Chemistry · Structure Of Atom · NEET
Wavelength (λ) is the distance between two next crests of a wave, measured in metres (m), nanometres (nm) or angstroms (Å). Frequency (ν) is how many full waves pass a point in one second, measured in hertz (Hz = 1/s). They are two different things but connected by c = νλ. Since c is fixed, a bigger wavelength always means a smaller frequency.
Wavenumber (ν̄, said 'nu-bar') is the number of waves present in one unit of length. Formula: ν̄ = 1/λ. Its usual unit is m⁻¹ or cm⁻¹. It is NOT the same as frequency. Frequency counts waves per second (time); wavenumber counts waves per metre (distance). NEET loves this trap, so remember: frequency uses time, wavenumber uses length.
Use c = νλ with c = 3 × 10⁸ m/s. To get wavelength: λ = c/ν. To get frequency: ν = c/λ. Always convert units first — turn kHz into Hz (×10³), MHz into Hz (×10⁶), and nm into metres (×10⁻⁹) before you divide. Wrong unit conversion is the number one mistake here.
High frequency light has more energy. Energy is directly proportional to frequency (E = hν) and inversely proportional to wavelength. So short wavelength = high frequency = high energy (like UV and X-rays). Long wavelength = low frequency = low energy (like radio waves). Full energy formula comes in the next topic, Planck's quantum theory.
When light moves from vacuum into a denser medium, its speed and wavelength both decrease, but its frequency stays the same. Frequency depends only on the source. For NEET numericals, unless told otherwise, always use vacuum speed c = 3 × 10⁸ m/s.
A particular station of All India Radio, New Delhi, broadcasts on a frequency of 1368 kHz. The wavelength of the electromagnetic radiation emitted by the transmitter is (Speed of light, c = 3 × 10⁸ m s⁻¹):
The ratio of the wavelengths of the light absorbed by a hydrogen atom when it undergoes n=2 → n=3 and n=4 → n=6 transitions, respectively, is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
c = νλ, where c = 3 × 10⁸ m/s in vacuum, ν is frequency in Hz, and λ is wavelength in metres. This means λ = c/ν and ν = c/λ.
Wavenumber (ν̄ = 1/λ) has the unit m⁻¹ in SI, but chemists very often use cm⁻¹. It tells you how many waves fit into one unit of length.
No. Frequency (ν) is waves per second (unit Hz). Wavenumber (ν̄) is waves per metre (unit m⁻¹). They relate as ν̄ = ν/c. NEET often tests this exact difference.
Almost every atomic structure numerical — radio waves, spectral lines, photon energy — starts from c = νλ. Master it first, because Planck's E = hν and the hydrogen spectrum both build on it.
