Reflection of a Ray of Light

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Have you already checked out An Introduction to Reflection, Refraction and Dispersion?

It is the opening page of our Reflection, Refraction and Dispersion Series and contains masses of useful information. This is the table of contents:

• Introducing reflection
• Types of reflection
• Introducing refraction
• Calculating the angle of refraction
• Introducing chromatic dispersion
• Introducing refraction and wavelength

Overview of this page

• This page provides an introduction to reflection.
• Related topics including refraction and dispersion are covered on other pages of this series.
• Introductions to refractive index and the law of refraction (sometimes called Snell’s law) also appear on subsequent pages.

About the diagram

• You will notice that this diagram looks at reflection but for simplicity ignores refraction and dispersion.
• It looks at the path of white light rather than at the paths of the different wavelengths that white light contains.
• The diagram shows an incident ray of white light approaching the boundary between air and glass.
• When the ray strikes the boundary between the air and the glass it bounces back off the surface of the glass because it is highly reflective.
• The diagram shows that the angle of incidence and angle of reflection are the same.
• Both the angle of incidence and the angle of reflection are measured between the ray and the normal (the dotted green line).

Reflection

• Reflection takes place when incoming light strikes the surface of a medium and the light bounces off and returns into the medium from which it originated.
•  Reflection is predictable and always obeys three rules (the laws of reflection):
  • The incident ray, the reflected ray and the normal all lie in the same plane.
  • The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes with the normal.
  • The reflected ray and the incident ray always appear on opposite sides of the normal.
• Reflection takes place when light is neither absorbed by an opaque medium nor transmitted through a transparent medium.

Types of reflection

• When sunlight strikes window glass, some light is reflected and some is transmitted through the glass into the room beyond.
• The type of glass made for picture framing is designed to reflect some wavelengths and to transmit others.
• When light illuminates objects and then goes on to strike a mirror, the reflected image can be seen by an observer.
• A reflected image contains objects that we recognise and is made up of visible wavelengths of light and their corresponding colours.
• If a reflecting surface is very smooth, light waves remain in the same order as they bounce off the surface, producing a specular reflection.
• A diffuse reflection, in which no image is visible, results from light reflecting off a rough surface and light waves scattering in all directions.
• Reflection is independent of the optical density of the medium through which incident light travels or of the medium it bounces off.

Incident light

• Incident light refers to incoming light that is travelling towards an object or medium.

White light

• White light is the name given to visible light that contains all wavelengths of the visible spectrum at equal intensities.
• The Sun emits white light because sunlight contains equal amounts of all of the wavelengths of the visible spectrum.
• As light travels through a vacuum or a medium it is described as white light if it contains all the wavelengths of visible light.
• As light travels through the air it is invisible to our eyes.
• White light is what an observer sees when all the colours that make up the visible spectrum strike a white or neutral coloured surface.

Angle of incidence

• The angle of incidence measures the angle at which incoming light strikes a surface.
• The angle of incidence is measured between a ray of incoming light and an imaginary line called the normal.

Angle of reflection

• The angle of reflection measures the angle at which reflected light bounces off a surface.
• The angle of reflection is measured between a ray of light which has been reflected off a surface and an imaginary line called the normal.

The normal

•  In optics, the normal is an imaginary line drawn on a ray diagram perpendicular to, so at a right angle to (90⁰), to the boundary between two media.
• If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.