Clarification of wavelength and How it is different from frequency and significance

What is wavelength? How it is different from frequency? what is its significance?

Definition of wave length is, distance traveled by the wave in one cycle.

Frequency is, number of cycles per second
Why we have to tell always wavelength, it is almost similar to frequency. How we measure the wavelength. Especially when we are using the wavelength in electronics.
(2) Are we able to measure the wavelength physically.
(3) Some times, in book , they tell, 1mm wave length, 1um wavelength. What is it called 1mm wavelength and 1um wavelength.
Can you clarify above clarification.

They are just different ways of describing the same thing. If you could see a signal, it's wavelength would be the distance in metres between the same point on adjacent cycles, in other words the length of one cycle. The frequency is the number of those cycles that would pass a fixed reference position in one second. That's why frequency is measured in cycles per second or to use it's SI unit "Hertz". We use both because sometimes it is easier to work with one than the other. Example, how long is a 1MHz full-wave antenna? If we described it as 300m it would be far easier to visualize although they are the same thing. Frequency is easier in calculations because it is mathematically simple to refer to it in radians per second where it is compatible with other kinds of measurement.

2. Yes, in most cases. In the case of a low to medium frequency antenna for example, is possible to measure the voltage and current along it's length. For higher frequencies where the wavelength is shorter and therefore more difficult to measure physically, we use micrometers and absorbtion or reflection techniques to quantify the smaller distances.

3. 1mm = 300GHz, 1um = 300,000GHz. Wavelength = speed of light / frequency.

Brian.