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Ch 1 light physics class 10th notes with example and pyq questions
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Chapter – 10Light
Introduction to Light
• Light is a form of energy due to which we are able to see the objects which emits light
for example objects like sun, lamp, candle emits light of their own and thus they are
known as luminous objects.
• There are objects like table , chair etc. which are not luminous objects and still we are
able to see them and this happens because they reflects lights which falls on them
from a luminous object like sun, lamp etc. and when this reflected light reaches our
eyes we are able to see such non luminous objects.
• Light rays basically consist of electromagnetic waves which do not require any
material medium (like solid, liquid or gas) for their propagation.
• Wavelength of visible light waves is very small and is of the order
of 4×10−7m4×10−7m to 8×10−7m8×10−7m.
• Speed of light waves depends on the medium through which they pass as speed of
light in air is slightly less than the speed of light in
vacuum 8×108m/s8×108m/s same way speed of light in water and glass is much
less than that in air.
• When light falls on the surface of an object it can either be
1. Absorbed:- If an object absorbs all the light falling on it , then it will appear
perfectly black for example a blackboard
2. Transmitted: - An object is said to transmit light if it allows light to pass
through itself and such objects are transparent.
3. Reflected:- If an object sends back light rays falling on its surface then it is
said to have reflected the light
Reflection of Light
• The process of sending back light rays which falls on the surface of an object is
called REFLECTION of light
• Silver metal is one of the best reflectors of light.
• Mirrors we use on our dressing tables in our home are plane mirrors.
• A ray of light is the straight line along which the light travelled and a bundle of light
rays is called a beam of light.
• Laws of Reflection of light
1. The angle of incidence is equal to the angle of reflection, and
2. The incident ray, the reflected ray and the normal to the mirror at the point of
incidence all lie in the same plane.
• These laws of reflection are applicable to all types of reflecting surfaces including
spherical surfaces
Real and Virtual images
• An image is formed when the light rays coming from an object meet at a point after
reflection from a mirror (or refraction from lens).
, • The images are of two types
1. Real Images:- Real images are formed when rays of light that comes from an
object (or source) meets at a point after reflection from a mirror (or refraction
from a lens). Real images can be formed on a screen and can be seen with the
eyes.
2. Virtual images:- Virtual image is an image in which the outgoing rays from an
object do not meet at a point. It will appear to meet at a point in or behind the
optical device (i.e., a mirror) but they do not actually meet after reflection
from a mirror (or refraction from a lens). A plane mirror always forms virtual
images.
Characteristics of images formed by mirrors:-
• (a) Images formed by mirrors are always virtual and erect
(b) Size of image is always equal to the size of the object and the image is laterally
inverted.
(c) The images formed by the plane mirror are as far behind the mirror as the object in
front of the mirror.
• Lateral inversion:- If an object is placed in front of the mirror, then the right side of
the object appears to be the left side and left side of the object appears to be the right
side of this image. This change of sides of an object and its mirror image is called
lateral inversion.
Spherical Mirrors
• The reflecting surface of a spherical mirror may be curved inwards or outwards.
• Spherical mirrors are of two types
1. Concave mirror: - In a concave mirror reflection of light takes place at the concave
surface or bent-in surface as shown below in the
figure.
2. Convex mirror:- In a convex mirror reflection of light takes place at the convex
, surface or bent out surface as shown below in the figure
• Commonly used terms about Spherical mirrors :-
1. Center of curvature: - The reflecting surface of a spherical mirror forms a part
of a sphere. This sphere has a center. This point is called the center of
curvature of the spherical mirror. It is represented by the letter C. Please note
that the center of curvature is not a part of the mirror. It lies outside its
reflecting surface. The center of curvature of a concave mirror lies in front of
it. However, it lies behind the mirror in case of a convex mirror as shown
above in the figure 2.
2. Radius of curvature: - The radius of the sphere of which the reflecting surface
of a spherical mirror forms a part, is called the radius of curvature of the
mirror. It is represented by the letter R.
3. Pole: - The center of a spherical mirror is called its pole and is represented by
letter P as can be seen in figure 2.
4. Principle axis: - Straight line passing through the pole and the center of
curvature of a spherical mirror is called principle axis of the mirror.
5. Aperture of the mirror: - Portion of the mirror from which reflection of light
actually takes place is called the aperture of the mirror. Aperture of the mirror
actually represents the size of the mirror.
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