# What is the difference between a white object and a mirror?

I was taught that something which reflects all the colors of light is white. The function of a mirror is the same, it also reflects all light. What's the difference?

Update:
But what if the white object is purely plain and does not scatter light? Will it be a mirror?

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No, it's an excellent question! Galileo analyzed this question in his Dialogue and uses the scattering answer given below to arrive at the conclusion that the Moon's surface is not like a shiny mirror (as people believed at that time). – recipriversexclusion Apr 6 '12 at 15:08

## 6 Answers

The difference is the direction the light is emitted in. Mirrors 'bounce' light in a predictable direction, white objects scatter light.

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+1 You beat me by a few seconds with a clear explanation in one sentence. – Frédéric Grosshans Dec 15 '10 at 17:03

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@dan, @Frédéric: you both beat me by few seconds but I realized that there is something missing in both your answers, so I posted mine anyway. I hope no one will mind :-) – Marek Dec 15 '10 at 17:10

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@Marek, If you find you're not fastest, most full seems like a reasonable stance. – dan_waterworth Dec 15 '10 at 17:12

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@LifeH2O, I think the problem you have understanding is because of your concept of colour. A colour is a set of frequencies of light. When you see a green object, it's because all of the other frequencies have been absorbed by the object and only green light is re-emitted. With a white object, all frequencies of light are re-emitted. By this definition, pure mirrors are white because they re-emit all frequencies of light. With this in mind I think you may understand some of the answers a little better. – dan_waterworth Dec 16 '10 at 10:19

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Thanks, You should add this detail in your answer. – LifeH2O Dec 16 '10 at 20:36
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A white object reflects the light in all the directions, independently of the original direction. It is called a diffuse reflection. If you shine a beam of light onto a white surface, it is scattered in all the directions.

On the other hand, a mirror reflect the light symmetrically to the input direction, with no scattering. This is what allows us to see "through" the mirror.

Edit to add: "Normal" coloured object all have a diffuse reflection, their colour being given by the mix of wavelength they reflect. To better understand the difference between the two kind of reflections, you can also read the related question : "Why can you have shiny black objects?" (and the answers !)

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The physical surface may not necessarily have to be rough, most materials are actually translucent, i.e. the light penetrates some depth before being scattered. Think of snow. Sunlight usually penetrates several centimeters before being scattered out. So it is not the surface roughness, but the photon by photon variability of the scattering depth (plus many will be scattered multiple times before exiting). Or for a larger example, light being scattered from a fog bank. A highly conductive material, like metal prevents this from happening and reflects a plane wave with a plane wave. – Omega Centauri Dec 15 '10 at 18:31
I think there is some other difference, the thing on the backside of mirror which cause reflection will continue to reflect even if it is not plain. Right? – LifeH2O Dec 16 '10 at 10:02
@Omega: very good point. But you'd still need the surface to be plain. So I am saying this is most important feature. A metallic surface which is not polished will not act as a mirror. This statement can be verified by just looking around. – Marek Dec 16 '10 at 10:16

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@Omega, as for the snow: it has fractal structure visible by a naked eye (if you'll observe the snowflake really closely). So it is definitely bumpy from the point of view of the visible light and therefore the light will be scattered already on the surface, notwithstanding any penetration. Therefore it's not a good counter-example against "roughness criterion" at all. – Marek Dec 16 '10 at 10:19

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@ Omega A real white has to have a rough surface. If You have eg a glossy white laquer, there is a combination of specular reflectance at the surface and diffuse from "inside" the laquer. Take for example practical white standards: some powder (often MdO) is compressed against a glas plate, the latter is then removed. This MgO-"tablet" then is used as a reference when measuring "whiteness" of paper or pulp. – Georg Nov 14 '11 at 20:18

Other answers are correct except that they forget to mention why white objects behave differently than mirrors. Well, generic white objects have very rough bumpy surfaces. This causes scattering.

On the other hand, mirror has a very even and polished surface so that the simple law for specular reflection of incoming rays holds.

Of course, you can also obtain any kind of roughness in between. Actually, you can produce a mirror by starting with a generic white object and polishing it.

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People say I'm a generic white object. Can I become a mirror? – Mark Eichenlaub Dec 15 '10 at 17:23
@Mark: I hate these trick questions :-) – Marek Dec 15 '10 at 18:27
You mean if the white object is purely plain like a mirror, it will act like a mirror? – LifeH2O Dec 16 '10 at 10:01

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Well, the question doesn't make sense because such an object wouldn't be white anymore. But if you mean white as "reflective in visible range" then my answer is yes. And to clarify: plain from the point of view of the visible light (that is on the scale of microns). It will still be rough on the atomic level, of course. – Marek Dec 16 '10 at 10:11

To complicate matters, a reasonably smooth flat surface, white or otherwise, becomes a mirror at grazing incidence (the angle of incidence, and the angle of reflection, are close to 90$^\circ$ from the normal direction). This effect often finds application in focusing x-rays, but it works at visible and other wavelengths too. You can see this by putting your eyelevel just above a desk or table, and looking for the reflection of an object placed a cm above the surface at a distance of a few tens of cm away.

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I think that maybe its not really on how smooth the surface is because white paint can be as smooth and yet not act like a mirror. Then again snow acts like shattered glass where broken individual snowflakes reflect sunlight making snow look white. It's mainly in the absorption spectrum or the molecular/bonding structure that determines the color of an object, and maybe how the object scatters light, which the electron levels either absorb, reflect, transparent, and/or refract a little bit, some, most, and/or all light. I do think that maybe white objects reflect and scatter white light in all and random directions while mirrors reflect all light in an evenly straight direction. But I do notice this. Let's say white light coming from the original light source, such as the sun or a light bulb, comes into a plant. The chlorophyll in the leaves will absorb most of the colors in the light and will reflect and scatter mainly green light. When the green light reaches our eyes we see the plant that it is green. If the plant is next to a mirror then the mirror will only reflect the green light from the plant in an evenly straight direction and once it reaches our eyes then we see another image of the green plant in the mirror. If the plant is next to a white object such as a white wall then the white object may absorb the green light reflecting out of the plant, unlike a mirror, but the white object will also reflect and scatter white light from the original light source. So we see the wall as white but we don't see another image of the green plant in the wall. Maybe there are other ways that a mirror is different from a white object but I hope I answered the question.

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White color is associated with reflected (not absorbed) light. White paint usually includes a titanium oxide component, whose absorption is in UV.

The difference between a shiny surface and a Lambertian surface is its roughness, if light is reflected in a collective manner it look shiny. Any smooth surface is shiny given a grazing angle.

A mirror is usually coated with a metal surface, silver for example. Silver's metallic behavior makes most of the light reflected back (about %95 percent reflection, which is why it has a greyish tint), however it becomes transparent for UV light.

A mirror can be engineered to have more reflectance than silver by using multi-layer reflection and interference, using dielectric materials instead of metal. They are called dielectric mirrors.

To sum up, white color is due to non absorbed white light, reflection is due to non transmitted light.

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