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CBSE Class-12 Physics Quick Revision Notes
Chapter-09: Ray Optics and Optical Instruments
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Laws of Reflection:
The reflection at a plane surface always takes place in
accordance with the following two laws:
(i) The incident ray, the reflected ray and normal to
surface at the point of incidence all lie in the same plane
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e
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The
image formed I has the following
characteristics:
(i) The size of image is equal to the size of
object
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e
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(iii) The image is virtual and erect
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Reflection of Light from Spherical Mirror:
a) A spherical mirror is a part cut from a hollow sphere
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c) The reflection at spherical mirror also takes place in accordance with the laws of
reflection
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In other words, the distances measured toward the right of the
origin are positive
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In other words, the distances measured towards the left of origin are taken as
negative
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Focal Length of a Spherical Mirror:
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b) The focal length of a concave mirror is positive and that of a convex mirror is positive
and that of a convex mirror is negative
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Principal Axis of the Mirror:
The straight line joining the pole and the centre of curvature of spherical mirror
extended on both sides is called principal axis of the mirror
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Magnification:
It is defined as the ratio of the size of the image to that of the object
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Magnification, m is positive, implies that the image is real and inverted
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Refraction:
When a ray of light falls on the boundary separating the two media, there is a change in
direction of ray
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Laws of Refraction
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(ii) For the same pair of media and the same colur of
light, the ratio of the sine of the angle of incidence to the
sine of the angle of refraction is constant i
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,
sin i
= a µb
sin r
Where a µb is a constant known as Refractive Index of the
medium b with respect to the medium a, i is the angle incidence in medium a and r is the
angle of refraction in medium b
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Principle of Reversibility of Light:
As light follows a reversible path,
sin r
= b µa
sin i
Multiplying we get,
sin i sin r
x
=1
a µb X b µ a =
sin r sin i
1
a µb =
b µa
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Methods to Determine Refractive Index of a Medium:
Refractive index of a medium can also be determined from the following:
Velocity of light in air
(i) µ =
Velocity of light in the medium
1
sinc
Where c is the critical angle
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Apparent Depth of a Liquid:
If the object be placed at the bottom of a transparent medium, say water, and viewed
from above, it will appear higher than it actually is
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Refraction through a Thin Lens
If R1 and R2 are radii of curvature of first and second refracting surfaces of a thin lens of
focal length f, then lens-makers formula is
1 µ2 − µ1 1 1
=
−
f µ1 R1 R2
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Power of a Lens:
The power of a lens P is its ability to deviate the ray towards axis
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placed in contact of each
other is,
1 1 1 1
= ' + ' + ' +
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The refractive index μ of the prism is
A + δm
sin
2
µ=
A
sin
2
Dispersion:
The splitting of white light into constituent colours is called the dispersion
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Optical Instruments:
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Human Eye:
a) It is the most familiar and complicated optical instrument provided by nature to
living beings
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b) The light is focused by the eye lens on the retina
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d) Brain finally processes the information
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b) The magnifying power of a simple microscope is
D
M = 1+
f
c) The magnifying power, M of a compound microscope is
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v D
M = M 0 xM e = 1 +
u
fe
Where, Mo and Me denotes the linear magnifying of the objective and eye lens
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b) For the final image is formed at the least distance of distant vision, the magnifying
power is
f
F
M = 0 (1 + e )
fe
D
c) The resolving power of a telescope
1
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