Class 10th cbse board chapter first Reflection of light -I

1. PHYSICS
CLASS - 10th
REFLECTION OF LIGHT
PART-I
BY R K CHAUDHRI

2. LIGHT - 'Seeing' is one of the most common things we do. When light from an object
enters our eyes, we see the object. This light can be that emitted by the object as in the case
of an electric bulb or a red0hot iron nail. It can be also be the light bouncing off an object
like a book. An object that emits light is a source of light during the day, the sun acts as a
natural source of light. Candles, oil lamps and electric bulbs are source of light made by us.
Light is a form of Energy. Which can enable us to see
Properties of Light-
 Light tends to travel in straight line.
 Speed of light in vaccum/Air 3x108m/sec.
 It does not required any medium
SOURCE OF LIGHT
Luminous and Non-luminous Sources
A source of light objects such as the sun and the stars that emits light of their own are called
self-luminous source
Objects such as Moon ,tables, and chairs that do not emit light of their own are called Non-
luminous sources. Non-luminous bodies reflect light that falls on them, and hence become
bodies visible.
Point Source - A point source of light is obtained by placing a screen with a pinhole in
front of a lighted candle. The pinhole acts as a point source of light. A bulb of any other
source is extended source of light
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3. IMAGE- When light, travelling in a medium, falls on an object or encounters a boundry
leading into a second medium, one or all of the following three phenomena must happen
 It may be transmitted through the object/medium almost completely.
 Only a part of it may be transmitted and the rest absorbed or speed out.
 It may not be allowed to pass through the object/medium at all.
Light travelling in a medium is reflected by a surface when the light in which it was travelling
is returned by the surface to the medium.(Mirror )
Light is refracted when it cross over from one optical medium to another (Lens )
When a number of rays, starting from a point, undergo reflection or refraction and
thereafter meet at another point, the second point is said to be image of the first point .
OR
It is a point where at least two light rays actually meet or appear to meet.
Image is two types
 Real Image
 Virtual Image
Real Image - It is formed when light rays actually meet
o Can be obtained on screen
o Inverted Example Concave Mirror
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4. Virtual Image-It formed when light rays appear to meet
o Can't be obtained on Screen
o Erect Like Plane mirror, convex mirror
Reflection of light - When light falls on a smooth surface, a part of it gets reflected Fig
shows a ray of light being reflected from a flat, smooth surface. The ray AO is incident on
the surface at point O and is reflected along OB. The line ON is the normal to the surface at
O. The angle AON that the incident ray makes with normal is called the angle of incidence it
denoted I and the angle BON that the reflected ray makes with the normal is called the
angle of reflection it is denoted by r.
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5. LAWS OF REFLECTION - There is two law of reflection
1-The first law of reflection states that, during reflection the incident ray, the reflected ray,
and the normal drawn to reflecting surface at the point of incidence, all lie in the same
plane.
MM'- plane reflecting surface
AO - Incident ray
OB - Reflected ray
ON - Normal
O- Point of incidence
AON = I = angle of incidence
BON = r = angle of reflection
2-The second law of reflection states that during reflection, the angle of incidence is equal
to the angle of reflection, that is, i = r
Reflection from plane mirror- The plane mirror commonly used as looking glasses.
A plane mirror is made from a glass plate. A few mm thick. One surface of the glass is
polished to high degree of smoothness, forming the front of the mirror. The back side is Ag,
i.e coated with a thin layer of Al or some other shiny
Properties of Plane mirror
 A plane mirror forms a virtual image. Such an image is not formed by actual
intersection of light rays and cannot be formed on a screen
 The image formed by a plane mirror is erect. So, the image of a person formed by a
plane mirror has the head at the top and the feet at the botom.
 The image is formed as far behind the mirror as the object is in front of it.
 The size of the image formed by a plane mirror is equal to that of object.
 The image is laterally inverted. This means that in the image formed by a plane mirror,
the left and right sides are reversed.
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6. Lateral Inversion
The image of an object is seen in a plane mirror, the right part of the objcet appears as its left
and the left part of the object appears as its right part. Or the sides of the image undergo
inversion. This side reversal in the image is called lateral inversion
Spherical Mirrors-
Spherical mirror are special type of curved mirrors in which the surface of the mirror is a part
of a sphere. The rear-view mirrorss in cars, scooters, etc., are spherical mirrors. A hollow
glass sphere .such a portion has two dissimilar surfaces. The hollow surface that is on the
same side as the centre of the original sphere is called the concave surface. Which bulges out
is called the convex surface . If the convex surface is polished and the concave surface is
silvered and vice versa
Spherical Mirror Terms
Pole- The central point on the surface of the mirror is called its pole
Centre of curvature and radius of curvature- The centre of the sphere of which the
spherical mirror is a part is called the centre of curvature of the mirror C . The radius of this
sphere is called the radius of curvature of the mirror R.
Principal axis- The line joining the pole and the centre of curvature is called the
principal axis of the mirror
Focus- Centre and pole mid point is called focus every ray focus on this is called focus
point F.
- the distance between focus and pole is called focus length f .
XY is affective diameter is called Aperture
Relation between focal length and radius of curvature- The focal length f
and the radius of curvature r of a spherical mirror of small aperture are
f = r/2
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7. Practice question
a. What is meant by self-luminous object? Give an example
b. Distinguish between real image and virtual image.
c. What is plane mirror?
d. What is reflection of light?
e. What is plane of incidence?
f. Mention the characteristics of an image fomed by a plane mirror.
g. What is lateral inversion?
h. A clock when viewed in a plane mirror shows 4.30. What is correct time?
i. What is the relation between focal length f and radius of curvature?
Reference book –
NCERT, HCV ,L. Singh (S. Chand Publication)
Raghvendra Kumar Chaudhari
rkchaudhry222@gmail.com
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