Gurukul 24×7 We Teach You To Excel [632161]

Gurukul 24×7 – We Teach You To Excel
Light, Reflection and Refraction
1.Light travels in a straight line.Can be seen from the sharp shadow of the opeque object
casted by source of light
2.Light is both a matter made up of photons and also consist electromagnetic waves.Different
waves have different frequencies and colors.So it exhibits properties of both matter and
waves.
3.The light incident on an object can be reflected, absorbed and transmitted through the
object.
4.The color of the object is color of the light reflected by the object.
5.A dull/unpolished object absorbs most of the light falling on it and hence reflects less
percent of incident light. Example- black board is black since it absorbs most of the incident
light
6.Transparent object transmits most of the light falling on it.Example- glass
7.A bright object reflects most of the incident light.Example- polished silver mirror
8.Reflection of light is the phenomenon of sending the incident light rays back.
9.Surface of silver can be easily scratched and it soon becomes rough.So it is made by
depositing a thin layer of silver on one side of plane glass sheet .The silver layer is protected
by red paint.Light rays travel through the glass and reflects from silver side.
10.Angle of incidence and of angle of reflection are measured with the normal to the mirror at
the point of incidence.
11.First law of reflection: Incident ray, normal to the mirror and reflected ray all lie in same
plane.
12.Second law of Reflection: Angle of incidence=angle of reflection
N R
I Q

O

P
NO =normal to PQ
IO=incident ray
OR=reflected ray
IO,NO and OR lie in one plane in which normal is made I.e. plane NOQ and NOP
ION=Angle of incidence=NOR (angle of reflection)
*PQ can be surface of plane or curved mirror.
So if the angle x is made with the plane of the mirror, angle of incidence=angle made with the
normal=90-x
13.We use silver for making mirrors because it is a good reflector of light.
14.How is an image formed?
●A number of light rays from the object will be incident on the mirror.
●The light rays incident on the mirror will be reflected according to laws of reflection.
●The point/points where the reflected rays meet or appear to come from will be the point

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where the image is formed.
●When the reflected rays actually meet, we get a real image that can be captured or
observed or obtained on a screen.For eg: images on cinema screen.
●When reflected rays do not actually meet but appear to come/diverge from a point, we
get a virtual image that can't be captured on a screen.Example : Convex mirror, plane
mirror
eye

Image Location (from where reflected rays appear
to diverge from–Virtual Image)

eye
In order to see the image of an object in a mirror, you must sight at the image; when you sight at the
image, light will come to your eye along that line of sight.
Reflection by plane mirror
1.Image size =size of object
2.Distance of object in front of the mirror=Distance of (virtual) image behind the mirror.
3.Lateral inversion of the image formed.
4.Image formed is always erect
Spherical Mirrors – Concave and Convex Mirrors –obtained by cutting a part of the hollow sphere
as shown.So center of the sphere from which the curved or spherical mirror is obtained is called the
center of curvature of the spherical mirror. Center of curvature doesn't lie on the curved mirror.
●The pole lies at the center of the surface of curved surface.
●Greater the radius of curvature of the sphere, greater the focul length (R=2F)
●Shorter focul length implies larger converging/diverging power
●Magnification= imagedistance
objectdistance=h2
h1=-image distance from pole
object distance from pole=−v
u (be careful with
the sign of distances)
●Mirror Formula for Spherical mirrors:

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1
object distance from pole1
image distance from pole=1
u1
v=1
focullengthf (be
careful with the sign of distances)
●Thickness of the lens is inversely proportional to its focul length .If the lens is thick then
it means it has been made from a sphere with smaller radius so it has smaller focul
length.Similarly if the lens is thin then it has been made from a sphere of larger radius, so it
is has been made from a large sphere. You need to imagine this in your mind or just draw 2
circles of different sizes on a piece of paper.
Rules to remember
1.Incident rays from the object that are parallel to principal axis, pass through the focal point
after being reflected from the mirror.
2.Incident rays from the object that pass through the focus, become parralel after being
reflected from the mirror.
3.Incident ray passing through the center of curvature, are reflected back through the center
of curvature. (Rays passing through the center of curvature are normal to the lens surface
because then the ray is along the radius and tangent along the lens boundary will be normal
to the radius)
Concave mirror – (Converging mirror)
1.Real focus on left side of the pole (Focul length is -ve)
2.Real and inverted image is formed except for the case when object is between F and P
3.Virtual and erect image is produced when object is between F and P
4.Produces a powerful parallel beam of light when object is placed on focus.Because of this,
it is used in torches,vehicle head lights etc.
5.Used in -solar cookers ,In solar furnaces -because they converge the solar light to its focus
6.by dentist, in shaving mirrors, in make up mirros because they give virtual enlarged image
when object I brought close to it (between F and P)
Convex mirror – (Diverging mirror)

