Light (Reflection and Refraction) Notes
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1. Ray of Light :
A line drawn in the direction of propagation of light is called a ray of light.
2. Beam of Light :
A group of rays of light emitted by a source of light is called a beam of
light. A light beam is of three types.
(i) Parallel beam : A group of light rays parallel to each other is known as parallel beam of
(ii) Divergent beam : A group of light rays spreading out from a source of light is called
divergent beam of light.
(iii) Convergent beam : A group of light rays meeting at a point is called convergent beam 8.
3. Reflection of Light : There are some surfaces which have ability to send the light back in
the same medium when light strikes it. This
9. phenomena of sending the light back in the same
medium by a surface is called reflection of light
(i) The incident ray, the reflected ray and the normal at the point of incidence, all lie in a same
(ii) The angle of incidence is always equal to the angle of reflection, ∠i = ∠r.
4. Image : When light rays meet or appear to meet after reflection from a mirror,then it is
called an image.
1. Real Image : It is a kind of image which is formed by actual intersection of light rays after
2. Virtual Image : It is a kind of image which is formed by producing the reflected rays
backward after reflection.
5. Plane Mirror : Plane mirror is a piece of glass whose one side is polished by using silver
paint, which is covered by a coating of red paint to protect the silver layer.
6. Spherical Mirrors : It is part of hollow glass sphere whose one surface is polished.
There are two types of spherical mirror.
(i) Concave Mirror : It is a spherical mirror whose outer surface is polished and inner or
concave side is reflecting surface.
(ii) Convex Mirror : It is a spherical mirror whose inner is polished and outer side or convex
side is the reflecting surface.
Principal Focus :
A point on the principal axis of a spherical mirror where the rays of light parallel to the
principal axis meet or appear to meet after reflection from the mirror.
Focal Length :
The distance between the pole (P) and principal focus(F) of a spherical mirror is called the
focal length of the mirror. It is denoted by f.
Uses of Concave Mirror :
(i) It is used as a shaving mirror because when it is placed close to the face, it forms a large
(ii) It is used in solar heating devices like solar cooker, because it converges Sun’s rays over a
small area to produce high temperature.
(iii) It is used for security checking purposes.
10. Uses of
Convex Mirror :
(i) It is used as rear view mirror in automobiles because it gives erect image as well as
diminished due to which Pt has wider field of view.
(ii) It is also used in street lights.
11. Mirror Formula :
It is a relation between distance of object, distance ofimage from the pole ofthe mirror and it’s
focal length, i.e., relation between ‘u’, ‘v and It is given by
12. Magnification :
It is defined as the ratio of height of image to the height of the object. It is
denoted by letter m.
Laws of Refraction :
(i) The incident ray, the refracted ray and the normal at the point of incidence all lie in the
(ii) When a ray of light undergoes refraction then the ratio of sine of angle of incidence to
the sine of angle of refraction is constant.
14. The Refractive Index :
The refractive index of medium 2 with respect to medium is given
by the ratio of the speed of light in medium 1 and the speed of light in medium 2. This is
usually represented by the symbol n21. This can be expressed in an equation form as
15. Refraction by spherical lenses :
Lens is a transparent medium which is formed by
joining two pieces of spherical glass. There are two types of lenses.
(i) Convex Lens : It is a lens which is thicker at the centre and thinner at the edges.
(ii) Concave Lens : It is a lens which is thinner at the centre and thicker at the edges.
16. Terms related to a lens
Optical Centre of Lens : It is the centre of the lens through which light can pass without
Principal Axis : It is the line passing through optical centre and is perpendicular to the
line joining its edges.
Principal Focus : It is a point on the principal axis where all light rays which are parallel
to principal axis either converge or appear to diverge from, after refraction.
17. Lens formula :
18. Magnification :
Magnification, m = h2/h1
Ratio of height of image to the height of object.
It is also given by v/u i.e., Ratio of distance of image to the distance of object.
19. Power of a lens :
A beam of light parallel to principal axis either gets converged or
diverged after refraction by a lens. Some lenses converge the beam of light to a small extent
and some lenses converge it to a large extent. This ability of lens to converge or diverge a beam
of light is known as the power of lens.
Si unit of power of lens is dioptre : One dioptre is the power of a lens whose focal length is 1
Power of a combination of two or more lenses :
If two or more lenses are placed together to form a combined lens then the power of this
combined lens is equal to the sum of the powers of individual lenses.
P = P1 + P2 + P3 + ……