The infrared wave is the part of the electromagnetic spectrum that comes right before visible light (when ordered from low to high frequencies). It is a transverse progressive wave, and so to find the frequency of this infrared wave from its wavelength and speed, we have to use the equation for wave speed:

From the question above, we have speed v = 2.2 x 108 m/s, and wavelength λ = 1.5 x 10-6 m. Therefore, the frequency f is:

Upon using your scientific calculator, you’ll notice your answer contains a factor of 1014 - this is what we call the standard form, which is a neat way of writing very large numbers in a convenient form. All that’s left is to round this off to 2 significant figures:

- Two magnets attracting each other
- An apple falling from a tree
- Charges produced by friction
- None of these