Reflection of waves

Reflection of wave takes place when a wave strike an obstacle. The wave will change its direction of propagation when it is reflected.

Characteristics of Reflection of waves:

  1. It obeys the Law of Reflection.
  2. The wavelength, λ of the reflected wave is the same as that of the incident waves.
  3. The frequency, f  of the reflected waves is the same as that of the incident waves.
  4. Therefore the speed, v of the reflected waves is the same as that of the incident waves.
The phenomenon of reflection of waves obeys the Laws of reflection:

  1. the incident wave, the reflected wave and the Normal lie in the same plane which is perpendicular to the reflecting surface at the point of incident.

  2. The angle of incident, i is equal to the angle of reflection, r.

 Angle of Reflection
= Angle of Incidence

 Angles are measured with
 respect to the normal line
 (the perpendicular line).

Reflection of water waves

The phenomenon of reflection of water waves can be investigated using a ripple tank. The water waves are produced by a vibrating source on the water surface.

A horizontal bar will produced a plane wave.  While a round source will produce circular wave

Wavelength of a circular wave.   

Study more about water wave here.         

Video clips on ripple tank

Virtual lab on wave

Being a transverse wave, the crest of the water wave acts as a convex lens that converge the light ray and produced a bright area on the white paper. While the trough acts a concave lens that diverge light ray and produced a dark area on the white paper. Hence a series of bright and dark regions are produced on the white paper. A mechanical stroboscope is used to observed the pattern of the reflected waves. The moving waves will appear to be stationary when the frequency of the stroboscope is the same as the frequency of the water waves. The frequency of the stroboscope = np, where n = the number of slits on the stroboscope and p = the frequency of rotation of the stroboscope (the number of the stroboscope is turned in a second). View some photographs taken by stroboscopic camera.

Characteristics of reflection of wave:
  1. angle of reflection = angle of incidence
  2. wavelength, frequency and speed of the wave do not change after reflection.
  3. direction of propagation and velocity of wave changes.

Video clips on:

Reflection of plane wave on a plane reflector Reflection of circular wave on a plane reflector
Reflection of plane wave on a circular reflector  

Reflection of light waves

 

Reflection of light on a Plain mirror

Light rays from an object strike the surface of a plane mirror. The rays are reflected on the mirror and appear to come from behind the mirror. The reflected rays are drawn as if they are from the image. The image is virtual. Therefore the rays behind the mirror do not exist. They are virtual rays and are represented by dotted lines. The continuous lines from the mirror to the eye indicates the reflected rays. The distance between the object and the mirror is the same as the distance between the image and the mirror.

Characteristics of the image of a plane mirror:

  1. Virtual
  2. Same size as the object
  3. laterally inverted

Application of reflection of light on plane mirror

Defence & safety

Telecommunication
  1. A periscope is an optical instrument used to view objects beyond obstacles. Light waves from an object which is incident on a plane mirror in the periscope are reflected twice before entering the eyes of the observer.

  1. The rear view mirror and side mirror in a car are used to view cars behind and at the side while overtaking another car or making a turn. The mirrors reflect light waves from the cars and objects into the driver's eye.
  2. The lamp of a car emit light waves with minimum dispersion. The light bulb is placed at the focal point of the parabolic reflector of the car lamp so that the reflected light waves are parallel to the principal axis of the reflector. Parallel light waves have a further coverage (shines further).
  1. Optical fibres have many advantages compared to conventional cables in the transmission of information. Optical fibres are light, flexible, electrically non-conducting and can transmit much more information.
  2. Infrared waves from a remote control of electrical equipment (TV / Radio) are reflected objects in the surroundings and received by the TV or radio.

Reflection of a curved mirror

If a concave mirror is thought of as being a slice of a sphere, then there would be a line passing through the center of the sphere and attaching to the mirror in the exact center of the mirror. This line is known as the principal axis. The point in the center of the sphere from which the mirror was sliced is known as the center of curvature and is denoted by the letter C in the diagram below. The point on the mirror's surface where the principal axis meets the mirror is known as the vertex and is denoted by the letter A in the diagram below. The vertex is the geometric center of the mirror. Midway between the vertex and the center of curvature is a point known as the focal point; the focal point is denoted by the letter F in the diagram below. The distance from the vertex to the center of curvature is known as the radius of curvature (represented by R). The radius of curvature is the radius of the sphere from which the mirror was cut. Finally, the distance from the mirror to the focal point is known as the focal length (represented by f). Since the focal point is the midpoint of the line segment adjoining the vertex and the center of curvature, the focal length would be one-half the radius of curvature. Concave mirror Characteristics of image
  1. Light rays from very far object are parallel. A real image is formed at the Principle Focus.

 
  • Real
  • formed at Principle Focus

 
  1. Object placed > 2f
  • Real
  • Diminished
  • Inverted
 
  1. Object placed at 2f

 

 
  • Real
  • same size as the object
  • Inverted
Rules of reflection of curved mirror
  1. Incident light rays that is parallel to the principle axis is reflected towards the point Principle Focus, F.
  2. Any light ray that passes through the Principle Focus, F is reflected parallel to the principle axis.
  3. Image is form at the point of intersection of 2 reflected rays.

Look at the animation below.
  1. Object placed between f and 2f

 
  • Real
  • Diminished in size
  • Inverted

 
  1. Object placed at F

 

 Image is formed at infinity
 
  1. Object is placed < f.
  • Virtual
  • Magnified
  • Upright

At this position, concave mirror acts as a magnifying mirror.

 
Convex mirror

 Image is always virtual, diminished, upright irrespective of where the object is placed.

Application of Reflection of a curved mirror

Convex Mirror

Concave Mirror
The image of a convex mirror is always Virtual, Diminished, upright. The image of a concave mirror depends on the object distance.
A convex mirror has a wider field of vision than a plane mirror. Therefore it is used as a surveillance mirror and a blind corner mirror which is placed at a sharp bend on a road.

A concave mirror converge all parallel light rays to a point along its principle axis called Focal point.

A convex mirror is used as a side mirror of a car because it gives an upright image and a wider field of vision than a plane mirror. Concave mirror with long focal lengths can be used as makeup mirror, shaving mirror or a dentist's mirror as they form magnified and upright image.
A dentist mirror The lamps of a car emit light waves with minimum dispersion. The light bulb is placed at the focal point of the parabolic reflector of the car lamp so that the reflected light waves are parallel to the principal axis of the reflector. Parallel light waves shine far and have a further coverage.
 

Parallel light rays reflected and focused on oil-filled pipes placed at the Principle focus of the parabolic mirror, which collect heat, make steam, propel generator turbines.

Application of the reflection of sound waves

  1. Sonar (Sound Navigation and Ranging). Sonar is used to detect underwater objects (corals / fishes) or to determine the depth of the water by means of an echo. Sonar equipment emits a high frequency sound signal which is reflected by the object in the water. The reflected sound wave is received by the sonar receiver. The time taken for the echo to return is used to determine the distance of the object below the water surface.

    

  1. The phenomenon of the reflection of sound wave is used to determine the distance between 2 objects, eg. depth of a cave, well or width of a valley.