Reflection & Refraction – Flat Boundary

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The diagram shows an incident ray of light approaching the boundary between air and glass.


  • When the ray strikes the boundary between air and glass some of the light bounces off the surface of the glass because it is highly reflective.
  • The diagram demonstrates that the angle of incidence and angle of reflection are the same.
  • The angles of incidence and reflection are both measured between the ray and the normal (the dotted green line).
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Reflection & Refraction - Flat Boundary

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
  1. Yes! As light crosses the boundary from a faster medium to a slower medium it bends towards the normal.
    Refraction refers to the way light changes both direction and speed as it travels from one transparent medium into another.
    Yes! When light leaves a vacuum or travels from one transparent medium into another, it undergoes refraction causing it to change both direction and speed.

About the diagram

Overview of this page

  • This page provides an introduction to a situation in which both reflection and refraction take place at a curved boundary between two transparent media.
  • It looks at the path of white light rather than at the paths of the different wavelengths that white light contains.
  • Related topics including dispersion are covered on other pages of this series.
  • Introductions to the terms refractive index and the law of refraction (sometimes called Snell’s law) also appear on later pages in the series.

An overview of 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 takes place when light is neither absorbed by an opaque medium nor transmitted through a transparent medium.

An overview of refraction

  • Refraction refers to the way that light (electromagnetic radiation) changes direction and speed as it travels from one transparent medium into another.
  • Refraction takes place as light travels across the boundary between different transparent media and is a result of their different optical properties.
  • When light is refracted its path bends and so changes direction.
  • The effect of refraction on the path of a ray of light is measured by the difference between the angle of incidence and the angle of reflection.
  • As light travels across the interface between different media (such as between air and glass) it changes speed.
  • Depending on the media through which light is refracted, its speed can increase or decrease.

An overview of reflection and refraction

  • When light strikes the boundary between two different media it may be partially reflected and partially refracted.
  • If both reflection and refraction take place:
    • A proportion of the light bounces off the surface of the new medium it encounters and returns into the medium from which it originated.
    • A proportion crosses the boundary and undergoes refraction, so changes speed and direction.

The diagram

The diagram shows an incident ray of light approaching the curved boundary between air and glass.

  • When the ray strikes the boundary between air and glass partial reflection and partial refraction takes place. This means that a proportion of the light bounces off the surface of the glass and returns into the air whilst the rest undergoes refraction.
  • When incident light strikes a curved surface the normal is drawn at a tangent to the curve.
  • In geometry, a tangent to a curve is a straight line that touches but does not intersect the curve at that point. It can be defined as a line through a pair of infinitely close points on a curve.

More about reflection

Reflection takes place when incoming light strikes the surface of a medium, some wavelengths are obstructed, and the wavefront bounces off and returns into the medium from which it originated.

  • The laws of reflection are as follows:
    • 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 same normal.
    • The reflected ray and the incident ray are on the 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.

More about refraction

  • When light crosses the boundary between two different transparent media it undergoes refraction.
  • The effect of refraction is that light changes speed along with its direction of travel.
  • The result of the change in direction is that rays either bend towards or away from the normal.
  • As the speed of light changes so does its wavelength but frequency and so the colour an observer sees remains the same.
  • The normal is an imaginary line drawn on a ray diagram at right angles (perpendicular) to the boundary between two media.
  • The change between the angle of incidence and the angle of refraction of a ray of light is always measured between the ray and the normal.
  • Whether light bends towards or away from the normal depends on the difference in optical density of the new medium it encounters.
  • An incident ray of light is refracted towards the normal and slows down when it travels from air into glass. Compared with air, glass is a slower, more optically dense medium (with the higher refractive index).
  • An incident ray of light is refracted away from the normal and speeds up when it travels from glass into air. Compared with glass, air is a faster, less optically dense medium (with a lower refractive index).

Calculating the angle of refraction

  • The direction in which a ray bends, and the precise angle, can be calculated if the type and refractive indices of both media are known.
  • The effect of refraction can be calculated using a neat little equation called the law of refraction (also known as Snell’s law).
  • If three of the variables are known, the law of refraction can be used to calculate the fourth.
  • Tables of refractive indices are available for common materials so that the change in direction of a ray can be calculated.
  • Tables of refractive indices for common materials often provide both the refractive index for white light as well as indices for specific wavelengths.

For an explanation of the refractive index (index of refraction) of a medium see: Refractive Index Explained.

