CLASS X | SCIENCE | LIGHT
Notes prepared by Subhankar Karmakar
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CLASS- X; PHYSICS; CHAPTER-1
Topic: REFLECTION OF LIGHT
LECTURE-1
Characteristics of Light:
1. Light is a form of energy.
2. Light always travelled in a straight line which is called Rectilinear Propagation of Light.
3. Light can travel through a medium as well as through vacuum also.
4. Light carries Energy from one point to another point.
5. Physics of light is called Optics.
6. Light exhibits two phenomena called Reflection and Refraction of Light.
REFLECTION OF LIGHT
• When a ray of light travelling in one medium falls on the surface of the second medium and turned back into the first medium, then it is called reflection of light.
Important characteristics of reflection of light:
• The objects having polished, shining surfaces reflect more light than objects having unpolished, dull surfaces.
• Silver metal is one of the best reflectors of light.
• Ordinary mirrors are made by depositing a thin layer of silver metal on the back side of a plane glass sheet. The Silver layer is then protected by a coat of red paint. The reflection of light in a plane mirror takes place at the Silver surface in it.
• A ray of light it is the straight line along which light travels.
• A bundle of light rays is called a beam of light.
INCIDENT RAY: The ray of light travelling in one medium when falls on the surface of the second medium is called incident ray.
REFLECTED RAY: The ray of light which after striking the surface of the second medium and turned back into the first medium is called reflected Ray.
POINT OF INCIDENCE: The point where incident ray strikes the surface of the second medium is called point of Incidence.
NORMAL: Perpendicular to the surface of the second medium drawn at the point of Incidence is called Normal.
ANGLE OF INCIDENCE: The angle between incident ray and normal is called angle of incidence. It is denoted by ∠i.
ANGLE OF REFLECTION: The angle between reflected ray and normal is called angle of reflection. It is denoted by ∠r.
LAWS OF REFLECTION:
Reflection of light takes place according to the following two laws:
First law of reflection:
1. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
Second law of reflection:
2. The angle of incidence is always equal to the angle of reflection, i.e., ∠i = ∠r.
• A ray of light which is incident normally or perpendicularly on a mirror, is reflected back along the same path ( because the angle of incidence as equal as the angle of reflection for such a Ray of light are zero).
• The laws of reflection of light applied to all kinds of mirrors, plane mirrors as well as spherical mirrors.
LECTURE-2
Regular reflection:
• In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction. Regular reflection of light occurs from smooth surfaces like that of a plane mirror or highly polished metal surfaces. A plane mirror always produces regular reflection of light.
Diffuse reflection of light:
• In diffuse reflection, a parallel beam of incident light is reflected in different directions. The diffuse reflection of light takes place from rough surfaces like paper, cardboard, chalk, table, chair and unpolished metal surfaces.
Objects: Anything which gives out light rays either its own or reflected by it, is called an object. For example, a bulb, a candle, a pinhead, an arrow etc.
Images: Image is an optical appearance produced when light rays coming from an object are reflected from a mirror for refracted through a lens. For example, when we look into a mirror, we see the image of our face, while watching a movie in the cinema hall, we see the images of actors and actresses on the cinema screen.
Types of images:
There are two types of images
a. Real image
b. Virtual image
Real image: The image which can be obtained on a screen is called a real image. The image formed on a cinema screen is an example of real image.
Virtual image: The image which cannot be obtained on a screen is called a virtual image. A virtual image can be seen only by looking into a mirror or a lens. The image of our face in a plane mirror is an example of virtual image. A virtual image is just an illusion.
LECTURE-3
Formation of image in a plane mirror:
The two reflected rays AX and BY are diverging from each other so they cannot meet under left side of the mirror. Let us produce the reflected rays AX and BY backwards. They meet at point I behind the mirror. Now, when the reflected rays AX and BY enter the eye of a person at position E, the eye sees the rays of light in the straight line direction in which the reflected rays enter it. So, the person looking into the mirror from position E sees the reflected rays as if they are coming from the point I behind the mirror. Thus, point I is the image of the object O formed by the plane mirror.
• The nature of image formed by a plane mirror is virtual and erect. The size of the image formed by a plane mirror is equal to that of the object.
• The image formed in a plane mirror is at the same distance behind the mirror as the object is in front of the mirror.
Lateral inversion:
• When an object is placed in front of a plane mirror, then the right side of object appears to become the left side of image, and the left side of object appears to become the right side of image. This change of sides of an object and its mirror image is called lateral inversion.
• The phenomenon lateral inversion is due to the reflection of light.
Characteristics of an image formed by a plane mirror:
a. The image formed in a plane mirror is virtual. It cannot be received on a screen.
b. The image formed in a plane mirror is erect. It is the same side up as the object.
c. The image in a plane mirror is of the same size as the object.
d. The image formed by a plane mirror is at the same distance behind the mirror as the object is in front of the mirror.
e. the image formed in a plane mirror is laterally inverted.
Uses of plane mirrors:
a. Plane mirrors are used to see ourselves. The mirrors on our dressing table and in bathroom are plane mirrors.
b. Plane mirrors are fixed on the inside walls of certain shop to make them look bigger.
c. Plane mirrors are fitted at blind turns of some busy roads so that drivers can see the vehicle coming from the other side and prevent accidents.
d. Plane mirrors are used in making periscopes.
LECTURE-4
SPHERICAL MIRRORS:
A spherical mirror is a reflecting surface which forms part of a hollow sphere.
(a) A hollow sphere cut by a plane (b) concave mirror (c) convex mirror
Spherical mirrors are of two types:
(i) Concave Mirror: A spherical mirror in which the outer bulged surface is silvered polished and the reflection of light takes place from the inner hollow surface is called a concave mirror.
(ii) Convex Mirror: A spherical mirror in which the inner hollow surface is silvered polished and the reflection of light takes place from the outer bulged surface is called convex mirror.
Terms related with Spherical Mirrors
1. POLE: It is the middle point P of the spherical mirror.
2. CENTRE OF CURVATURE: It is the centre C of the sphere of which the mirror forms a part.
3. RADIUS OF CURVATURE: It is the radius ( R= AC or BC) of the sphere of which the mirror forms a part.
4. PRINCIPAL AXIS: The line PC passing through the pole and the centre of curvature of the mirror is called its principal axis.
5. APERTURE: It is the diameter AB of the circular boundary of the spherical mirror.
6. PRINCIPAL FOCUS: A narrow beam of light parallel to the principal axis either actually converges to or appears to diverge from a point F on the principal axis after reflection from the spherical mirror. This point is called the principal focus of the mirror.
CONCAVE MIRROR:
7. Principal focus of a concave mirror:
The principal focus of a concave mirror is a point on its principal axis to which all other light rays which are parallel and close to the axis, converge after reflection from the concave mirror
8. Focus of a concave mirror:
A concave mirror has a real focus. The focus of a concave mirror is in front of the mirror.
9. Focal length of a concave mirror:
The focal length of a concave mirror is the distance between its pole and principal focus. It is denoted by f.
10. Relation between radius of curvature (R) and focal length (f) of a spherical mirror:
The focal length of a spherical mirror is equal to half of its radius of curvature. It is true for both concave and convex mirror.
∴ f = R/2
Principal focus and focal length of a convex mirror:
• A convex mirror has virtual focus, focus of a convex mirror is situated behind the mirror.
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