Ray Optics - 2 PPT Physics Class 12

 Page 1


RAY OPTICS - II 
1. Refraction through a Prism
2. Expression for Refractive Index of Prism
3. Dispersion 
4. Angular Dispersion and Dispersive Power
5. Blue Colour of the Sky and Red Colour of the Sun
6. Compound Microscope
7. Astronomical Telescope (Normal Adjustment)
8. Astronomical Telescope (Image at LDDV)
9. Newtonian Telescope (Reflecting Type)
10.Resolving Power of Microscope and Telescope
Page 2


RAY OPTICS - II 
1. Refraction through a Prism
2. Expression for Refractive Index of Prism
3. Dispersion 
4. Angular Dispersion and Dispersive Power
5. Blue Colour of the Sky and Red Colour of the Sun
6. Compound Microscope
7. Astronomical Telescope (Normal Adjustment)
8. Astronomical Telescope (Image at LDDV)
9. Newtonian Telescope (Reflecting Type)
10.Resolving Power of Microscope and Telescope
Refraction of Light through Prism:
A
Refracting Surfaces
Prism
i
d
A
B C
e
O
P
Q
r
1
r
2
N
1
N
2
D
In quadrilateral  APOQ,
A + O = 180° …….(1) 
(since N
1
and N
2
are normal)
In triangle OPQ,
r
1 
+ r
2
+ O = 180° …….(2) 
In triangle DPQ,
d = (i - r
1
) + (e - r
2
)
d = (i + e) – (r
1
+ r
2
)   …….(3) 
From (1) and (2),
A = r
1 
+ r
2
From (3),
d = (i + e) – (A)
or
i + e = A + d
µ
Sum of angle of incidence and angle 
of emergence is equal to the sum of 
angle of prism and angle of deviation.
Page 3


RAY OPTICS - II 
1. Refraction through a Prism
2. Expression for Refractive Index of Prism
3. Dispersion 
4. Angular Dispersion and Dispersive Power
5. Blue Colour of the Sky and Red Colour of the Sun
6. Compound Microscope
7. Astronomical Telescope (Normal Adjustment)
8. Astronomical Telescope (Image at LDDV)
9. Newtonian Telescope (Reflecting Type)
10.Resolving Power of Microscope and Telescope
Refraction of Light through Prism:
A
Refracting Surfaces
Prism
i
d
A
B C
e
O
P
Q
r
1
r
2
N
1
N
2
D
In quadrilateral  APOQ,
A + O = 180° …….(1) 
(since N
1
and N
2
are normal)
In triangle OPQ,
r
1 
+ r
2
+ O = 180° …….(2) 
In triangle DPQ,
d = (i - r
1
) + (e - r
2
)
d = (i + e) – (r
1
+ r
2
)   …….(3) 
From (1) and (2),
A = r
1 
+ r
2
From (3),
d = (i + e) – (A)
or
i + e = A + d
µ
Sum of angle of incidence and angle 
of emergence is equal to the sum of 
angle of prism and angle of deviation.
Variation of angle of deviation with angle of incidence:
d
i
0
i = e
d
m
When angle of incidence increases,            
the angle of deviation decreases.  
At a particular value of angle of incidence 
the angle of deviation becomes minimum 
and is called ‘angle of minimum deviation’.
At d
m
, i = e and      r
1 
= r
2 
= r (say)
After minimum deviation, angle of deviation 
increases with angle of incidence.
Refractive Index of Material of Prism:
A = r
1
+ r
2
A = 2r
r = A / 2
i + e = A + d
2 i = A + d
m
i = (A + d
m
) / 2
According to Snell’s law,
sin i
µ = 
sin r
1
sin i
sin r
=
µ =
sin
(A + d
m
)
2
sin
A
2
Page 4


