical mirror gives the relative extent to which
the image of an object is magnified with respect to the object size. It is expressed
as the ratio of the height of the image to the height of the object.
Magnification is positive for virtual and erect image whereas it is negative for real
and inverted image.
