# Why an Object Appears White

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This is one of a set of 10 diagrams exploring why objects appear to be different colours to an observer.

There are always three key factors that affect the colour of an object.

• The first is the type of light source and what happens to the light on its journey towards an object.
• The second is what happens when light strikes different types of objects and materials.
• The third concerns factors related to the observer which affect what they see and how things appear.
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## Description

#### Why an Object Appears White

###### TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
Yes! Every observer has a unique view of the world because: Each one of us sees the world from a different physical location and so has a unique point of view Every one of us has different life experiences including educational, social and cultural factors that affect how we see the world.
Because different wavelengths of light are reflected off the surface of objects. Every surface has unique properties.
A human observer is a person who watches something from their own unique point of view.

This is one of a set of 10 diagrams exploring why objects appear to be different colours to an observer.

There are always three key factors that affect the colour of an object.

• The first is the type of light source and what happens to the light on its journey towards an object.
• The second is what happens when light strikes different types of objects and materials.
• The third concerns factors related to the observer which affect what they see and how things appear.

When light strikes an object at least one of the following things happens:

• Reflection. The term reflection refers to a situation where light strikes the surface of an object and some wavelengths are obstructed and bounce off. If the surface is smooth, light is reflected away at the same angle as it hits the surface. The term reflection refers then to what happens to wavelengths of light that are neither absorbed (by an opaque medium) nor transmitted (through a transparent medium).
• Scattering. Scattering takes place when light waves are reflected in random directions at the boundary between two media. Scattering can also take place when light strikes particles or other irregularities within a medium through which light propagates.
• Absorption. When light strikes an opaque medium the wavelengths that are not reflected are absorbed and their energy is converted to heat.
• Transmission. In optics, transmission refers to the passage of electromagnetic radiation through a medium.
• Refraction. The term refraction refers to the way a light wave changes direction and speed as it travels from one medium to another.
• Dispersion. Chromatic dispersion refers to the way that light separates into its component wavelengths and the colours corresponding with each wavelength become visible.

To make sense of these diagrams it’s important to be clear about the following:

Light sources:

• In the context of a discussion of visible light, a light source is a natural or man-made object that emits a range of wavelengths – each corresponding with a different colour.
• The light source in these diagrams is the Sun.
• Sunlight includes all visible wavelengths. The diagrams explore what happens to the wavelengths of ROYGBV. An arrow is used to represent each incident ray.
• The diagram uses coloured balls to represent the object the incident rays strike. The wavelengths which are absorbed, reflected or transmitted (through the transparent ball) are shown by arrows.
• The observer is represented by an eye and the colour of the object as seen by the observer is shown in a thought bubble.
• Use the text to check exactly which colours are absorbed, reflected or transmitted.

Objects:

• Every object, material, medium or substance that we can see is made of matter of one kind or another. The key differentiating factor is the elements and molecules they are constructed from.
• You will have come across the elements that make up the periodic table.
• A close look at molecules reveals that they are made up of atoms composed of electrons surrounding a nucleus of protons and electrons.
• In a nutshell, different elements and molecules react to light in different ways because of their atomic structure and the particular way they combine to form mixtures or compounds.
• In the case of an opaque object, it is the molecules that form its surface that determine what happens when light strikes it. Translucent and transparent objects behave differently because light can travel through them.
• Another factor that needs to be taken into account when light strikes an object is surface finish. A smooth and polished surface behaves differently from one that is rough, textured or covered in ripples.

Observers:

• There are many different factors that can influence an observer’s colour perception.
• Perhaps the most important factor is the colour of nearby objects.
• Another important factor is to do with the well-being of an observer. Health, medications, mood, emotions or fatigue can all affect the eye, vision and perception.
• A further factor is the environment in which colours are observed, the type of object and colour associations.
• Two different observers may see colour differently because of life experience including educational, social and cultural factors.

Other useful facts:

• Material: The matter from which a thing is or can be made.
• Material thing: Something formed or consisting of matter.
• Matter: (in physics) That which occupies space and possesses mass as distinct from energy.
• Medium: A substance that carries a wave (or disturbance) from one location to another.
• Object: a material thing that can be seen and touched.
• Substance: a particular kind of matter with uniform properties.

#### Some key terms

Reflection takes place when incoming light strikes the surface of a medium, obstructing some wavelengths which bounce back into the medium from which they originated.

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

If the reflecting surface is very smooth, the reflected light is called specular or regular reflection.

Specular reflection occurs when light waves reflect off a smooth surface such as a mirror. The arrangement of the waves remains the same and an image of objects that the light has already encountered become visible to an observer.

Diffuse reflection takes place when light reflects off a rough surface. In this case, scattering takes place and waves are reflected randomly in all directions and so no image is produced.

• A light source is a natural or man-made object that emits one or more wavelengths of light.
• Natural light sources include:
• The Sun is the most important natural light source in our lives and emits every wavelength of light in the visible spectrum.
• Celestial sources of light include other stars, comets and meteors.
• Other natural sources of light include lightning, volcanoes and forest fires.
• There are also bio-luminescent light sources including some species of fish and insects as well as types of bacteria and algae.
• Man-made light sources of the most simple type include natural tars and resins, wax candles, lamps that burn oil, fats or paraffin and gas lamps
• Tungsten lights: These are a type of incandescent source which means they radiate light when electricity is used to heat a filament inside a glass bulb.
• Halogen bulbs: These are more efficient and long-lasting versions of incandescent tungsten lamps and produce a very uniform bright light throughout the bulb’s lifetime.
• Fluorescent lights: These are non-incandescent sources of light. They generally work by passing electricity through a glass tube of gas such as mercury, neon, argon or xenon instead of a filament. Fluorescent lamps are very efficient at emitting visible light, produce less waste heat, and typically last much longer than incandescent lamps.
• LED lights: An LED (Light Emitting Diode) is an electroluminescent light source. It produces light by passing an electrical charge across the junction of a semiconductor. An LED light typically emits a single colour of light which is composed of a very narrow range of wavelengths.
• Made-made lights can emit a single wavelength, bands of wavelengths or combinations of wavelengths.

The perception of colour by an observer results from properties of light that are visible to the human eye. The visual experience of colour is associated with terms like red, blue and yellow.

• The observation of colour depends on:
• The range and intensity of wavelengths of visible light emitted by a light source, and the various media and materials it encounters on its journey to the retina of a human eye
• Optical phenomena such as absorption, dispersion, diffraction, polarization, reflection, refraction, scattering and transmission.
• Predispositions of an observer, such as their personal and social experience, health and state of mind.

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.

• The wavelength of an electromagnetic wave is measured in metres.
• Each type of electromagnetic radiation, such as radio waves, visible light and gamma waves,  forms a band of wavelengths on the electromagnetic spectrum.
• The visible part of the electromagnetic spectrum is composed of the range of wavelengths that correspond with all the different colours we see in the world.
• Human beings don’t see wavelengths of visible light, but they do see the spectral colours that correspond with each wavelength and the other colours produced when different wavelengths are combined.
• The wavelength of visible light is measured in nanometres. There are 1,000,000,000 nanometres to a metre.

Sunlight is light emitted by the Sun and is also called daylight or visible light.

• Sunlight is only one form of electromagnetic radiation emitted by the Sun.
• Sunlight is only a very small part of the electromagnetic spectrum.
• Sunlight is the form of electromagnetic radiation that our eyes are sensitive to.
• Other types of electromagnetic radiation that we are sensitive to, but cannot see, are infrared radiation that we feel as heat and ultraviolet radiation that causes sunburn.