Light_Refraction By Praveen Sir.ppt for 10th standard

as per cbse syllabus, 10th refraction of light

1. LIGHT -REFRACTION
1.Refraction of Light
2.Laws of Refraction
3.Refractive Index
4.Refraction through a Parallel Slab
5.Refractive Indices of Different Media
6.Principle of Reversibility of Light
7.Refraction through a Compound Slab
8.Apparent Depth of a Liquid
9.Total Internal Reflection
10.Refraction by Spherical Lenses
11.Image Formation by a Convex Lens
12.Image Formation by a Concave Lens
13.New Cartesian Sign Conventions
14.Lens Formula, Linear Magnification & Power of a Lens
Created by Praveen Prabhakar Sir, Physics (TGT), DAV Jamalpur

2. Refraction of Light:
Refraction is the phenomenon of change in the path (direction) of light as
it travels from one transparent medium to another (when the ray of light is
incident obliquely).
It can also be defined as the phenomenon of change in speed of light
from one transparent medium to another.
Rarer
Rarer
Denser
N
N
r
i
r
i
Laws of Refraction:
I Law:The incident ray, the normal to
the refracting surface at the point of
incidence and the refracted ray all lie in
the same plane.
II Law:For a given pair of media and for
light of a given wavelength, the ratio of
the sine of the angle of incidence to the
sine of the angle of refraction is a
constant.(Snell’s Law)
μ

4. i
i
r
r
A
B
C
D
N N
AB –Incident wavefront
CD –Refracted wavefront
XY –Refracting surface
E
F
G
X
Y
c, μ
1
v, μ
2
Denser
Rarer
Bending of Light

5. Refractive Index:
Refractive index of the 2
nd
medium with respect to the 1
st
medium is
defined as the ratio of the sine of angle of incidence in the 1
st
medium to
the sine of angle of refraction in the 2
nd
medium.
Refractive index of the 2nd medium with respect to the 1st medium is also
defined as the ratio of the speed of light in the 1
st
medium to the speed of
light in the 2
nd
medium.
(The constant
1μ
2is called refractive index of the
medium, iis the angle of incidence and ris the
angle of refraction.)
sin i
sin r
1μ
2=μ
21=
Speed of light in 1
st
medium
Speed of light in 2
nd
medium
1μ
2=μ
21=
If the 1
st
medium is vacuum or air, then the refractive index is called
‘absolute refractive index’.
If ‘c’ is the speed of light in air and ‘v’ is the speed of light in the medium,
then the absolute refractive index is given by
Speed of light in air
Speed of light in medium
μ
m= =
c
v

6. Refraction through a Parallel Glass Slab:
Rarer
medium (a)
Denser
medium (b)
N
N
r
1
i
1
i
2
r
2
M
t
δ
y
sin i
1
aμ
b=
sin r
1
sin i
2
bμ
a=
sin r
2
But
aμ
bx
bμ
a= 1
sin i
1
sin r
1
sin i
2
sin r
2
x
= 1
It implies that i
1 = r
2 and i
2= r
1
since i
1≠r
1and i
2≠r
2.
μ
Rarer
medium (a)
TIPS:
1.μof optically rarer medium is lower and that of a denser medium is higher.
2.μof denser medium w.r.t. rarer medium is more than 1 and that of rarer
medium w.r.t. denser medium is less than 1.(μ
air=μ
vacuum= 1)
3.In refraction, the velocity and wavelength of light change.
4.In refraction, the frequency and phase of light do not change.
5.
aμ
m= c
a/ c
m and
aμ
m= λ
a/ λ
m

7. Material Medium Refractive
Index
Canada balsam 1.53
Rock salt 1.54
Carbon disulphide1.63
Dense flint glass1.65
Ruby 1.71
Sapphire 1.77
Diamamond 2.42
Material Medium Refractive
Index
Air 1.0003
Ice 1.31
Water 1.33
Alcohol 1.36
Kerosene 1.44
Fused quartz 1.46
Benzene 1.50
Crown glass 1.52
Refractive Index of different media

8. Rarer (a)
N
r
i
Denser (b)
sin i
aμ
b=
sin r
sin r
bμ
a=
sin i
aμ
bx
bμ
a= 1 or
aμ
b= 1 /
bμ
a
If a ray of light, after suffering any number of
reflections and/or refractions has its path
reversed at any stage, it travels back to the
source along the same path in the opposite
direction.
A natural consequence of the principle of reversibility is that the image and object
positions can be interchanged. These positions are called conjugate positions.
μ
Principle of Reversibility of Light: (Not in Syllabus)

