White objects appear white because they reflect most visible light wavelengths and absorb little. When white light (containing all visible wavelengths) shines on a white surface, almost all of the light is reflected back without being absorbed. This is why white objects look bright – they are reflecting a lot of light into your eyes.
However, there is more to the story than just visible light. The light we see is only a small part of the electromagnetic spectrum. There are many other wavelengths of electromagnetic radiation that our eyes cannot see, such as ultraviolet light, infrared light, microwaves and radio waves. So while white objects reflect most visible light, what do they do with these other types of radiation?
Infrared radiation is emitted by all objects based on their temperature. Hotter objects emit more infrared than cooler objects. Our eyes cannot see infrared light, but we can feel it as heat. Infrared radiation is the primary way that heat transfers between objects.
When infrared radiation shines on a white surface, the majority of it is reflected back rather than absorbed. This means white objects generally stay cooler in sunlight than darker colored objects which absorb more infrared.
For example, if you wear a black shirt outside on a sunny day, it will absorb a lot of the infrared radiation from the sun and feel hot. But if you wear a white shirt, it will reflect most of that infrared and stay cooler.
This effect is very apparent for automobiles. Darker colored cars left in the sun get much hotter inside than lighter colored cars. The dark interiors absorb a lot of infrared radiation and reach scorching temperatures. White car interiors mostly reflect it and remain closer to the ambient temperature.
So in terms of infrared radiation, the majority is reflected by white objects rather than absorbed. White stays cooler because it does not readily absorb heat from infrared.
Visible Light Absorption
As mentioned earlier, white objects reflect nearly all visible light wavelengths. However, there is a small amount of light that white materials will absorb rather than reflect.
What makes something appear white is that it reflects light relatively evenly across the visible spectrum. But there are small variations in the reflectivity for different wavelengths. Most whites absorb a touch more in the blue/violet end of the spectrum.
This is why white objects can take on a very slight yellowish or brownish tint over time. The blue wavelengths get absorbed a bit more than other colors. This is often seen in white paints, plastics, paper and fabrics that are exposed to sunlight for long periods. They gradually yellow as the blue frequencies get absorbed.
So white reflects almost all visible light, but absorbs just a tiny fraction more in the bluish range. This gives aged whites a slight yellow/brown appearance. But the absorption is very minimal compared to darker colors.
Light from the sun contains a significant amount of ultraviolet (UV) radiation. This is categorized into three wavelength ranges:
- UVA – Not absorbed much by the atmosphere or skin. Penetrates deeply and causes aging/wrinkling of skin.
- UVB – Mostly absorbed by the atmosphere but some gets through. Causes sunburns and skin cancer.
- UVC – Completely absorbed by the atmosphere before reaching earth. Germicidal.
Most white pigments and materials reflect nearly all UVA and UVB wavelengths. Some nanoparticle white pigments even reflect up to 99% of UV radiation. On the other hand, many colored pigments and dyes absorb high amounts of UV.
Over time, UV exposure causes colored materials to visibly fade, whereas whites show exceptional resistance. For example, the vivid colors of paints, fabrics and plastics will weather and grow dull after prolonged sunlight exposure. But white versions of the same materials retain their bright appearance much longer against UV degradation.
So in summary, white strongly reflects nearly all wavelengths of ultraviolet radiation. It withstands UV exposure better than most colors.
Microwaves occupy a portion of the electromagnetic spectrum with longer wavelengths than visible light or ultraviolet. Their wavelengths range from about 1 millimeter to 1 meter.
Microwaves have the ability to penetrate and pass through some materials that are opaque to visible light. For example, microwave radiation can pass through plastic, paper and ceramic materials. This allows microwave ovens to heat food inside containers.
White materials like plastics, paper and ceramics reflect microwaves rather than absorbing them. This is why microwave-safe containers are often white or light colored. Darker containers absorb more microwaves and become hotter, potentially leaching chemicals into food.
Metals, on the other hand, reflect microwaves exceptionally well. This includes metals like aluminum that are coated white. Metals keep microwave radiation confined inside the oven cavity and prevent exposure.
So white materials such as plastics, paper and ceramics reflect some microwave radiation but not nearly as well as metals. Of all materials, metals are the best microwave reflectors.
Radio waves have even longer wavelengths than microwaves, ranging from 1 millimeter to over 50 kilometers. They are produced by radio transmitters and used for radio/television broadcasts, mobile phones and wireless networks.
Most everyday materials are essentially transparent to radio waves. Whether a material is white or any other color makes little difference – radio waves pass right through with minimal reflection or absorption.
The exception is metals. Due to their electrical conductivity, metals interact with radio waves differently than other materials. Very thick metal can reflect lower radio frequencies. This property is utilized in Faraday cages which shield devices from external radio interference.
So in summary, white objects do not show significant reflective or absorptive properties toward radio waves, except for thick metal surfaces. Radio waves mostly pass through unimpeded regardless of an object’s color.
To summarize the various behaviors:
- Infrared radiation (heat energy) is strongly reflected by white rather than absorbed.
- Visible light is almost entirely reflected by white, with a very small amount of blue/violet absorption.
- Ultraviolet radiation is very strongly reflected by white with little absorption.
- Microwaves are moderately reflected by white materials such as plastic, paper and ceramic.
- Radio waves pass through white objects with minimal reflection or absorption.
So white tends to reflect most wavelengths along the electromagnetic spectrum, from infrared to ultraviolet and microwaves. The exceptions are visible light which is almost completely reflected, and radio waves which pass through unaffected.
This high degree of reflectivity across many types of radiation is why white objects stay cooler in the sun compared to darker colors. By reflecting heat energy rather than absorbing it, white materials like paints, roofs and car interiors reduce heating from the sun.
White is especially effective at reflecting and shielding against ultraviolet radiation. This protects white surfaces from UV damage and fading over time. That’s why white paint holds up better to sun exposure compared to darker paint colors.
In summary, lighter white colors tend to reflect heat, visible light and UV radiation across a wide range of wavelengths. This keeps white objects cooler and provides protection from solar damage compared to materials that absorb more radiation. So when considering heating and cooling, white tends to be the most reflective major color on the spectrum.
|Radiation Type||White Behavior|
|Infrared||Reflects most, absorbs little|
|Visible Light||Reflects almost all, absorbs trace blue/violet|
|Ultraviolet||Reflects most, absorbs very little|
|Radio Waves||Neither reflects nor absorbs significantly|