Wavefronts, Practice problems, FAQs

Have you thrown a stone in a pond? As the stone dips in the water it produces disturbance in the water. This disturbance travels in the medium in the form of waves. If you notice, the circular ring type shape moves in the radial direction and its radius keeps on increasing. If you draw an imaginary circle with the center at the point where the stone is dropped, it will represent a wavefront. Let's understand what a wavefront is!

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Table of content

What is Wavefront ?
Types of wavefront
Features of wavefront
Practice problems
FAQs

What is Wavefront ?

A wavefront is defined as the locus of all the points in the same phase on a wave in the medium of wave propagation. In the figure below a plane sinusoidal wave is traveling in a direction, if we draw a line connecting the crests of the wave, then it will represent a wave front.

A wavefront is perpendicular to the direction of wave propagation. The distance between the two wave fronts in phase represents the same type of motion of particle is equal to wavelength . Depending upon the type of source, the wave front can be of different types such as spherical wavefront, cylindrical wavefront and planar wavefront.

Types of wavefront

The wavefront can be of following types -

Spherical wavefront - A source having dimension much less than the distance of an observer, then it is called a point source. A spherical wavefront is produced by the point source. A wave produced from this source travels in 3 dimensions. The wave will be in the same phase at points equidistant from the source. So the wavefront by point source is spherical in nature. The figure shows a spherical wave front.

Example - Sound waves produce spherical wave fronts.

Cylindrical wavefront - The cylindrical wave front is produced by a line source. When a wave is produced by a line source, it travels in a radial direction of the cylinder. The points equidistant from the axis of the cylinder are in the same phase, so the locus of these points will be a cylindrical wavefront. The figure shows a cylindrical wave front.

Examples - light rays coming from a thin tube light which acts as a line source.

Plane wavefront - When the wave is produced by the source which is at a large distance, then the wavefront can be considered a planar one. If we consider a small part of a cylindrical and spherical wavefront, then it can also be said as a plane wave front. Figure shows a plane wave front.

Example - light rays coming from the sun.

Features of wavefront

The Wavefront have following features -

The Wavefront is always perpendicular to the propagation of waves.
Every point on the wavefront works as a source of secondary waves.
Wavefront travels with the wave as time passes.
If the speed of different points on the wavefront changes due to change in medium, then the shape of the wavefront may also change. For example, a lens can change the planar wavefront to a spherical one and vice versa.

Practice problems

Q. Write a difference between spherical and cylindrical wavefront.

A. Spherical wavefront is generally produced by a point source and a cylindrical wavefront is generally produced by a line source.

Q. Write an incident when a plane wavefront is converted into a spherical wavefront.

A. In the young's double slit experiment, when light falls on the slits, it has a planar wavefront and after passing the slit it has a spherical wavefront.

Q. What are the three types of wavefront ?

A. The three types of wavefront are -

Cylindrical wavefront
Spherical wavefront
Plane wavefront

Q. What is the huygens’s principle ?

A. According to this, every point on the wavefront is considered as the source of the secondary wave.

Video Explanation:

FAQs

Q. What is the angle between the wave velocity and wavefront?
A. The wavefronts are at 90 degrees with the velocity of the wave propagation.

Q. Is the wavefront always a surface?
A. No, wavefronts can be a point. Line wavefront is also possible depending on the dimension of the wave.

Q. Write an example of a spherical wavefront.
A. Wavefront of a sound wave or the wavefronts emanating from a point light source is an example of a spherical wavefront.

Q. Can we observe the wavefront?
A. Wavefront is not a tangible physical object, but just an imaginary surface. So it can not be observed as such. This concept of wavefront is used to define many optical phenomena.