9. Refraction through a Compound Slab: (Not in Syllabus)
Rarer (a)
Rarer (a)
Denser
(b)
N
N
μ
b
r
1
i
1
r
1
r
2
r
2
i
1
Denser
(c)
μ
c
N
sin i
1
aμ
b=
sin r
1
sin r
1
bμ
c=
sin r
2
aμ
bx
bμ
cx
cμ
a= 1
sin r
2
cμ
a=
sin i
1
aμ
bx
bμ
c=
aμ
cor
bμ
c=
aμ
c /
aμ
bor
μ
a
μ
c >μ
b

10. Apparent Depth of a Liquid:(Not in Syllabus)
Rarer (a)
Denser (b)
O
O’
N
μ
b
h
r
h
a
i
r
r
i
sin i
bμ
a=
sin r
sin r
aμ
b=
sin i
or
h
r
aμ
b=
h
a
=
Real depth
Apparent depth
Apparent Depth of a Number of
Immiscible Liquids:
h
a = ∑ h
i / μ
i
i = 1
n
Apparent Shift:
Apparent shift = h
r-h
a= h
r –(h
r/ μ)
= h
r[ 1 -1/μ]
TIPS:
1.If the observer is in rarer medium and the object is in denser medium then
h
a< h
r.(To a bird, the fish appears to be nearer than actual depth.)
2.If the observer is in denser medium and the object is in rarer medium then
h
a> h
r.(To a fish, the bird appears to be farther than actual height.)
μ
a

11. Total Internal Reflection:(Not in Syllabus)
Total Internal Reflection (TIR) is the phenomenon of complete reflection of
light back into the same medium for angles of incidence greater than the
critical angle of that medium.
N N N N
O
r = 90°
i
c i > i
c
i
Rarer
(air)
Denser
(glass)
μ
g
μ
a
Conditions for TIR:
1.The incident ray must be in optically denser medium.
2.The angle of incidence in the denser medium must be greater than the
critical angle for the pair of media in contact.

12. Relation between Critical Angle and Refractive Index:
(Not in Syllabus)
Critical angle is the angle of incidence in the denser medium for which the
angle of refraction in the rarer medium is 90°.
sin i
gμ
a=
sin r
sin i
c
=
sin 90°
= sin i
c
or
1
aμ
g=
gμ
a
1
aμ
g=
sin i
c
or
1
sin i
c=
aμ
g
λ
g
sin i
c=
λ
a
Also
Redcolour hasmaximumvalue of critical
angle andVioletcolour hasminimum
value of critical angle since,
1
sin i
c=
aμ
g
=
1
a + (b/ λ
2
)
Applications of T I R:
1.Mirage formation
2.Looming
3.Totally reflecting Prisms
4.Optical Fibres
5.Sparkling of Diamonds

13. Refraction by Spherical Lenses
Lenses whose refracting surfaces are spherical are called ‘spherical lenses’.
A spherical lens whose refracting surfaces are bulging outwards at the
centre is called a ‘double convex lens’. It is thicker in the middle compared
to the edges.
A spherical lens whose refracting surfaces are curved inwards at the
centre is called a ‘double concave lens’.It is thinner in the middle
compared to the edges.
Different types of Spherical Lenses
Double
Convex
Double
Concave
Plano-
convex
Plano-
concave
Convexo-
concave
Concavo-
Convex
Meniscus lenses

14. First Principal Focus:
First Principal Focus is the point on the principal axis of the lens at which if
an object is placed, the image would be formed at infinity.
F
1
f
1
F
2
f
2
Second Principal Focus:
Second Principal Focus is the point on the principal axis of the lens at
which the image is formed when the object is kept at infinity.
F
2
f
2
F
1
f
1

15. Concave LensConvex Lens
f
R
M
N
O
CF
O
C
f
F
M
N
R
CFC F
XX’ XX’
Optic Centre (O)is the central point of a lens.
Centre of curvature (C)is the centre of the imaginary sphere from which
spherical lens is cut out. There are two centres of curvature on either side of
the lens.
Radius of curvature (R)is the distance between the optic centre and the centre
of curvature.
Principal axis (XPX’)is the line passing through the optic centre and the centre
of curvature and extending to ∞. It is the normal to the lens at the pole.

16. Principal Focus (F)is the point on the principal axis at which the incident rays
of light parallel to principal axis either really pass through or appear to
pass through after getting refracted from the lens. There are two foci for
the lens.
Focal length (f)is the distance between the optic centre and the principal
focus.
Radius of curvature is approximately twice the focal length. R ≈ 2f
Aperture (MN)is the diameter of the refracting surface. Note that it is not the
diameter of the sphere from which the lens is cut out.

