Page 163 - NCERT Science Class 10 English Medium

The SI unit of power of a lens is ‘dioptre’. It is denoted by the letter D.
If f is expressed in metres, then, power is expressed in dioptres. Thus,
–1
1 dioptre is the power of a lens whose focal length is 1 metre. 1D = 1m .
You may note that the power of a convex lens is positive and that of a
concave lens is negative.
Opticians prescribe corrective lenses indicating their powers. Let us
say the lens prescribed has power equal to + 2.0 D. This means the lens
prescribed is convex. The focal length of the lens is + 0.50 m. Similarly,
a lens of power – 2.5 D has a focal length of – 0.40 m. The lens is concave.

Many optical instruments consist of a number of lenses. They are combined to increase
the magnification and sharpness of the image. The net power (P ) of the lenses placed
More to Know! P = P + P + P + …
in contact is given by the algebraic sum of the individual powers P , P , P , … as
2
1
3
3
2
1
The use of powers, instead of focal lengths, for lenses is quite convenient for opticians.
During eye-testing, an optician puts several different combinations of corrective lenses
of known power, in contact, inside the testing spectacles’ frame. The optician calculates
the power of the lens required by simple algebraic addition. For example, a combination
of two lenses of power + 2.0 D and + 0.25 D is equivalent to a single lens of power + 2.25 D.
The simple additive property of the powers of lenses can be used to design lens systems
to minimise certain defects in images produced by a single lens. Such a lens system,
consisting of several lenses, in contact, is commonly used in the design of lenses of
camera, microscopes and telescopes.

Q U E S T I O N S

1. Define 1 dioptre of power of a lens.
2. A convex lens forms a real and inverted image of a needle at a distance
of 50 cm from it. Where is the needle placed in front of the convex lens ?
if the image is equal to the size of the object? Also, find the power of the
lens.
3. Find the power of a concave lens of focal length 2 m.

What you have learnt

n Light seems to travel in straight lines.
n Mirrors and lenses form images of objects. Images can be either real or virtual,
depending on the position of the object.
n The reflecting surfaces, of all types, obey the laws of reflection. The refracting
surfaces obey the laws of refraction.
n New Cartesian Sign Conventions are followed for spherical mirrors and lenses.

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