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1.Virtual focus on right side of the pole (focul length is +ve)
2.Always gives virtual, erect and diminished image of the object
3.If object moves away from the pole of convex mirror, image becomes smaller in size and
moves away from the pole.
4.Convex mirror has a wide field or range of view since it always produces diminished
images. It is therefore used in rear-view mirrors or side-mirrors of vehicles.
REFRACTION
Figure 1
1.When particles of light interact with the electrons of the atoms of the medium then particles
of the medium absorb energy from light particles called photon.The electrons remit some
energy in the form of light or electromagnetic wave.Since the interaction between photons
and electrons of the medium is different,the speed of light varies in different medium.
2.Light travels with different speed in different medium depending upon its optical
density.Speed of light is more in an optically rarer medium than in optically denser medium.
3.Refraction occurs at the boundry of the two mediums
4.Due to refraction light bends towards or away from the normal when it travels from one
medium to another.It moves towards the normal when travels from optically rarer to denser
medium and away from the normal when it travels from optically denser medium to
optically rarer medium.
Snell's Law
if light travels from medium 1 to medium 2 , then the relation between angle of incidence on the
boundary of medium 1 and angle of reftaction in medoum 2 is give by
sini
sinr=refractive index of medium 2
refractive index of medium 1=n2
n1=refractive index of medium 2 with respect to
medium 1
Refractive index is a measure of the optical density of the medium.Higher the refractive index,
higher the optical density.

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*Refractive index of air and vaccum is 1.So when medium 1 is air or vaccum, sini/sinr=absolute
refractive index of medium 2.
So for figure 1, sinQ1
sinQ2=nglass
nair–Equation 1
If ray refracts at B with angle Q3, then snell's law when light travels into air from point B will be
A
Q1
O

B
Q

sinQ2
sinQ3=nair
nglass–––Equation 2 P
Multiplying equation 1 and 2, sinQ1
sinQ3=1 => Q3=Q1
Q3 is called emergent angle (e) .
Emergent angle=Incident angle only when two faces where refraction occurs are parallel to each
other.Otherwise angle COB !=angle OBD=Q2
5.Refraction depends upon the angle of incidence and the nature of the medium.
6.Refractive index of the medium depends upon the frequency of incident light (or its color).
7.Perpedicular distance between incident and emergent ray is called lateral displacement.
Speed of the light is inversely proportional to the refractive index of the medium. =>
Speed of light in medium 2
speed of light in medium 1=n1
n2
8.Speed of light in vaccum = 3×108
Convex Lens and Concave LensQ2

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A double convex lens or convex lens (biconvex lens) as well as concave lens can be thought of as
originally being a part of the sphere but both forned in different manner as shown.
Refraction through convex lens:

*Refraction occurs twice at the boundries between are and glass..
*Normal to incident and refracted rays is drawn from the center of the spheres of which the lenses
are a part of.
*Optical center is the center of the convex of concave lens and is different from the center of the
sphere from which the lenses are made.
Magnification formula for lenses:
m=h2/h1= v/u
Lens formula:

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1/v -1/u=1/f
Concave Mirror Concave Lens
Focus in front of the mirror (on left side)
because parallel incident rays meet at a point on
the left after reflection. So focul length is
negative. Focus on left side (or same side as the object) of
the lens because parallel rays incident on the
concave lens appear to diverge from point on the
left after refraction.Focul length is negative
Since parallel rays actually meet after being
reflected so focus point is real.Since parallel rays appear to diverge after
refraction so focus point is virtual.
Forms both virtual or real image depending on
position of the object.Always forms virtual image.
Convex Mirror Convex Lens
Focus behind the mirror because parallel
incident rays appear to come from that point
after reflection from convex mirrror.Focul length
Is +veFocus on right side (or other side of the object)
of the lens because parallel rays incident on the
convex meet at a point on the right side (or
other side of the object) after refraction.Focul
length is +ve
Since parallel rays appear to come/diverge from
focus point after being reflected so focus point is
virtual.Since parallel rays actually meet after being
being refracted to focus point is real.
Forms only virtual image. Forms both real or virtual image depending on
the position of the object.
Mirror Lens
Real image is formed in front of the mirror (on
left or same side as the object) because on that
side only converging rays actually meet while
virtual image is formed behind the mirror (on
right or opposite side of the object ) since rays
cannot pass through the mirror so appear to
diverge from a virtual image.Real image is formed on right (or opposite side
of the object) since rays pass through lens
converge to meet on the other side while Virtual
image is formed on the left (same side as the
object) since rays passing through the lens are
diverged and cannot meet so appear to come
from virtual image.
Mirror Formula : 1/v+1/u=1/f Lens Formula: 1/v-1/u=1/f
Magnification Relation with Object & Image
Distance:
h2 / h1 =-v / uMagnification Relation with Object & Image
Distance:
h2 / h1 =v / u