5. Electromagnetic Spectrum

 

The electromagnetic spectrum is the range of all possible electromagnetic radiations whether it is visible or invisible. Nearly all objects in the universe, except black holes, emit, reflect or transmit some light. The distribution of this light along the electromagnetic spectrum is determined by the objects composition. Several types of spectra can be distinguished depending upon the nature of the radiation which varies on three physical properties :-

>> Wavelength of the EMR wave (Ʋ)

>> Frequency of the EMR wave (𝛌)

>> Energy of their photons (E)

A periodic wave has an area of elevation and an area of depression. Each area always reaches to their corresponding positive and negative peak values on the either sides of their base line and reaches back to zero value in order to complete an oscillation of the wave. The peak value of wave on the positive side is called crest and the peak value corresponding to the negative side is known as trough.

The wavelength is the spatial period of a periodic wave and it is the distance over which the shape of the wave repeats. It is the distance between two consecutive corresponding points of the same phase on the wave such as two adjacent crests or two adjacent troughs or two adjacent zero crossings. The inverse of the wavelength is called the spatial frequency.

Frequency is the number of occurrences of a repeating event of a wave per unit of time. The wavelength is inversely proportional to frequency of the wave, that is; the waves with higher frequencies have shorter wavelengths whereas the waves with lower frequencies have longer wavelengths. Wavelength depends on the medium that a wave travels through. That is why, when a white light passes through water or glass, it gets splits up into a visible spectrum containing seven colours in the order of 'violet, indigo, blue, green, yellow, orange and red (VIBGYOR).

 

In the visible spectrum, red light has the longest wavelength and the lowest frequency while the violet light has the shortest wavelength and the highest frequency. This visible spectrum is a portion of the whole EM spectrum that is visible to the human eyes.

The EM spectrum is the range of frequencies of EM radiations along with their respective wavelengths and the photon energies. The EM spectrum covers EM waves with frequencies ranging from below one Hertz to above 10²⁵ Hertz , corresponding to wavelengths from thousands of kilometers down to a fraction of the size of an atomic nucleus. The limit for long wavelengths is the size of the universe itself while it is thought that the short wavelength limit is in the vicinity of the Planck's length. This frequency range is divided into separate bands and the EM waves within each frequency band are called by different names from which one of the those bands is what we earlier discussed about, the visible light.

Visible light is a small portion of the spectrum which having a range of wavelengths ranging from 400nm to 780nm along with the corresponding frequency range from 750THz to 380THz that can be perceived by the human eye and this range of the EM spectrum which can be detected by the human eyes is called the visible spectrum.

 

But in EM spectrum, along with this visible spectrum, we are dealing with the light waves of frequencies and wavelengths range beyond the visible spectrum. There are many frequency bands which are lower and higher than the visible spectrum. In EM spectrum, every EM waves of every frequencies except from visible spectrum are invisible.

In visible spectrum band, red light wave has the lowest frequency. But in EM spectrum, the light waves of spectrum band having frequency range lower than that of red light are called Infrared waves. The light waves of frequency range lower than that of the Infrared waves are called Microwaves. The light waves of frequency band lower than the microwaves are called Radiowaves. Hence, the infrared waves, the microwaves and the radiowaves are the invisible EM waves which has the frequencies lower than the frequency range of the visible light waves.

While considering about the higher frequency bands, the violet has the highest frequency in the visible spectrum. But in EM spectrum, the light waves of spectrum band having frequency range higher than the violet light are called Ultraviolet waves. The light waves of frequency range higher than that of the ultraviolet waves are called X-ray waves. The light waves of frequency band higher than that of the X-ray waves are called Gamma waves. Cosmic waves are the light waves having frequency range higher than the gamma waves. Hence, the ultraviolet waves, X-ray waves, the gamma waves and the cosmic waves are the invisible EM waves having frequency band higher than that of the visible light.