Answer:
λ = 1.86 x 10⁻⁴ m = 186 μm
Step-by-step explanation:
The relationship between the wavelength and the frequency of a wave is given by the following equation:

where,
λ = wavelength of infrared radiation = ?
c = speed of infrared radiation = speed of light = 3 x 10⁸ m/s
f = frequency of infrared radiation = 1.61 THz = 1.61 x 10¹² Hz
Therefore,

λ = 1.86 x 10⁻⁴ m = 186 μm