# The Naked Scientists Forum

### Author Topic: What makes objects have certain colours?  (Read 1553 times)

#### thedoc

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##### What makes objects have certain colours?
« on: 12/05/2015 04:50:02 »

I'm a twelve year old girl and we recently learned about colours and white light and i found this topic very interesting. Thirsty for more information I was wondering whether you could answer this question for me: How do objects know which colours to absorb and which colours to reflect ?

I found your website quite interesting and useful...

What do you think?
« Last Edit: 12/05/2015 04:50:02 by _system »

#### Colin2B

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##### Re: What makes objects have certain colours?
« Reply #1 on: 12/05/2015 07:57:20 »

I'm a twelve year old girl and we recently learned about colours and white light and i found this topic very interesting. Thirsty for more information I was wondering whether you could answer this question for me: How do objects know which colours to absorb and which colours to reflect ?

I found your website quite interesting and useful...

What do you think?
Were you told that light can be described as waves or vibrations in the same way as sound?

With sound we can talk about it's wavelength for example on a piano the low notes have a long wavelengths and the high notes a short wavelength. This is similar to the waves on the sea, the big rollers have a long wavelength - distance between the high peaks, whereas ripples have a short distance between peaks, short wavelength. The colours of light also have different wavelengths, red is longer than blue.

It helps if I describe an example with sound. If you have a violin or guitar try singing into the sound hole with your finger lightly touching the bridge. If you start with a low note and slide the notes higher you will notice some notes are making the strings vibrate and the instrument will start to make the same sound you are singing - the note is reflected back to you

A similar thing happens when light hits a surface, but in this case it's not the strings that move it's the electrons in the atoms. If they move when hit by a certain colour eg red we will see that colour just as you could hear the note of the string. We say that colour is reflected.
Some atoms don't reflect the colour but absorb the energy in the light so those colours aren't reflected. If an object absorbs all the colours of the spectrum (rainbow) it will appear black, if it reflects them all it will appear white.

I hope that helps, it sounds as though you are really  interested in finding out about how things work.
« Last Edit: 12/05/2015 08:19:56 by Colin2B »

#### evan_au

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##### Re: What makes objects have certain colours?
« Reply #2 on: 12/05/2015 12:25:08 »
Wikipedia is a good place to start: http://en.wikipedia.org/wiki/Color

I have heard it said that there are about a dozen different ways to produce colours (it's a pity I can't remember them all, but I'll give it a try...)
• Hot things glow, like the sun, or a candle. This produces many colours, all mixed together, that we call "white light". Objects that are at different temperatures have different mixes of colours. Surprisingly, this is called "black body radiation", because even something that is black like soot will glow if you get it hot enough. Different stars have different colours.
• Dyes and pigments absorb certain colours from sunlight, and the non-absorbed ones are reflected, giving the sensation of colour to your eye (this is what Colin was describing)
• Rain or a glass prism can separate light from the Sun into its many colours to form a rainbow (without absorbing any of them)
• Small dust particles in the atmosphere scatter short wavelengths strongly, allowing the longer wavelengths to pass through, giving us a blue sky in the day, and oranges and reds at sunrise and sunset.
• A soap bubble or a film of oil on water produce colours because the thickness of the film is close to the wavelength of light. Some colours pass through, other colours are reflected, and some are absorbed
• If you crunch some hard sugar candies in a dark room, you get a flash of light. Quickly peeling sticky tape or postal envelopes (remember them) can produce the same effect.
• Some materials have a crystal structure which is close to a specific wavelength of light, reflecting a single colour intensely, and absorbing all the rest. Some butterfly wings work this way.
• Fluorescent materials absorb light at one frequency, and then later emit it at a different frequency. Fluorescent lights and many white LED lights use this effect.
• Electrons dropping into place in an atom release light of certain specific frequencies, depending on the material. Red, Green and blue LED lights generate light like this, as do the yellow street lights still used on some highways.
• Lasers store energy temporarily, and then release it when other light arrives, with precisely the same frequency. This produces light of a single, very pure colour, and is used in CD players, DVDs and the internet.
• The blue glow in a nuclear reactor comes from tiny particles which move very quickly, and exceed the normal speed of light in water. Warning: Don't try this one at home!
• Fast-moving electrons bent in a magnetic field produce light. This effect is used in your microwave oven at home.
• Radioactive materials can produce "invisible" gamma rays (see next post on "invisible" colours). Don't try this one, either!

Some sources of colour use a mixture of the methods listed above, for example stained-glass windows use a combination of pigments and scattering.
« Last Edit: 12/05/2015 18:33:00 by evan_au »

#### evan_au

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##### Re: What makes objects have certain colours?
« Reply #3 on: 12/05/2015 18:29:36 »
As well as the colours you can see, there are many "invisible" colours that you can't see; some of them you can feel. This large range of colours was predicted by a Scotsman, James Clerk Maxwell, 150 years ago.

In a rainbow, there are colours just off each end of the rainbow, where it looks like there is no colour. But scientists discovered and named:
• Infra-Red, just past red in the spectrum. You can feel this as heat given off by a radiator at the other end of the room. Some snakes use infra-red to find their prey at night, and is used in many TV remote-controls.
• Ultra-Violet, just past violet in the spectrum. This is sometimes used for killing bacteria, and making things glow in the dark at a theme park. You can feel the effects of the Sun's ultraviolet after a day outdoors, as the itch of sunburn; this indicates that the ultraviolet has damaged your skin, so use a protective sunscreen!

There is now a whole spectrum of invisible "light" which we use for a variety of purposes:
• Microwave Ovens
• X-Rays to examine your teeth for cavities
• Gamma rays used to detect healing bones that don't show up on X-Rays

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##### Re: What makes objects have certain colours?
« Reply #3 on: 12/05/2015 18:29:36 »