Chemistry · Structure Of Atom · NEET
c is the speed of light = 3 × 10⁸ metres per second (m/s) in vacuum. It is a fixed number for all electromagnetic waves. ν (the Greek letter nu, not the English 'v') is the frequency, measured in hertz (Hz), which means how many waves pass a point in one second. λ (the Greek letter lambda) is the wavelength, the length of one full wave, measured in metres (m). Simple check: (m/s) = (1/s) × (m). The units balance, so the formula is correct.
They are INVERSELY proportional. This confuses many students. Rearrange the formula: λ = c/ν. Since c is a constant, a bigger frequency ν makes the wavelength λ smaller, and a smaller frequency makes λ bigger. So high frequency = short wavelength (like X-rays), and low frequency = long wavelength (like radio waves). They act like a seesaw.
Just rearrange the same formula. To find wavelength: λ = c/ν. To find frequency: ν = c/λ. Always put c = 3 × 10⁸ m/s. Warning: convert everything to SI units first. If frequency is given in kHz, multiply by 10³; if in MHz, multiply by 10⁶. If wavelength is in nm, multiply by 10⁻⁹ to make it metres.
No, it is NOT velocity. ν is the Greek small letter 'nu', and it stands for frequency. It just looks like the English letter v, which is why students mix it up. In this chapter c is the speed (velocity of light), and ν is the frequency. Never write speed = frequency × wavelength as v = vλ — that is meaningless. Keep c for speed and ν for frequency.
The idea (speed = frequency × wavelength) works for all waves, but in the NEET Structure of Atom chapter, c = νλ is only for electromagnetic radiation (light, X-rays, radio waves, etc.) travelling in vacuum, where the speed is always 3 × 10⁸ m/s. For sound the speed is different, so you would not use c. For NEET numericals in this chapter, always assume electromagnetic waves and c = 3 × 10⁸ m/s.
Wavenumber (written ν̄, 'nu bar') is 1/λ — the number of waves per unit length, usually in m⁻¹ or cm⁻¹. Do not confuse it with frequency ν. From c = νλ you can also write ν = c × ν̄, because ν̄ = 1/λ. So frequency = speed × wavenumber. NEET sometimes gives wavenumber to trick you; just remember ν̄ = 1/λ.
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]
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It links the speed of light (c), frequency (ν) and wavelength (λ) of any electromagnetic wave. NEET uses it to find one quantity when the other two are given, and it is the base for photon energy (E = hν = hc/λ).
Always take the speed of light c = 3 × 10⁸ m/s in vacuum, unless the question gives a more exact value like 2.998 × 10⁸ m/s.
Frequency ν is in hertz (Hz = per second, s⁻¹). Wavelength λ is in metres (m). Speed c is in metres per second (m/s).
Frequency becomes half. Because c = νλ is constant, wavelength and frequency are inversely proportional — double one and the other halves.
c = νλ is the first step in almost every Structure of Atom numerical about light, photons and spectra. Getting it fast and correct saves time and directly earns 1–2 marks most years.
