Wave Front

The link between waves and light rays is the subject of wave optics, also known as physical optics. It is concerned with studying diverse phenomena such as polarisation, diffraction, interference and other occurrences when rays cannot represent geometric optics. 

The branch of optics that deals with the behaviour of light and its wave characteristics is known as wave optics. When studying and analysing waves, the Huygens principle is one of the most important principles to remember. Huygens concept essentially aids us in comprehending and describing wave motion.

Wavefront

The location of all points of the medium that vibrate in the same phase is characterised as a wavefront.

The shape of the disturbance source determines the shape of the wavefront.

A wavefront is always normal to the light rays, and it never propagates backwards.

Wavefronts can be one of three sorts, depending on the shape of the light source.

Spherical wavefront  

The wavefronts of light that emerge from a point source are spherical.

Cylindrical wavefront

The wavefronts are cylindrical when the light source is linear. The source is equidistant from all of the places.

Plane wavefront

The wavefronts of light originating from a very far-away source are flat. Because the amplitude of a plane wavefront remains constant, the intensity of the wavefront stays unchanged.

Three types of wavefronts have different aptitudes and intensities.

Wavefront types and their amplitude (A) and intensity (I)

Spherical wavefront

A ∝ 1/r

I ∝ 1/r2

Planar wavefront

A ∝ r0

I ∝ r02

Cylindrical wavefront

A ∝ 1/√r

I ∝ 1/r

What is wave normal, and what does it mean?

The “wave normal” is a perpendicular drawn to the surface of a wavefront at any point in the direction of light propagation. A ray of light is the direction in which light travels. As a result, a wave normal is the same as a light ray.

Shape of wavefronts

The shape of wavefronts can be altered with the use of a lens. The notion of wavefronts in reflection and refraction is explained in the following sections. There are two wavefront shapes: wavefront for reflection and wavefront for refraction. 

Wavefronts for reflection

If light falls on a plane mirror:

When plane wavefronts are reflected on a plane mirror, the reflected light’s wave front has the same form as the original wavefront.

If light falls on a concave mirror or convex mirror:

When a plane wavefront hits a concave mirror or convex mirror, the reflected light has a spherical shape.

Wavefronts for refraction

If light shines on a level surface, the following happens:

When a plane wavefront collides with a plane surface, the refracted ray has a plane wavefront as well.

When light falls on curved surfaces, the following happens:

If a plane wavefront collides with a converging (or) diverging lens, the resulting light will have a spherical wavefront.

Huygens wave theory

No one had questioned Newton’s corpuscular theory until Christopher Huygens, in the early 18th century, put forth his theory of wave optics in engineering physics. 

According to Huygens, light is a wave. He imagined a wave crest moving forward by visualising each point along the crest as a source point for little, circular, expanding wavelets that expand at the same rate as the wave. The contour of the advancing wave is determined by the surface tangent to these wavelets. 

According to Huygens’ principle,

(I) Every point on a wavefront emits secondary wavelets and operates as a light source.

(ii) With the speed of light, secondary wavelets spread in all directions in space (vacuum).

(iii) The new position of the wavefront is determined by the envelope of secondary wavelets along the forward direction after a set period.

Light reflection, refraction, interference and diffraction were all explained by Huygens’ wave optics theory. However, he was unable to explain: 

• Polarisation, because Huygens regarded light waves to be longitudinal mechanical disturbances.
• Compton Effect, photoelectric effect and black body radiation.
• The ether, a hypothetical medium that was never discovered. But  we now know the light propagates through vacuum.

Later, another brilliant mind came up with his theory. Let’s take a look! 

Thomas Young theory

Thomas Young (1773-1829) was the one who finally conducted a thorough investigation for wave interference effects in light and correctly evaluated the results. 

He noticed double-slit diffraction of light, as well as a variety of other diffraction effects, all of which indicated that light had wave interference effects and that visible light waves had extremely short wavelengths. 

The proof that light was an electromagnetic wave by researcher Heinrich Hertz and theorist James Clerk Maxwell was the pinnacle achievement.

One bright evening, Maxwell is believed to have told his wife about his discovery, telling her that she was the only other person on the planet who understood starlight.

Conclusion

According to the Huygens principle, every point on a wavefront is a generator of wavelets. At the same rate as the source wave, these wavelets expand out in a forward direction. The new wavefront is a straight line that runs parallel to all wavelets. A wavefront is a surface on which the phase of an optical wave remains constant.

The principal’s results include: 

• The Huygens wave theory that established the concept of light reflection.
• The idea also endorsed the concept of light refraction.
• It proved both the concepts of light interference and light diffraction.