find the wavelength of electromagnetic waves of frequency 5 ×10^19`Hz` in free space. give its two applications.

Find the wavelength of electromagnetic waves of frequency 4 x 10^9 Hz in free space. Give its two applications.

A needle of sewing machine moves in path of amplitude 2 cm and has a frequency at 10 Hz. Then what is its acceleration after (1/40) second after it has crossed its mean position ?