The focal length (f ) can be calculated by formula instead of using a graph. Calculations
Tabulation of Data
formula f u v
The focal length can be found without using a graph. For each pair of readings, sub the values of u and v into the uv .
. Calculate the average to find the focal (f ) of the mirror. Table 13.7
u / cm v / cm f / cm average focal length
POWER OF A LENS
P 1 f
P power f focal length
POWER OF A COMPOUND LENS
P P1 P2 P power of the compound lens
P1 power of lens 1 P2 power of lens 2
Compound lenses
FOCAL LENGTH OF A COMPOUND LENS
f f1f2 f1 f2
f focal length of the compound lens
f1 focal length of lens 1 f2 focal length of lens 2
The power of a compound is equal to the algebraic sum of the powers of each of the lenses that make it up.
The reason we specify the ‘algebraic’ sum is that it is vital to include the ‘’ or ‘’ for the power depending on whether we’re dealing with converging or diverging lenses.
We can work out the focal length of the compound lens by noting that:
COMPOUND LENSES Compound lenses are combinations of two or more lenses in contact with each other.
Power of a Lens
POWER OF A LENS The power of a lens is the inverse of the focal length.
It is a scalar quantity with an SI unit of per metre (m1) or dioptre (m1). It does, however, have a ‘sign’ associated with it. Converging lenses have a positive focal length and therefore a positive power. Diverging lenses have a negative power.
