Reflection & Refraction – 90 deg to 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 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.
- In this diagram, the incident ray of light approaches the boundary between air and glass at an angle of incidence is 00.
- The reflected ray bounces off the surface in line with the incident ray, at a 00 angle to the normal.
- The refracted ray crosses the surface in line with the incident ray, at a 00 angle to the normal.
Description
Reflection & Refraction - 90 deg to Boundary
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About the diagram
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:
Overview of this page
- This page provides an introduction to a situation in which both reflection and refraction take place.
- 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.
Partial 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 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.
- In this diagram, the incident ray of light approaches the boundary between air and glass at an angle of incidence is 00 to the normal.
- The reflected ray bounces off the surface in line with the incident ray, at a 00 angle to the normal, and retraces its path.
- The refracted ray crosses the surface in line with the incident ray, at a 00 angle to the normal.
- In this unique case, refraction produces a change in speed but does not produce a change in direction.
Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels from one transparent medium to another.
- In all other cases, as incident light undergoes refraction as it travels from a faster medium to a slower medium it bends toward the normal.
- In this case, in which the incident light approaching the boundary is already in line with the normal, at an angle of incidence is 00, the change in direction is also 00.
- As light travels across the interface between different media and undergoes refraction it changes speed.
- This case is no different. Because the optical density of the glass is greater than that of air, the speed of light decreases.
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
Incident light refers to light that is travelling towards an object or medium.
- Incident light refers to light that is travelling towards an object or medium.
- Incident light may come from the Sun, an artificial source or may have already been reflected off another surface, such as a mirror.
- When incident light strikes a surface or object, it may be absorbed, reflected, refracted, transmitted or undergo any combination of these optical effects.
- Incident light is typically represented on a ray diagram as a straight line with an arrow to indicate its direction of propagation.
Optical density is a measure of how much a material resists and slows the transmission of light.
- The higher the optical density of a material, the slower light travels through it.
- The lower the optical density of a material, the faster light travels through it.
- A vacuum is not a medium and has zero optical density.
- Light travels through a vacuum at the maximum possible speed of light which is 299,792 kilometres per second.
- Optical density and refractive index are related properties.
- In general, materials with higher optical density tend to have higher refractive indices and vice versa.
- The greater the difference in refractive index between two materials, the more they will bend light when they come into contact.
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.
- See this diagram for an explanation: Reflection of a ray of light
- In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
- If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.
In physics and optics, a medium refers to any material through which light or other electromagnetic waves can travel. It’s essentially a substance that acts as a carrier for these waves.
- Light is a form of electromagnetic radiation, which travels in the form of waves. These waves consist of oscillating electric and magnetic fields.
- The properties of the medium, such as its density and composition, influence how light propagates through it.
- Different mediums can affect the speed, direction, and behaviour of light waves. For instance, light travels slower in water compared to a vacuum.
- Examples of Mediums:
- Transparent: Materials like air, glass, and water allow most light to pass through, with minimal absorption or scattering. These are good examples of mediums for light propagation.
- Translucent: Some materials, like frosted glass or thin paper, partially transmit light. They allow some light to pass through while diffusing or scattering the rest.
- Opaque: Materials like wood or metal block light completely. They don’t allow any light to travel through them.
The angle of refraction measures the angle to which light bends as it passes across the boundary between different media.
- The angle of refraction is measured between a ray of light and an imaginary line called the normal.
- In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
- See this diagram for an explanation: Refraction of a ray of light
- If the boundary between the media is curved, the normal is drawn perpendicular to the boundary.
If one line is normal to another, then it is at right angles to it.
In geometry, normal (a or the normal) refers to a line drawn perpendicular to a given line, plane or surface.
- How the normal appears in a geometric drawing depends on the circumstances:
- When light strikes a flat surface or plane, or the boundary between two surfaces, the normal is drawn perpendicular to the surface, forming a right angle (90°) with it.
- 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.
- When dealing with curved surfaces, such as those found on spheres or other three-dimensional objects, determining the normal requires a slightly different approach. Instead of simply drawing a line perpendicular to the surface as with a flat plane, draw the normal straight up from the point where light hits the surface.
- When considering a sphere, the normal line passes through the centre of the sphere. This is because, regardless of where light enters or exits the sphere, the normal represents the direction perpendicular to the surface at that point.
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.
- See this diagram for an explanation: Reflection of a ray of light
- In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
- If the boundary between the media is curved, then the normal is drawn at a tangent to the boundary.
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