9702/Oct Nov/13/2017/Q28

An electromagnetic wave travels in a straight line through a vacuum. The wave has a frequency
of 6.0 THz.

What is the number of wavelengths in a distance of 1.0 m along the wave?

A             5.0 × 10^–5

B             2.0 × 10¹

C             2.0 × 10⁴

D             5.0 × 10⁷

Solution:
Answer: C

λ = v/f

= (3.0 x 10⁸ ms^-1)/ (6.0 x 10¹² s^-1)

= 5.0 x 10^-5 m

No. of λ = 1.0m/ (5.0 x 10^-5 m)

= 20 000

= 2.0 x 10⁴

Reference: PYQ - Oct/Nov 2017 Paper 13 Q28