# Light - Reflection And Refraction

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```					                  10. Light - Reflection and Refraction

Laws of Reflection

(i)    i=r
(ii)   AO, OP, and OB are co-planar

Spherical Mirror
 Centre of curvature = Centre of the sphere of which the spherical mirror is a
part

 Pole = Mirror centre

 Focus = Where parallel rays (parallel to the principal axis) meet or appear to
meet after reflection
 Principal Axis= The imaginary line that runs through the pole and the center
of curvature of a spherical mirror.
 Distance of focus from the pole is half the radius of curvature.
 Concave mirror and Image
 All images are real and inverted, except when the object is between the
focus and the pole.
 Image size = object size when the object is at the centre of curvature

 Uses:
o Torch reflector
o Search light
o Dentist’s mirror
o Shaving mirror
 Convex mirror and Image
 No real image
 No inverted image
 All images are diminished
 Uses:
o Rear-view mirror
o Security mirror
Mirror formula
1 1 1
 
f v u
For concave mirror, f = –ve and for convex mirror, f = +ve
Image height v
 Magnification                  
Object height u
For real image, v = –ve
Virtual image, v = +ve

 Refraction Laws
(i) AO, OB, and MON
are co-planar                Incident ray,
refracted ray, and
sin i                     normal to the
(ii)         = constant          interface at
sin r
(Snell’s law)             incident point are
co-planar

 Refractive index (RI)
velocity of light in medium I v1
2
1    ( of 2 w.r.t. 1) =                                 
velocity of light in medium II v 2
(Absolute RI when medium I = Vacuum = air)
(Light speed {vacuum} = 3 × 108 m/s)

Medium (Optically denser) =  >
1

Optically rarer =  < 1

path of a ray when there is no change in
medium.
medium
 Centre of curvature = Centre of the sphere of which the lens surfaces is a part of
(Same as Spherical mirror)
 Focus = Where parallel rays meet after refraction (On principal axis = principal
focus)
 Convex lens and Image
 Virtual and erect images – when the object is placed between F1 and the
optical centre (Magnifying glass)
 Image size = object size when object at 2F (=Centre of curvature)

 Concave lens and Image
 Virtual and erect at all object positions
 Lens Formula
For   concave lens f = –ve
convex lens f = +ve
 Magnification
Im age height v
m                (Same as mirror)
Object height u
 Lens power
1      f = –ve for concave

P Unit dioptre  
f  in m 

**

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