Reflection of Waves

Reflection of Waves - Reflection of Sound Waves and Light Waves

Reflection is that the modification or change in direction of a wavefront at the associate interface between 2 totally different media so the wavefront returns into the medium from where it originated. Common examples can be of the reflection of sunshine, sound and water waves. The law of reflection denotes that for a mirror-like reflection the angle at that the wave incident on the surface is equal to the angle at that it gets reflected. Mirrors exhibit specular reflection.

In this unit, we will talk about the reflection of light waves (specular reflection) and sound waves.


The law of reflection of sound waves states that the angle of incidence is always equal to the angle of reflection. But unlike reflection of light on a highly smooth surface, in the reflection of the sound wave, a part of the incident wave gets transmitted to the medium where it hits. This wave that gets absorbed or transmitted to the medium is called a transmitted wave. 

Constructive and destructive interferences are produced by the interference of reflected wave with the incident wave. This can cause resonances referred to as standing waves in rooms. It additionally means the sound intensity close to a tough surface is increased as a result of the reflected wave that is added to the incident wave, giving a pressure amplitude that is twice as nice during a thinner "pressure zone" close to the surface. This is utilized in pressure zone microphones to extend sensitivity. Reflection of waves in strings and air columns are responsible for the resonant standing waves in those systems.

Diagrammatic Representation of laws of reflection

Pressure Variation

Phase Change upon Reflection

The section of the reflected sound waves from hard surfaces and therefore the reflection of string waves from their ends determines whether or not the interference of the reflected and incident waves can be constructive or destructive. For string waves at the ends of strings, there's a reversal of section and it plays a vital role in manufacturing resonance in strings. The reflected wave and the incident wave get imposed to each other while moving in opposite directions, the mechanism of propagation is gone, and the resulting vibration is called a standing wave.

When sound waves in air (pressure waves) encounter a tough surface, there is no phase change upon reflection. That is, once the high a part of a wave hits the wall, it will be reflected as high pressure, not a reversed phase which would be low pressure. Keep in mind that once we remark the pressure related to a wave, a positive or "high" pressure is one that is above the ambient atmospheric pressure and a negative or "low" pressure is simply one that's below air pressure. A wall is defined as having the next "acoustic impedance" than the air, and when a wave encounters a medium of higher acoustic impedance there is no phase change upon reflection.

On the other hand, if a sound wave hits an air boundary within a solid, a section reversal will be experienced by the pressure wave that gets reflected back into the solid from the boundary of the air- a hard-hitting half reflective as an unaggressive region. That is, reflections of a lower impedance medium will be reversed in phase.

Besides manifesting itself within the "pressure zone" in the air close to a tough surface, the character of the reflections contributes to standing waves in rooms and within the air columns that make up musical instruments.

Production of a standing wave in an air column involves reflection from both the closed end and the open end of the column.

Variation of phase due to reflection

The conditions that are responsible for a phase change on one end but not on the other end can also be projected with a string if one imagines that the loose end of a string is constrained and is allowed to move only transverse to the string. The loose end would be a representation of an interface with a smaller effective impedance and would not result in phase change for the transverse wave. Thus, we can say that the mechanism involved in the spring and air column is just the opposite of one another.

Pressure Zone

The sound intensity close to a tough surface is increased as a result of the reflected wave adding to the incident wave, giving a pressure amplitude that's double to the thin "pressure zone" near the surface. This is employed in the pressure zone microphones to extend sensitivity. The doubling of pressure provides a six-unit (6 decibels) increase within the signal picked up by the electro-acoustic transducer.

This is an effort to predict the development of the pressure zone in terms of the dynamics of the air molecules concerned in transporting the sound energy. The air molecules are in fact in perpetual motion simply due to the thermal energy and have energy as a result of the air pressure. The energy used in sound transport is very less compared to the overall energy. Presuming the collisions with the wall to be elastic, no energy is lost within the collisions.

Viewing the gathering of molecules as a "fluid", we will invoke the concept that the interior pressure of a fluid may be considered as a quantity of energy density. The energy of the molecules reflected from the wall adds to that of the molecules approaching the wall in the volume which is very near to the wall, resulting in the doubling of the energy density and that in turn increases the pressure related to the wave.


Reflection of a light wave is either reflective (mirror-like) or diffusive (retaining the energy, however losing the image) depending on the character of the surface it falls.

A mirror provides the foremost common model for reflective light wave reflection and generally consists of a glass sheet with a gold coating wherever the many reflections happen. Reflection is increased in metals by suppression of wave propagation on the far side their skin depths. Reflection conjointly happens at the surface of clear media, like water or glass.

In the diagram, a lightweight ray PO strikes a vertical mirror at purpose O, and therefore the reflected ray is OQ. O is perpendicular to the mirror and is called the normal. The angle of incidence, θi and therefore the angle of reflection, θr is formed with the normal and both are always equal i.e. (θi = θr)

Reflection of light

In fact, reflection light wave could occur whenever light travels from a medium of a given ratio into a medium with a special ratio. This ratio decides the optical density of the medium. In the most general case, an exact fraction of the light is reflected from the surface, and also the remaining part is refracted (when the surface is not absolutely smooth). Thus, in majority cases, a quantity of the light wave is reflected, and a part of it is refracted during a given state of affairs. This is analogous to the method resistance mate in an electrical circuit causes reflection of signals. Total internal reflection of light from an optically denser medium happens if the angle of incidence is bigger than the critical angle. A critical angle is defined as the angle of incidence beyond which rays of light passing through an optically denser medium to the surface of an optically thinner medium no longer gets refracted but gets totally reflected.

Total internal reflection is employed as a means of focusing waves that can't effectively be reflected by common means. X-ray telescopes are made by making a converging "tunnel" for the waves. As the waves move at a low angle with the surface of this tunnel they're mirrored toward the main focus purpose (or toward another interaction with the tunnel surface, ultimately being focussed to the detector at the focal point). A conventional reflector would be useless because the X-rays would merely undergo the supposed reflection.

Specular reflection forms images. Reflection from a flat surface forms a mirror image, that seems to be reversed from left to right, as a result, we tend to do mental calculation if any words are projected on the mirror. Letters appear reversed on the mirror for this reason, except the symmetrical letters like A, H, O etc. The magnified or diminished image can be produced by light reflection on a curved surface. These types of mirrors have optical power and they are spherical or parabolic in nature. 

Critical angle and total internal reflection

Laws of the refection of light

Considering the reflection of light on a very smooth reflecting surface, the laws of reflection can be stated as follows:

  • 1) The incident ray (i), the normal (n) to the surface of reflection and the reflected ray(r) all lie on the same plane at the point of incidence.

  • 2) The incident angle formed between the incident ray and the normal is always equal to the reflected ray formed by the normal and the reflected ray.

  • 3) The incident ray and the reflected ray are always on the opposite side of the normal.