Lens is a section of transparent refractive material of two
surface of definite geometrical shape of which one surface must be spherical.
Lens is generally of two types: (i) Convex lens (ii) Concave lens.
When a lens is thicker at the middle than at the edges, it
is called a convex lens or a converging lens. When the lens is thicker at the
edges than in the middle, it is called as concave lens or diverging lens.
Some terms regarding a lens.
Power of a lens
Power of a lens is its capacity to deviate a ray. It is
measured as the reciprocal of the focal length in meters, i.e. p= 1 /f
SI unit of power is dioptre (D).
Power of a convex lens is positive and that of a concave
lens is negative.
If two lenses are placed in contact, then the power of
combination is equal to the sum of powers of individual lenses.
Change in the power of a lens: if a lens is dipped in a
liquid, its focal length and power both change. This change depends upon the
refractive indices of lens and the liquid. If a lens of refractive index μ is
dipped in a liquid of refractive index μ’ then the following three situations
are possible.
(i)
μ> μ’ i.e. lens is dipped in a liquid of
smaller fractive index like a lens of glass (μ = 1.5) is dipped in water (μ’ =
1.33). Then the focal length of the lens increases and the power of the lens
decreases.
(ii)
μ= μ’ i.e. lens is dipped in a liquid of equal
refractive index then the focal length of the lens becomes infinite i.e. its
power becomes zero. The lens and the liquid behave as a single medium.
(iii)
μ< μ’ i.e. lens is dipped in a liquid of higher
refractive index the focal length
increases i.e. power decreases as well
as the nature of the lens also changes i.e. convex lens behave as concave lens
and vice-versa. For example, an air bubble trapped in water or glass appears as
convex but behaves as concave lens. Similarly a convex lens of glass ( μ = 1.5)
when dipped in carbon disulphide ( μ’ = 1.68), it behaves as a concave lens.
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