Ray Optics Class 12 Class Notes for CBSE boards preparation
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Course
Physics
Institution
Sixth Year / 12th Grade
These are the easy and effective notes for someone who wants to prepare for chapter RAY OPTICS for Class 12 CBSE boards. These notes are generated by expert faculties.
Credit- VEDANTU
1. RECTILINEAR PROPAGATION OF LIGHT As shown in the figure, the angle between reflected ray and
incident ray is180 – 2i where i is the angle of incidence. Maximum
It is a well established fact that light is a wave. Although, a light deviation is 180°, when angle of incident i is zero.
wave spreads as it moves away from its source, we can approximate
its path as a straight line. Under this approximation, we show light 2.3 Law of Reflection in Vector Form
as a ray and the study of light as a ray is called ray optics or
geometrical optics. Say unit vector along incident ray = û .
1.1 Ray Unit vector along normal = n̂
The straight line path along which light travels in a homogeneous Unit vector along reflected ray = r̂
medium is called a ray. Then r̂ û 2 û . n̂ n̂
2. REFLECTION OF LIGHT
The phenomenon in which a light ray is sent back into the same
medium from which it is coming, on interaction with a boundary,
is called reflection. The boundary can be a rigid surface or just an
interface between two media.
2.1 Law of Reflection Laws of reflection remain the same whether the reflected surface is
plane or curved.
We have few angles to define before considering law of reflection
(i) Angle of incidence : The angle which the incident ray
makes with normal at the point of incidence.
(ii) Angle of reflection : The angle which the reflected ray
makes with normal at the point of incidence.
A reflected ray lies in the plane of incidence and has an angle of
reflection equal to the angle of incidence. i = r.
2.2 Deviation
2.4 Reflection by a plane surface
When a ray of light suffers reflection, its path is changed. The
angle between its direction after reflection and the direction before Suppose a reflecting surface is rotated by an angle T (say
reflection is called the deviation. anticlockwise), keeping the incident ray fixed then the reflect ray
rotates by 2T along the same sense, i.e., anticlockwise.
, RAY OPTICS
Magnification of a plane mirror is unity.
The image is formed behind the mirror. It is erect. Virtual and
2.5 Reflection from plane mirror laterally inverted.
When an object is placed in front of a plane mirror, its image can Image formation by two inclined mirrors, inclined at angle
be seen behind the mirror. The distance of the object from the = T [0, 180°]
mirror is equal to the distance of the image from the mirror. The object and all its images will always lie on a circle, having
center at the point of intersection of the two inclined mirrors, in a
two dimensional view.
3. OTHER IMPORTANT INFORMATIONS
(i) When the object moves with speed u towards (or away) from
the plane mirror then image also moves toward (or away) with
speed u. But relative speed of image w.r.t. object is 2u.
(ii) When mirror moves towards the stationary object with speed
u, the image will move with speed 2u.
, RAY OPTICS
4. SPHERICAL MIRRORS Paraxial rays : Rays which are close to principal axis and make
small angles with it, i.e., they are nearly parallel to the axis, are
A spherical mirror is a part of sphere. If one of the surfaces is
called paraxial rays. Our treatment of spherical mirrors will be
silvered, the other surface acts as the reflecting surface. When
restricted to such rays which means we shall consider only mirrors
convex face is silvered, and the reflecting surface is concave, the
of small aperture. In diagrams, however, they will be made larger
mirror is called a concave mirror. When its concave face is silvered
for clarity.
and convex face is the reflecting face, the mirror is called a convex
mirror. Images formed by spherical mirrors
Let us consider various cases depending on the nature of the
object and the image
(i) Real object and real image
(a)
Before the discussion of reflection by curved mirrors, you shall
carefully comprehend the meaning of following terms
(i) Centre of curvature : Centre of curvature is the centre of
sphere of which, the mirror is a part.
(ii) Radius of curvature : Radius of curvature is the radius of
sphere of which, the mirror is a part.
(b)
(iii) Pole of mirror : Pole is the geometric centre of the mirror.
(iv) Principal axis : Principal axis is the line passing through the
pole and centre of curvature.
(v) Normal : Any line joining the mirror to its centre of curvature
is a normal. (ii) Real object and virtual image
(a)
(b)
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