mirrors

Light is the form of energy having both wave and particle nature. Speed of light in vacuum is 3 lakhs km/s
Reflection of Light by plane and Spherical Mirrors
Mirror is a object having one highly polished surface and other painted. Polished surface of mirror reflects most of the
light falling on it and form image.
The phenomenon of the bouncing back of light from polished surface is called reflection.
The laws of reflection of light:
(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.
These laws of reflection are applicable to all types of reflecting surfaces including spherical surfaces.
Mirror having plane polished surface is called plane mirror.
Magnification = hi/ho = +1
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 .The image is laterally inverted.
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The exchange of right and left portion of object into image formed by plane mirror is called lateral inversion.
Spherical Mirror: Mirror having curve reflecting surface are called Spherical Mirror. A spherical mirror can be made
from a hollow spherical ball of glass.
There are two types of spherical mirrors (i) Concave mirrors (ii) Convex mirrors
A concave mirror is a spherical mirror whose reflecting surface is curved inwards.
A convex mirror is a spherical mirror whose reflecting surface is curved outwards.
Compare the characteristics of the image on the two surfaces of spoon:
The inward surface of the steel bowl or a spoon acts as a concave mirror, while its outer surface acts as a convex
mirror.
The centre of the reflecting surface of a spherical mirror is a point called the pole. The pole is usually represented by
the letter P.
The centre of sphere of which spherical mirror is a part is called the centre of curvature of the spherical mirror. It is
represented by the letter C.
The straight line joining the pole (P) and the centre of curvature (C) is termed as the principal axis.
The distance between the centre of curvature and pole is known as the radius of curvature.
Focus The focus (F) is the point on the principal axis of a spherical mirror where all the incident rays parallel to the
principal axis meet (real) or appear to meet (virtual) after reflection.
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The different ways in which a ray of light is reflected from a spherical mirror are:
Case I: When the incident light ray is parallel to the principal axis. In this case, the reflected ray will pass through the
focus of a concave mirror, or it appears to pass through the focus of a convex mirror. (see above image)
Case II: When the incident light ray passes through the focus of a concave mirror, or appears to pass through the
focus of a convex mirror.
Case III: When the incident ray passes through or appears to pass through the centre of curvature. In this
case, light after reflecting from the spherical surface moves back in the same path. This happens because light is
incident perpendicularly on the mirror surface.
Case IV: When the incident ray is normal to the reflecting surface In this case, the incident light ray will be
reflected back by the reflecting surface of the spherical mirror, as in the case of plane mirror.
Ponder over It: Four spherical mirrors of radius of curvature R1, R2, R3, and R4 (R1 > R3 > R2 > R4) are placed
against the sunlight. Try to obtain the bright spot on a paper sheet for each mirror. Which mirror forms the brightest
spot at a maximum distance from the pole of the mirror? Explain.
For spherical mirrors the focal length is half the radius of curvature. Light parallel to the principal axis (rays from sun)
are converged at the focus. So, larger the radius of curvature larger the focal length, hence, the focus point is at a
greater distance from the pole.
We have, R1 > R3 > R2 > R4 ? R1 is the largest radius of curvature. So, mirror with radius of curvature R forms the
brightest spot at a maximum distance from the pole of the mirror.
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Activity to find focus of concave mirror:
Hold a concave mirror in your hand and direct its reflecting surface towards the Sun. Adjust the concave mirror in such
a way you find bright , sharp spot of light 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. The distance of this image from the position of the
mirror gives the approximate value of focal length of the mirror.
Q. What type of images formed when object moves from infinity to mirror?
Ans: The type of image formed by a concave mirror depends on the position of object in front of the mirror. There are
six positions of the object:
Case–1: Object is in between P and F : Image formed is :
(i) Behind the mirror (ii) virtual and erect and
(iii) larger than the object (or magnified)
Case–2: Object is at the focus(F): The image formed is
(i) at infinity (ii) real