Introduction to Wave Optics

Wave Optics

Physical optics, often known as wave optics, is a discipline of optics that examines interference, diffraction, polarisation, and other phenomena for which the geometric optics ray approximation is not applicable. The relationship between waves and light rays is well described by wave optics. Light is a kind of energy that flows through a medium as a transverse wave, according to the wave theory of light.

At any given time, the wavefront is the location of all particles vibrating in just the same frequency. To conclude, a wavefront is a surface on which all vibrating particles have almost the same frequency at any one point.

Theory of Wave Optics 

The field of wave optics has witnessed a renowned debate between two significant scientific societies dedicated to studying the nature of light. One society believes that light is a particle, while the other believes it is a wave. Both ray optics and wave optics are incredibly important as they are based on rectilinear propagation of light and deal with mirrors, lenses, reflection, refraction, etc. 

Sir Isaac Newton is a well-known supporter of the particle nature of light, proposing a corpuscular theory in which “light consists of extremely light and tiny particles known as corpuscles that travel at extremely high speeds from the source of light to create a sensation of vision by reflecting on the retina of the eye.” 

Newton was able to describe reflection and refraction using this theory, but he was unable to explain the causes of interference, diffraction, and polarisation. The main flaw in Newton’s corpuscular theory was that it failed to explain why the velocity of light in denser media was lower than in vacuum.

Then, Huygens came up with his theory in engineering physics of wave optics.  

Huygens Wave Theory

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

Light, according to Huygens, 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. Huygens’ creation of reflection and refraction is depicted in the diagram below.

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. But 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 which we now know can propagate light in a 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 what starlight was.

Wavefront and Wave Normal

What is Wavefront?

A wavefront is a line or surface in the course of wave motion that has the same phase of disturbances at all points. Depending on the source of light, wavefronts might be one of three types:

• Spherical wavefront
• Cylindrical wavefront
• Plane wavefront

Let’s discuss each of them in detail!

• Spherical Wave Front

In an isotropic medium, when a point source emits waves in three dimensions, the wavefronts are spheres positioned on the foundation. One such wavefront is a spherical wavefront.

Lightwave amplitude, A ∝ 1/r

Light waves intensity, I ∝ 1/r2

• Cylindrical Wave Front

All locations equidistant from a linear source lie on the surface of a cylinder when the source of light is linear. A cylindrical wavefront is the name for such a wavefront.

• Plane Wave Front

A plane wavefront is created when a fraction of a spherical or cylindrical wavefront arrives from a faraway source, such as infinity.

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 shapes of wavefronts, namely 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 wavefront has the same form as the original wavefront.

If light falls on a concave mirror; convex mirror: When a plane wavefront hits a concave 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.

Conclusion 

Wave optics in engineering physics (also known as physical optics) studies how light behaves when it has wave characteristics. We studied wave optics, theories (Huygens and Young theory), wavefront, wave normal, and other topics here.