17. Rays to be considered for drawing Ray Diagram
The intersection of at least two refracted rays give the position of image of the
point object.
Any two of the following rays can be considered for locating the image.
1. A ray parallel to the principal axis, after refraction from a convex lens,
passes through the principal focus on the other side of the lens. In case of
a concave lens, the ray appears to diverge from the principal focus on the
same side of the lens.
O
CFC F
XX’
O
C F CF
XX’

18. 2. A ray passing through the principal focusof a convex lens or a ray which is
directed towards the principal focus of a concave lens, after refraction, will
emerge parallel to the principal axis.
3. A ray passing through the optic centre of a convex or concave lens, after
refraction, will emerge without any deviation.
O
CFC F
XX’
O
C F CF
XX’
O
CFC F
XX’
C F CF
XX’
O

19. Image formation by a convex lens
1) When object is placed at infinity:
•
O
B
•
2F
2
•
F
2
•
F
1
•
2F
1
Parallel
rays from ∞
i)Position of image: At F
2
ii)Nature of image : Real & inverted
iii) Size of image : Very small
(Highly Diminished)

20. 2) When object (AB) is placed beyond C
1(2F
1):
•
O
A
B
A’
B’
•
F
1
•
F
2
•
2F
2
•
2F
1
i)Position of image: Between C
2 (2F
2) & F
2
ii)Nature of image : Real & inverted
iii) Size of image : Smaller than object
(Diminished)

21. 3) When object (AB) is placed at C
1(2F
1):
•
O
A
B
A’
B’
•
F
2
•
2F
2
•
2F
1
•
F
1
i)Position of image: At C
2
(2F
2)
ii)Nature of image : Real & inverted
iii) Size of image : Same size as that of the object

22. •
O
A
B
A’
B’
•
F
1
•
2F
1
4) When object (AB) is placed between C
1(2F
1) & F
1:
•
F
2
•
2F
2
i)Position of image: Beyond C
2(2F
2)
ii)Nature of image : Real & inverted
iii) Size of image : Larger than object
(Enlarged)

23. 5) When object (AB) is placed at F
1:
•
O
A
B
•
F
2
•
2F
2
•
2F
1
•
F
1
i)Position of image: At ∞
ii)Nature of image : Real & inverted
iii) Size of image : Very large
(Highly enlarged)
Parallel rays
meet at ∞

24. 6) When object (AB) is placed between F
1& O: (Simple Microscope)
•
O
A
B
•
F
2
•
2F
2
•
2F
1
•
F
1
A’
B’
i)Position of image: On the same side as that of the object
ii)Nature of image : Virtual & erect
iii) Size of image : Larger than the object

25. Between O and FSame side of the
lens
Enlarged Virtual and erect
Image formation by a convex lens for different positions of the object
Beyond C Between F and CDiminished Real and inverted
At C At C Same size Real and inverted
Between F and CBeyond C Enlarged Real and inverted
At F At infinity Highly enlargedReal and inverted
Position of the
object
Position of the
image
Size of the image Nature of the
image
At infinity At F Highly
diminished
Real and inverted

26. Image formation by a concave lens
•
O
A
B
•
F
2
•
2F
2
•
2F
1
•
F
1
A’
B’
i)Position of image: On the same side as that of the object
ii)Nature of image : Virtual & erect
iii) Size of image : Smaller than the object

27. Sign Conventions for Refraction by Spherical Lenses
(New Cartesian Sign Convention)
1.The object is always placed to the left of the lens. i.e. the incident rays
from the object always move from left to right.
2.All distances parallel to the principal axis are measured from the optic
centre (O) of the lens.
3.All the distances measured to the right of the optic centre (along +ve x-
axis) are taken +ve while those measured to the left of the optic centre
(along -ve x-axis)are taken –ve.
4.Distances measured perpendicular to and above the principal axis (along
+ve y-axis) are taken +ve while those measured below the principal axis
(along –ve y-axis) are taken –ve.
Note:
While solving numerical problems, new Cartesian sign convention must be
used for substituting the known values of u, v, f, h and R.

28. •
O
Direction of
incident light
-ve + ve
+ ve
-ve
X
X’
Direction of
incident light
-ve + ve
+ ve
-ve
X
X’
Y
Y’
•
O
Y
Y’

29. f
•
R
u
O
A
B
A’
B’
M
v
•
2F
2
•
F
2
•
F
1
•
2F
1
Lens Formula
u –object distance
v –image distance
f –focal length of the mirror
1
v f
- =
1
1
u

30. Linear Magnification:
Linear magnification produced by a lens is defined as the ratio of the size of
the image to the size of the object.
Magnification in terms of v and f:
m=
f -v
f
Magnification in terms of u and f:
m=
f
f -u
More of Refraction in Higher Class…
Magnification produced by a lens is also defined as the ratio of the image
distance to object distance.
=
v
u
m=
h’
h
Power of a Lens:
Power of a lens is its ability to
bend a ray of light falling on it and
is reciprocal of its focal length.
When f is in metre, power is
measured in Dioptre (D).
P=
1
f