Previous Year Question

The colour of light is primarily determined by its Wavelength. Visible Light is the part of Electromagnetic Radiation that can be detected by the human eye. Its wavelength generally ranges from approximately 380 to 750 nanometres (nm).

The relationship between the wavelength and Frequency of light is expressed by the following equation—

c = ν × λ

Here, c is the speed of light in vacuum, ν is its frequency, and λ is its wavelength. The speed of light in vacuum is approximately 3 × 10⁸ m/s. Therefore, as wavelength increases, frequency decreases, and as wavelength decreases, frequency increases.

Approximate wavelengths of different colours of visible light:

Among visible colours, red light has the longest wavelength, while violet light has the shortest. Conversely, violet light has a higher frequency and energy per photon than red light. Photon energy is determined by E = h × ν, where h is Planck's constant and ν is the frequency of light.

Option (a): Amplitude represents the magnitude of oscillation of a light wave. Under the same conditions, greater amplitude increases light intensity but does not determine its colour.

Option (c): Intensity represents the rate of energy flow per unit area. It affects the Brightness of light rather than its fundamental spectral identity.

Option (d): Speed is the rate at which light propagates. All colours travel at the same speed in vacuum. When light enters a medium, its speed and wavelength may change, but its frequency remains unchanged. Therefore, changing the medium does not change the fundamental frequency-based colour of light.

The colour of Monochromatic Light is generally associated with its wavelength. From a physical perspective, frequency is a more fundamental identifier because it remains constant when the medium changes.