Electromagnetic Nature of Light

In 1856, James Clark Maxwell proposed that light and other forms of radiant energy propagate themselves in the form of waves, through space. These waves have electric and magnetic fields associated with them and are called electromagnetic waves or electromagnetic radiation. The important characteristics of these waves are:

• They consist of electric and magnetic fields that oscillate perpendicular to the direction the waves travel and are simultaneously perpendicular to each other.

• All electromagnetic waves travel with a velocity equal to that of light i.e. (3.0 x 10⁸m s^-1)

• Electromagnetic radiations do not require any medium for propagation for e.g., light from the sun travels through empty space to reach us.

Fig: 3.6 - Electromagnetic radiation wave

Wave motion

Waves may be considered as disturbances, which originate from some vibrating source and travel outwards as a continuous sequence of alternating up and down movements called crests and troughs respectively.

Fig: 3.7 - Wave motion

Characteristics of Wave Motion

Characteristics and problems

Wavelength (l)

The distance between two adjacent crests or two adjacent troughs is called wavelength. It is denoted by the Greek letter l (lambda) and is generally expressed in terms of Angstrom units denoted as Å.

(1Å = 10^-8 cm)

Frequency

The number of waves, which pass through a given point in one second is known as frequency. It is denoted by the Greek letter n (nu). The unit of frequency is cycle per second or simply reciprocal second (s^-1). A cycle is said to be completed when a wave consisting of one crest and one trough, passes through a point. Frequency is also denoted by the term Hz (hertz) and 1Hz = 1cycle per second.

Amplitude

The measure of the height of a wave or the depth of its trough is called amplitude. Amplitude determines the intensity or brightness of a wave. It is denoted by the letter 'A'.

Velocity

The distance traveled by a wave in one second is termed as its velocity, it is generally denoted by the letter 'c'.

Wave number

It is the number of wavelengths per centimeter. It is equal to the inverse of a wavelength expressed in centimeter.

The relationship that follows between wavelength, frequency and velocity is, velocity = wavelength x frequency

c = l x n.

Electromagnetic spectrum

While all electromagnetic waves travel at the same speed (i.e., velocity 'c' is constant), the type of their radiation differs in the duration of wavelength and consequently their frequencies.

An electromagnetic radiation with a long wavelength will have a low frequency and vice versa.

'The arrangement of different types of electromagnetic radiations in order of increasing wavelengths (and consequently decreasing frequencies) is known as electromagnetic spectrum'.

The different types of radiations arranged in an electromagnetic spectrum are: Cosmic rays, gamma rays, X-rays, ultra-violet rays, visible radiations, infrared radiations, micro waves and radio waves.

Fig: 3.8 - Electromagnetic spectrum

It is interesting to note that visible light, which the human eye detects, constitutes only a small portion of the total electromagnetic spectrum. The visible spectrum is shown separately. Violet colour has a radiation of minimum wavelength (380nm) and highest frequency (7.5 x 10¹⁴Hz), while red has a radiation of maximum wave length (760nm) and lowest frequency (4 x 10 ¹⁴ Hz). The frequencies and wavelengths of radiations corresponding to other visible colours lie between these two extreme values.

In the portion immediately below the visible region (decreasing order of frequency) the infra-red region produces heat radiations or high thermal effects. Below these frequencies lie the microwave region (around 10¹⁰ Hz) used for microwave cooking, telecommunications and radar. Lastly the radiowaves region (around 10⁶ Hz) is used in broad casting and has the lowest frequency.

In the region above the visible portion lie the ultraviolet region (around 10¹⁶ Hz) that includes the Sun's rays and the X-ray region. The region with the highest frequency (and lowest wave length) are Gamma rays, which originate from radioactive substances and finally the cosmic rays that originate from outer regions and fall on the Earth.

Problems

3. A radio station broadcasts on a frequency of 980 kHz (kilohertz). What is the electromagnetic radiation broadcasted by the radio station?

Solution

The frequency of the radiation emitted is

= 980 kHz = 980 x 10³ Hz

= 980 x 10³ s^-1

c = 3.0 x 10⁸ ms^-1

The radio station broadcasts electromagnetic radiation of wavelength = 306m

4. Name the different types of radiations present in the electromagnetic spectrum in the increasing order of wavelengths.

Solution

The different types of electromagnetic radiations in the increasing order of wavelengths are, gamma rays (g), X-rays, ultra-violet rays (U.V.), visible radiation, infrared radiation (IR), microwaves and radio waves.