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What is Refraction?

pencil in a glass of water demonstrating refraction of light

Refraction of Light Simply Explained

Refraction is responsible for bending light as it passes from one translucent or transparent medium to another.  Dispersion is the phenomenon that splits light into its component colours.

Refraction and dispersion are 'optical properties' or 'optical phenomena'.  They affect the way light interacts with a gemstone and play a crucial role in determining colour or perceived colour. 

Some optical properties such as lustre, colour and transparency (diaphaneity) can be easily seen.  Others are not so visible to the naked eye.

Refraction can be difficult to understand but this two minute video explains the basic concept very well.

With regards to colour, when light hits the surface of an object some is absorbed, the rest is reflected.  The reflected light is the colour we see.

The colour is not in the object itself, it's in the light that's shining on it and reflecting to our eyes.  

The light being reflected will form part of the colour we see.  That's why some colours can look different depending on the type of light.

Light that's absorbed by an object never reaches our eyes.  When no light is reflected we only see black or grey. 

Light is made up of wavelengths of individual colours.  White light also known as colourless light is made up of red, orange, yellow, green, blue, indigo and violet.  To the human eye the combined colours look white.  Daylight is white light.

Different materials absorb or reflect certain wavelengths of colour.  The gemstone peridot absorbs all colours except green.  With green being reflected that's the colour we see.

Look at peridot in a darkened room and our eyes perceive it to be black or grey.  That's because no light is being reflected. 

A black mineral or gemstone looks black because no wavelengths of colour are being reflected.

If a gemstone reflects all wavelengths of colour equally, we perceive it to be colourless or white.  That's because it's reflecting the same colours as the light that's hitting it.

The colour we see in a gemstone is dependent on the way light bends and separates as it passes through.

Light moves more slowly as it passes through a denser substance.  For example when it travels from air into water, through glass or through a gemstone.

When light changes speed it changes direction or in other words it bends.  The change in direction or "bending of light" as it passes from one medium to another is refraction.

A simple way to explain this phenomenon is with a pencil in a glass of water.  At the boundary where the two mediums meet the pencil appears to be misaligned.

This is because light is changing speed and bending as it moves from air into water.  It then continues in a straight line.pencil in a glass of water demonstrating refraction of lightA prism is a three dimensional transparent object with flat sides.  At least two sides have the same size and shape.  A prism can be a triangle, cube, hexagon or an octagon.

A prism works because the combined colours of white light slow down and travel through the object instead of being reflected back to our eyes.

When white light enters the prism it undergoes dispersion.  This means the light is separated into its component colours (red, orange, yellow, green, blue, indigo and violet).  Each colour has a different wavelength. 

As each one passes through the prism it bends or refracts at a slightly different angle depending on its wavelength. Red bends the least whilst violet bends the most.

This bending of light (refraction) causes the colours to spread out forming a spectrum of colours.

As the colours exit the prism they continue to travel in their separated paths.  This enables us to see each colour distinctively. 
a prism showing light passing through and coming out the other side as separate coloursThe same thing happens when we see a rainbow.  Each raindrop acts as a tiny prism that disperses light and reflects it back to your eye.

The splitting of light into its seven component colours is dispersion.  

For refraction and dispersion to occur, light must enter the object (or medium) at an angle and not head-on.  When light travels from one transparent medium to another the change in speed and direction or "bending of light" happens at the boundary between the two.  Think back to the pencil in the glass of water.

It's preferable to use the word "medium" because although refraction takes places as light passes through objects, it also happens when it passes from one "medium" to another.  An example being from air to water. 

If light approaches the boundary head-on there won't be any change in direction so no refraction/dispersion will take place.

This phenomenon is why we often observe striking visual effects when light passes through transparent objects like prisms, lenses and glass surfaces.

The bending of light due to refraction is what enables these objects to produce interesting optical effects.  Refraction plays a significant role in various aspects of everyday life and science.
an example of iridescence formed by a clear container on a kitchen worktopAnother example of refraction of light is a raindrop.  Raindrops create rainbows because each drop of water acts like a prism.  The rounded shape of the rainbow is caused because the prism (the raindrop) is also round.a rainbow with a subtle second rainbow just next to it

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