For an explanation of how to use the refractive index of a medium see: How to Use the Refractive Index of a Medium.

For an explanation of the Law of Refraction see: Snell’s Law of Refraction Explained.

Some key terms

As light crosses the boundary between two transparent media, the law of refraction (Snell’s law) states the relationship between the angle of incidence and angle of refraction of the light with reference to the refractive indices of both media as follows:

When electromagnetic radiation (light) of a specific frequency crosses the interface of any given pair of media, the ratio of the sines of the angles of incidence and the sines of the angles of refraction is a constant in every case.

  • Snell’s law deals with the fact that for an incident ray approaching the boundary of two media, the sine of the angle of incidence multiplied by the index of refraction of the first medium is equal to the sine of the angle of refraction multiplied by the index of refraction of the second medium.
  • Snell’s law deals with the fact that the sine of the angle of incidence to the sine of the angle of refraction is constant when a light ray passes across the boundary from one medium to another.
  • Snell’s law can be used to calculate the angle of incidence or refraction associated with the use of lenses, prisms and other everyday materials.
  • When using Snell’s law:
    • The angles of incidence and refraction are measured between the direction of a ray of light and the normal – where 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.
    • The wavelength of the incident light is accounted for.
    • The refractive indices used are selected for the pair of media concerned.
    • The speed of light is expressed in metres per second (m/s).

Wavelength is a measurement from any point on the path of a wave to the same point on its next oscillation. The measurement is made parallel to the centre-line of the wave.

White light is the name given to visible light that contains all wavelengths of the visible spectrum at equal intensities.

  • 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.
  • When we look around we see through the air because it is very transparent and light passes through it.
  • The term white light doesn’t mean light is white as it travels through the air.
  • One situation in which light becomes visible is when it reflects off the surface of an object.
  • When white light strikes a neutral coloured object and all wavelengths are reflected then it appears white to an observer.

Optical density is a measurement of the degree to which a refractive medium slows the transmission of light.

  • The optical density of a medium is not the same as its physical density.
  • The more optically dense a medium, the slower light travels through it.
  • The less optically dense (or rare) a material is, the faster light travels through it.
  • A vacuum has the least optical density and so light travels through it at a maximum speed of 299,792 kilometres per second.
  • Optical density accounts for the variation in refractive indices of different media.

https://en.wikipedia.org/wiki/Absorbance

If one line is normal to another, then it is at right angles. So in geometry, the normal is a line drawn perpendicular to and intersecting another line.

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.

  • Expressed more formally, in optics, the normal is a geometric construct, a line drawn perpendicular to the interface between two media at the point of contact. This conceptually defined reference line is crucial for characterizing various light-matter interactions, such as reflection, refraction, and absorption.
  • Light travels in a straight line through a vacuum or a transparent medium such as air, glass, or still water.
  • If light encounters a force, an obstacle or interacts with an object, a variety of optical phenomena may take place including absorption, dispersion, diffraction, polarization, reflection, refraction, scattering or transmission.
  • Optics treats light as a collection of rays that travel in straight lines and calculates the way in which they change direction (deviate) when encountering different optical phenomena.
  • When the normal is drawn on a ray diagram, it provides a reference against which the amount of deviation of the ray can be shown.
  • The normal is always drawn at right angles to a ray of incident light at the point where it arrives at the boundary with a transparent medium.
  • Expressed more formally, in optics, the normal is a geometric construct, a line drawn perpendicular to the interface between two media at the point of contact. This conceptually defined reference line is crucial for characterizing various light-matter interactions, such as reflection, refraction, and absorption.

The frequency of electromagnetic radiation (light) refers to the number of wave-cycles of an electromagnetic wave that pass a given point in a given amount of time.

The speed (or velocity) of a light wave is a measurement of how far it travels in a certain time.

  • The speed of light is measured in metres per second (m/s).
  • Light travels through a vacuum at 300,000 kilometres per second.
  • The exact speed at which light travels through a vacuum is 299,792,458 metres per second.
  • Light travels through other media at lower speeds.
  • A vacuum is a region of space that contains no matter.
  • Matter is anything that has mass and occupies space by having volume.
  • When discussing electromagnetic radiation the term medium (plural media) is used to refer to anything through which light propagates including empty space and any material that occupies space such as a solid, liquid or gas.
  • In other contexts empty space is not considered to be a medium because it does not contain matter.

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