Page 49 - Physics 10_Float

GEOMETRICAL OPTICS

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Convex lens Image
Screen

Stand
Metre rod

Fig.12.22: Approximate method of finding focal length of a convex lens

Power of a Lens

Power of a lens is defined as the reciprocal of its focal length
in metres. Thus
For your information
Power of a lens = P = 1 / focal length in metres Dioptres are handy to use
because if two thin lenses are
placed side by side, the total
The SI unit of power of a lens is “Dioptre”, denoted by a power is simply the sum of the
-1
symbol D. If f is expressed in metres so that 1 D = 1 m . Thus, individual powers. For
1 Dioptre is the power of a lens whose focal length is 1 metre. example, an ophthalmologist
places a 2.00 dioptre lens next
Because the focal length of a convex lens is positive,
to 0.35 dioptre lens and
therefore, its power is also positive. Whereas the power of a immediately knows that the
concave lens is negative, for it has negative focal length. power of the combination is
2.35 dioptres.
12.9 IMAGE FORMATION BY LENSES

In mirrors images are formed through reflection, but lenses
form images through refraction. This is explained with the
help of ray diagrams as follows:
Image formation in convex lens can be explained with the
help of three principal rays shown in Fig.12.23
Remember it
When dealing with diverging
1. The ray parallel to the principal axis passes through
lenses, you must be careful not
the focal point after refraction by the lens. to omit the negative sign
2. The ray passing through the optical centre passes associated with the focal
straight through the lens and remains undeviated. length and the image position.
3. The ray passing through the focal point becomes

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