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Concave Lenses
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 of a concave lens 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. Nowadays even simpler symbols for the concave lens is now used:
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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. 
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