Wavefronts

A wavefront represents the surface of constant phase in the propagation of a wave. In simple terms, it's the imaginary line or surface connecting points of a wave that are vibrating in sync. Whether it's light traveling from the sun or sound waves emanating from a speaker, wavefronts help us understand how energy spreads in a medium. For instance, when a stone is thrown into a pond, the ripples that spread outward form circular wavefronts. Similarly, sound waves from a ringing bell or light rays from a lamp also propagate in the form of wavefronts, demonstrating how energy is transferred across distances in everyday experiences. Understanding wavefronts helps in various technologies like radar, ultrasound imaging, and even predicting how tsunamis propagate in oceans.

Light also shows the wave nature. According to Huygens, each point source of light is a center of disturbance from which waves spread in all directions.

Wavefront

• The locus of all particles in a medium, vibrating in the same phase is called WaveFront (WF).
• The direction of propagation of light is perpendicular to the WF.
• The time taken by the light to travel from one wavefront to another is the same along any ray.
• The phase difference between various particles on the wavefront is zero.
• Various type of wavefront

1. Spherical WF- For a point source
2. Cylindrical WF- For line source
3. Plane WF- For parallel light rays

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Solved Examples Based on Wavefronts

Example 1: What is the type of the wavefront on earth for sunlight?

1) Spherical

2) Plane

3) Cylinderical

4) None of the above

Solution:

The sun is at very large distance from the earth. Assuming sun as spherical, it can be considered as point source situated at infinity. We can treat it like a point object as seen from the surface of earth.

Because of large distance, the radius of wavefront can be considered as large (infinity) and hence, wavefront is almost plane.

Example 2: Consider a point at the focal point of a convergent lens. Another convergent lens of short focal length is placed on the other side. What is the nature of the wavefronts emerging from the final image?

1) Spherical

2) Plane

3) Cylindrical

4) None of the above

Solution:

Orientation of wave front is perpendicular to ray. The ray diagram of the situation is as shown in figure:

Parellel rays incident on lens forms the image at the focal point of the lens. This image acts as object for the lens . Now, due to the converging lens . let final image formed is I which is point image. Hence the wavefront for this image will be of spherical symmetry.

Example 3: Wave front is

1) locus of all adjacent points at which the electric field is the same

2) locus of all adjacent points at which the phase of vibration of a physical quantity associated with the wave is the same

3) series of points on the wave with the same amplitude

4) series of points on the wave with the same frequency

Solution:

The locus of all particles in a medium, vibrating in the same phase is called WaveFront.

Hence, the answer is the option (2).

Summary

Wavefronts represent the locus of points vibrating in the same phase in wave propagation, and the direction of wave travel is perpendicular to the wavefront. Different types of wavefronts, such as spherical, cylindrical, and plane, occur based on the source of the wave. For example, sunlight forms a nearly plane wavefront due to the sun's large distance from Earth, while a converging lens creates a spherical wavefront at its focal point. Wavefronts play a crucial role in understanding wave behavior in optics and wave mechanics.