Exam Master CHSE ODISHA ~ Physics)

The path along which light travels is called ray of light. The direction of light rays are always perpendicular to the wavefront and in outwards direction from the source.

Ray Ray Cylindrical wavefront (line source) Spherical wavefronts (point source) (a) (b) Plane 1 Ray Plane wavefronts (c) Different types of wavefronts

Huygens' Principle

Huygens' principle is essentially a geometrical construction which gives the shape of the wavefront at any time that allows us to determine the shape of the wavefront at a later time.

According to Huygens' principle,

(i) Each point on the given wavefront (called primary wavefront) is the source of a secondary disturbance (called secondary wavelets) and the wavelets emanating from these points spread out in all directions with the speed of the wave.

(ii) A surface touching these secondary wavelets, tangentially in the forward direction at any instant gives the new wavefront at that instant. This is called secondary wavefront.

Some Important points Related to Huygens' Principle

• The laws of reflection and refraction can be verified using Huygens’ wave theory.

Huygens' wave theory successfully explains the phenomenon of interference, diffraction and polarisation.

As, frequency vis characteristic of the source, therefore

Wavelength is inversely proportional to refractive index u) of the medium and directly proportional to the phase speed or wave speed. Using the relation

to another.

Laws of Reflection on the Basis of Huygens' Principle

As shown in figure, consider a plane wavefront AR incident on the reflecting surface XY, both the wavefront and the reflecting surface being perpendicular to the plane of paper.

Incident wavefront

KD

Reflected wavefront N'

S

х

с

Let angles of incidence and reflection be i and T, respectively. In AABC and ADCB, we have

ZBAC = ZCDB [each is 90°)

BC = BC [common)

AC = BD [each is equal to ct

AABC = ADCB Hence, LABC = ZDCB

..

Ni

i=r or

i.e. the angle of incidence is equal to the angle of reflection. This proves the law of reflection.

Law of Refraction on the Basis of Huygens' Principle

1 remains the same as light travels from one medium T V=

Consider a plane wavefront AB incident on a plane surface XY, separating two media 1 and 2, as shown in figure. Let U1 and be the velocities of light in twe media, with v2 < V. V2

N Incident wavefront B S Vit X Rarer-V1 Y Denser-V2 r с Refracted wavefront

Let the . AC

angles of incidence and refraction be i andr respectively.

From right AABC, we have sin ZBAC = sini = BC

( λι

AD From right AADC, we have sin ZDCA = sinr = sin i BC uit sinr AC

Eva, we get i.e. v=

U2 U νλ λα = = VA 2

B

or

Y

U2 U 1

(a constant

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