How to Study Ray Optics in Physics?

3.  The branch of physics that deals with
light and vision,

4.  Lens is a peace of transparent medium
bounded by two curved surfaces or one
curve and one plane surface.

5. 1. Optical centre :
rays passes undeviated

6. 2. Centre of curvature & Radius of
curvature :

7. Principal focus , Principal plane , Focal
plane, Focal length :

10. 2
1 2
1
1 2
2
1
2 1
2 1
(1)
According to Snell’s law
sin
(2)
sin
1 2
sin
sin
sinr sin i
sin r r and sin i i
r i .. 3
i
r
From and
i
r
For very small angles

11. 2 1
2 2 1 1
2 1 1 2
2 1 1 2
From ODC i (4)
and from DIC r
r (5)
.4&5 3
( ) ( )
( ) 6
As , and are very small angles and expressed in ra
Sub in


dian
then form the diagram.
arcPD arcPD arcPD
PO Pl PC

12. 2 1 1 2
2 1 1 2
2 1 1 2
1 2 2 1
2 1
Substituting these values in equation 6 , we get
( )
( )
The factor is called as power of surface.
arcPD arcPD arcPD
PC PO PI
PC PO PI
R u v
u v R
R

13. 1
'
1
1 1
u v R
m - m
- =

14. 1 2 2 1
1 2
'
1
'
2
1 2
1
1, '
1 1
(1)
2
1 1
(2)
Adding equation 1 and 2 we can write
1 1 1 1
( 1)
for surface
u v R
Let and v v
u v R
For surface
v v R
v u R R

15. 
1 2
1 2
1 2
1 2
When u and v f.
1 1 1
( 1)
For concave lens R is negative and R is positive therefore,
1 1 1
( 1)
1 1 1
( 1)
f R R
f R R
f R R

17. 1. A plano convex lens is made of refractive
index 1.6. The radius of curvature of the
curved surface is 60 cm. The focal length
of the lens is
(a) 50 cm (b) 100 cm
(c) 200 cm (d) 400 cm

18. Ans:
(b) 100 cm
1 2
1 1 1
1
f R R
1 1 0.6 1
1.6 1
60 60 100
f 100 cm

19. 2. A convex lens has a focal length f. It is cut
into two parts along a line perpendicular to
principal axis. The focal length of each part
will be
(a) f/2 (b) f
(c) (d) 2f3
f
2

20. Ans:
(d) 2f
1 1 1 2
1 1 ..... i
f R R R
11 1 1
1 ..... ii
f ' R R
Divide i by ii
f '
2 f ' 2f.
f

21.  “Ratio of linear size of image to linear size
of object is called as linear
magnification.”

22.  “The ability of a lens to converge or
diverge the rays passing through it is
called as power of lens.”
 “Power of lens can also be defined as
reciprocal of focal length in meter.”

23.  The minimum distance of an object from
eye at which the object can clearly seen
without causing strain to the eye is called
as least distance of distinct vision (D) or
distance of distinct vision (DDV)

24. “The magnifying power of convex lens or a simple microscope is defined
as the ratio of angle subtended by the image at the eye (β) when seen
through lens, to the angle subtended by the object at the eye (α) when
the object is held at the distance of distinct vision and seen directly.”

25. 1
1
AB AB A B AB AB
&
AP D A P AP u
Magnifying power of simple microscope is,
AB / u
MP
AB / D
D
MP (11)
u
a = = b = = =
b
= =
a
= - - - - - -

26. 1 1 1
But
f v u
Applying new Cartesian sign conventions
1 1 1 1 1
f ( v) ( u) v u
1 1 1
u f v
Multiplying the above relation by D we can write
D D D
u f v
D D
MP
f v
= -
= - = - +
- -
= +
= +
= +

27. 1 1
MP D
f v
If the image is formed at distance of distinct vision
i.e. V D then
D D D
MP 1
f
:
v f
Wherepispowerof lens
If the image is formed
DP
at infinity
i.e. v then
D D
MP
f v
1
:
æ ö
÷ç= + ÷ç ÷çè ø
=
= + = + =
= ¥
=
+
+ =
Case 1
Case 2
D D D
f f
MP DP
+ =
¥
=

29.  “Magnifying power of compound
microscope is defined as “ratio of angle
subtended at the eye by final image (β)
to the angle subtended at the eye by
the object (α) when placed at DDV.”

30. If object is at DDV from objective then μ0 = D.
( )
( )
1 1
e 0
1 1 e 1 1
e
01 1
0
e
e
e
A B AB AB
and
u u D
A B / u A B D
MP
AB / D ABu
vA B
But M
AB u
D
& M
u
M.P. of compound microscope is, MP M x M
b = a = =
a
= = =
b
= =
=
=

31. 0 e
0
0
0 e
0
: If final image is formed at infinity then,
: If the final image is formed
MP M x M
.
1
MP M x M
1
at DDV then,
e
e
e
e
e
D
M
f
v D
MP
u f
D
M
f
v D
MP
u f
Case 1
Case 2

32. 0 0 0
0
0 0
0 0
0 0
0 0
0 0 0
0 0
0 0
0 0 0
0
0
1 1 1
multiplying by u
1
1
. . if image is at infinity
0
0
. . 1 is image is formed at DDV.
0 0
e
But
v u f
u u
v f
u u
v f
u u f
v f
v f
u u f
f D
M P
u f f
f D
M P
u f fe

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