Heat and Light

We have a set of plates painted in different colours. When exposed to sunlight for a reasonable amount of time they absorb energy and heat up. On touching them, you will find that the white plate is the coolest while the black is the hottest. Compare the degree of hotness among the plates of other colours. The amount of energy absorbed and radiated is decided by the colour. White is a good reflector while black is a good absorber of heat. Among other colours there is a gradation of temperature.

There are six plates of equal area and thickness, all , made out of the same material and mounted adjacent to one another with the same inclination to the ground.

The only difference between them is the colour with which they are painted. The colours used here are black, red, green, blue, yellow and white.

On exposing all of them to the sunlight for a reasonable time, we can touch them and notice that they are all heated up to different extents with black plate being the hottest and white plate being the coldest.

Why should it be so when all other parameters except the colour are the same for all the plates ?

Certainly, the colour has something to do with the rise in the temp of a painted plate. The colour of the paint what we see depends on what wavelength of the visible light is reflected by the pigment present in the paint.

This Visible light is that portion of the sunlight to which our eyes are sensitive. But the radiations from sun also comprises of Infra red radiations to a large extent (almost 50%) and Ultra Violet radiations. Infra red radiations cannot be seen but they cause heating effects.

When sunlight is incident on a painted surface, there are two factors which decides the amount of absorption of the solar radiation.

The pigment in the paint : If the pigment reflects blue and absorbs all other wave lengths in the visible range, we see it as blue. Energy of the radiation is inversely proportional to the wavelength. Blue has shorter wavelength and therefore higher energy. If blue is reflected then the energy absorbed in the visible range by the pigment is lesser compared to a pigment which reflects any other longer wavelength. The energy so absorbed by the pigment is partly transmitted to the metal sheet and partly re-emitted in the form of Infra red radiations (i.e., heat)

Since sunlight consists of IR rays & UV rays other than the visible light , the solar reflective index which is a measure of how much solar heat is reflected by a material also plays an important role here. Larger the solar reflective index lesser will be solar heat absorbed. These two factors decides the extent to which a painted metal plate gets heated up.

Black is a good absorber [ it reflects only 5% of the visible sunlight ] where as white is a good reflector [ it reflects almost 80% of the visible light ].

The light reflective index which is a measure of how much visible light is reflected by the pigment of the paint has a value greater than that of black & lesser than that of white for all other colours.

Therefore the plate painted with black will be the hottest and white will be the coolest and the plates painted with other colours will be having intermediate temperatures.