What is Refraction?

Refraction of Light 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 how light interacts with a gemstone and play a crucial role in determining its colour or perceived colour.

Some optical properties, such as lustre, colour, and transparency (diaphaneity), can be easily seen. Others are less visible to the naked eye.

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

With regard to colour, when light hits the surface of an object, some is absorbed, and 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 shining on it that's being reflected back 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.

The 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.

When looking at peridot in a darkened room, our eyes perceive it as 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 reflects the same colours as the light hitting it.

The colour of a gemstone depends on how 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 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 called refraction.

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

That's because light changes speed and bends as it moves from air into water. It then continues in a straight line.A prism is a three-dimensional transparent object with flat sides. At least two sides are the same size and shape. A prism can be a triangle, cube, hexagon or 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, which are red, orange, yellow, green, blue, indigo, and violet. Each colour has a different wavelength. 

As each colour passes through the prism, it bends or refracts at a slightly different angle depending on its wavelength. Red bends the least, 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 separate paths, which enables us to see each one individually. 
The 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. Raindrops create rainbows because each drop of water acts like a prism. The rounded shape of the rainbow is caused by the prism (the raindrop) also being round.

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 occurs when light passes through objects, it also happens when it passes from one 'medium' to another, such as from air to water. 

If light approaches the boundary head-on, there will be no change in direction, so no refraction/dispersion will occur.

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 enables these objects to produce interesting optical effects. Refraction plays a significant role in various aspects of everyday life and science.

Article Photos

The peridot in the first pop-up photo, and the double rainbow photo, which is clickable, are courtesy of Stan Celestian. 

The photo of the granite worktop was taken in my kitchen.