NCERT Lens Science Chapter Class 10 Note
A transparent material bound by two surfaces, of which one or both surfaces are spherical.
Convex lens:
It is a lens which is thicker at the center and thicker at the edges.
Concave lens:
It is a lens which is thicker at the center and thicker at the edges.
Terms related to a lens:
1)optical center:
It is the center of the lens through which high con pass without any deviation.
2)Principal Axis:
It is a line passing through the two centers of curvature of the sphere of which the lens is a part.
3)Principal focus:
It is a point on the principal axis where all light rays which are parallel to principal axis either convex or appear to diverge from after refraction.
3 Special rays for concave lens:
A ray incident parallel to the principal axis of a concave lens after refraction appear to come from the focus.
A ray directed towards the focus of a concave lens after refraction becomes parallel to the principal axis.
A ray incident through the optical center of the concave lens gets undeviated after refraction from the lens.
Image formation of a concave lens:
Position of object
Infinity
Between infinity and parallel center
Position of image
Focus
Between focus and optical center
Nature
Virtual,erect
Virtual,erect,diminished
Lens formula:
1/v - 1/u = 1/f
v = object distance
u = image distance
f = Focal length
Magnification:
The ratio of size of image to the height of the object is known as magnification.
m = hi /ho
m = v/u
hi /ho = v/u
Magnification for real and inverted – negative
Magnification for virtual and erect – positive
|m|= 1
|m|<1
|m|>1
h'>= h
h'< 1
h'>1
Power of lens:
It is defined as the reciprocal of the focal length.
P = 1/f (f in m)
P = 100/f (f in cm)
P = P1 + P2 + P3 ....
1/f = 1/f1 + 1/f2
Uses of lens:
- Astronomical Telescope - position of object at infinity w.r.t lens.
- Camera lens – position of the object is beyond 2F1
- Cinema Projector – Position between object is F1 and 2F1.
