NEET Physics Notes Magnetostatics EMI and AC, EM waves-Wave Optics-Wave Nature of Light
Wave Nature of Light
Wave Nature of Light
According to Huygen, light is a form of energy, which travels in the form of waves through a hypothetical medium ‘ether’. The medium was supposed to be all pervading, transparent, extremely light, perfectly elastic and an ideal fluid.
Light waves transmit energy as well as momentum and travel in the free space with a constant speed of 3 x 108 ms-1. However, in a material medium, their speed varies from medium to medium depending on the refractive index of the medium.
Wavefront
- A wavefront is the locus of all those points (either particles) which are vibrating ip the same phase. The shape of the wavefront depends on the nature and dimension of the source of light.
- In an isotropic medium, for a point source of light, the wavefront is spherical in nature.
- For a line (slit) source of light, the wavefront is cylindrical in shape.
- For a parallel beam of light, the wavefront is a plane wavefront.
Huygens’ Principle
Every point on a given wavefront, acts as secondary source of light and emits secondar y wavelets which travel in all directions with the speed of light in the medium. A surface touching all these secondary wavelets tangentially, in the forward direction, gives the new wavefront at that instant of time.
Laws of reflection and refraction can be determine by using Huygens’ principle.
Interference of Light
Interference of light is the phenomenon of redistribution of light energy in space when two light waves of same frequency (or same wavelength) emitted by two coherent sources, travelling in a given direction, superimpose on each other.
If a1 and a2 be the amplitudes of two light waves of same frequency and Φ be the phase difference between them, then the amplitude of resultant wave is given by
Condition for Constructive Interference
If at some point in space, the phase difference between two
Condition for Destructive Interference
Young’s Double Slit Experiment
The arrangement is shown in figure monochromatic light of one wavelength is used.
Bright and dark fringes are formed on the screen with central point O behaving as the central bright fringe, because for O, the path difference ∆ = 0.
For light waves reaching a point P, situated at a distance x from central point A, the path difference
- If
, then we get nth bright fringe.
Hence, position of bright fringes on the screen are - given by the relation,
If
, then we get the dark fringe. Hence, for nth dark fringe 
where, n = 1, 2,3,
Fringe Width
The separation between any two consecutive bright and dark fringes is called fringe width p.
Thus,
and for a given arrangement it is d
constant, i.e. all fringes are uniformly spaced. Moreover, fringe width β