RAY OPTICS - II 
1. Refraction through a Prism
2. Expression for Refractive Index of Prism
3. Dispersion 
4. Angular Dispersion and Dispersive Power
5. Blue Colour of the Sky and Red Colour of the Sun
6. Compound Microscope
7. Astronomical Telescope (Normal Adjustment)
8. Astronomical Telescope (Image at LDDV)
9. Newtonian Telescope (Reflecting Type)
10.Resolving Power of Microscope and Telescope
Refraction of Light through Prism:
A
Refracting Surfaces
Prism
i
d
A
B C
e
O
P
Q
r
1
r
2
N
1
N
2
D
In quadrilateral  APOQ,
A + O = 180° …….(1) 
(since N
1
and N
2
are normal)
In triangle OPQ,
r
1 
+ r
2
+ O = 180° …….(2) 
In triangle DPQ,
d = (i - r
1
) + (e - r
2
)
d = (i + e) – (r
1
+ r
2
)   …….(3) 
From (1) and (2),
A = r
1 
+ r
2
From (3),
d = (i + e) – (A)
or
i + e = A + d
µ
Sum of angle of incidence and angle 
of emergence is equal to the sum of 
angle of prism and angle of deviation.
Variation of angle of deviation with angle of incidence:
d
i
0
i = e
d
m
When angle of incidence increases,            
the angle of deviation decreases.  
At a particular value of angle of incidence 
the angle of deviation becomes minimum 
and is called ‘angle of minimum deviation’.
At d
m
, i = e and      r
1 
= r
2 
= r (say)
After minimum deviation, angle of deviation 
increases with angle of incidence.
Refractive Index of Material of Prism:
A = r
1
+ r
2
A = 2r
r = A / 2
i + e = A + d
2 i = A + d
m
i = (A + d
m
) / 2
According to Snell’s law,
sin i
µ = 
sin r
1
sin i
sin r
=
µ =
sin
(A + d
m
)
2
sin
A
2
Refraction by a Small-angled Prism for Small angle of Incidence:
sin i
µ = 
sin r
1
sin e
µ = 
sin r
2
and
If i is assumed to be small, then r
1
, r
2
and e will also be very small.  
So, replacing sines of the angles by angles themselves, we get
i
µ = 
r
1
and
e
µ = 
r
2
i + e = µ (r
1
+ r
2
) = µ A
But  i + e = A + d
So, A + d = µ A
or
d = A (µ – 1)
Page 5


RAY OPTICS - II 
1. Refraction through a Prism
2. Expression for Refractive Index of Prism
3. Dispersion 
4. Angular Dispersion and Dispersive Power
5. Blue Colour of the Sky and Red Colour of the Sun
6. Compound Microscope
7. Astronomical Telescope (Normal Adjustment)
8. Astronomical Telescope (Image at LDDV)
9. Newtonian Telescope (Reflecting Type)
10.Resolving Power of Microscope and Telescope
Refraction of Light through Prism:
A
Refracting Surfaces
Prism
i
d
A
B C
e
O
P
Q
r
1
r
2
N
1
N
2
D
In quadrilateral  APOQ,
A + O = 180° …….(1) 
(since N
1
and N
2
are normal)
In triangle OPQ,
r
1 
+ r
2
+ O = 180° …….(2) 
In triangle DPQ,
d = (i - r
1
) + (e - r
2
)
d = (i + e) – (r
1
+ r
2
)   …….(3) 
From (1) and (2),
A = r
1 
+ r
2
From (3),
d = (i + e) – (A)
or
i + e = A + d
µ
Sum of angle of incidence and angle 
of emergence is equal to the sum of 
angle of prism and angle of deviation.
Variation of angle of deviation with angle of incidence:
d
i
0
i = e
d
m
When angle of incidence increases,            
the angle of deviation decreases.  
At a particular value of angle of incidence 
the angle of deviation becomes minimum 
and is called ‘angle of minimum deviation’.
At d
m
, i = e and      r
1 
= r
2 
= r (say)
After minimum deviation, angle of deviation 
increases with angle of incidence.
Refractive Index of Material of Prism:
A = r
1
+ r
2
A = 2r
r = A / 2
i + e = A + d
2 i = A + d
m
i = (A + d
m
) / 2
According to Snell’s law,
sin i
µ = 
sin r
1
sin i
sin r
=
µ =
sin
(A + d
m
)
2
sin
A
2
Refraction by a Small-angled Prism for Small angle of Incidence:
sin i
µ = 
sin r
1
sin e
µ = 
sin r
2
and
If i is assumed to be small, then r
1
, r
2
and e will also be very small.  
So, replacing sines of the angles by angles themselves, we get
i
µ = 
r
1
and
e
µ = 
r
2
i + e = µ (r
1
+ r
2
) = µ A
But  i + e = A + d
So, A + d = µ A
or
d = A (µ – 1)
Dispersion of  White Light through Prism:
The phenomenon of  splitting a ray of white light into its constituent colours
(wavelengths) is called dispersion and the band of colours from violet to red
is called spectrum (VIBGYOR).
d
r
A
B C
D
White 
light
d
v
Cause of Dispersion:
sin i
µ
v
= 
sin r
v
sin i
µ
r
= 
sin r
r
and
Since  µ
v
> µ
r
,   r
r
> r
v
So, the colours are refracted at different 
angles and hence get separated.
Screen
N