The pole of the lens
is the centre of the lens. Rays drawn passing through this point are
not diverted, they continue in a straight line.
The focal length of
a lens is the distance between the pole of the lens and the focal point OR the perpendicular distance between the axis of the lens and tha focal plane.
The focal point or principal
focus is point from which rays of light travelling
near to, and parallel to, the principal axis seem to emerge from after
refraction by the lens. (The point all emerging rays can be 'traced back'
through).
In the above
diagram the refraction of the rays is shown at BOTH lens surfaces.
In the diagram
below a SYMBOL for the lens is used - showing the refraction at each
surface is then not necessary... it is shown happening at a 'line' -
in exam questions make sure you carefully look to see whether symbol
or lens drawing is required.
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