Common SI prefixes

Physical Quantity is a quantity that can be measured and it consists of a numerical magnitude (size) and a unit. (e.g. 5m where 5 is the numerical magnitude and m is the unit)

1) The seven base quantities and their SI units

Out of the seven, for O level Physics, you are required to learn the first 5 (length, mass, time, current and temperature).

2) Some common derived quantities and units

You can think of base quantities as the smallest building blocks (like Lego bricks). When you combined any base quantities to ‘build’ into something else, they becomes a derived quantity.The derived quantity is ‘derived’ or ‘gotten from’ when 2 or more quantities are combined together.

3) Prefixes for SI units

Prefixes are used to represent very large or small quantities.For example, instead of writing 1000000 m, it will be more convenient to write 1Mm, which means the same thing!

Units conversion involving prefixes

Prefixes are used to simplify the writing of very big or very small numbers.

For instance, instead of having to write 12 500 000 m every time, you can simplify it by using the prefix mega (M), hence it can also be written as 12.5 Mm. Similarly, 0.00054 s can be written as 0.54 ms.

If you are unsure or confused with the unit conversions, you can refer to the following videos.

1) Prefixes can be used for different physical quantities units.

2) Unit conversion with examples

3) Unit conversion for units of area and volume

4) Unit conversion for units of speed and density

2010SPp1q12 Lens - focal length definition

Answer: Option C

Focal length f is the distance between the focal point and the centre of the lens (optical centre).

Note that only when parallel rays of light enter a converging lens, the rays will converge to a point. That point is considered to be focal point F (principal focus). The distance between focal point F and the optical centre is the focal length f. Refer to below.

As none of the options is similar to the above definition, you have to consider that the rays from a distant (far away) object are considered parallel. Hence the sharp image formed on the screen is considered the forcal point F of the lens and the distance between the image and the optical centre is the focal length f.

If it is a close object, the rays entering are not considered parallel. Hence even if the rays converged to a point, that point is NOT focal point F and the distance between this converged point and the optical centre is NOT focal length f.