Visible light is a portion of the electromagnetic spectrum , which is a wave propagating through space and time and is a form of energy called radiant energy.

Like any form of energy, radiant energy can be converted either into heat or work. Light itself is not a form of heat. When light is absorbed by some object, the electromagnetic energy is converted into heat and the temperature of the object increases. Similarly, an object that is hot (for example the filament in an incandescent light bulb or the gas molecules in a star), will emit electromagnetic radiation. The sun heats the earth by emitting electromagnetic radiation, which travels through space and is absorbed by the earth, and it heats up as a result.

As an object becomes hotter, it will emit electromagnetic radiation of different wavelengths. The exact law that describes this is called Planck’s law. For an ideal object (which physicists refer to as a black-body) we can calculate the temperature that an object would need to be in order to emit light of a certain color. When the object’s temperature increases the peak wavelength of the emitted light shifts to shorter wavelengths.

A black-body below about 500 degrees Celsius only emits electromagnetic energy in the infrared region.

At around 500 Celsius objects will begin to emit light of a dim red glow.

Between 1000-1200 Celsius (the temperature of lava) objects appear to glow orange.

At around 5000 Celsius objects appear white, and hotter than 6000 Celsius they appear blue.

However, it is important to realize that the temperature is not the only reason that flames or lights could appear various colors. Usually, the chemistry also plays an important role.

Instead of absorbing electromagnetic radiation as heat, the electrons in the atoms can absorb the energy and jump to a higher energy level. When the electron falls back to its normal energy level, it emits light of a particular wavelength. Each atom has a certain frequency at which it absorbs and emits light and that can be more important than the temperature in determining the color. So different chemicals can burn at different colors even though the temperature of the flame is the same. For example, Barium produces a green flame (seen in fireworks) and the blue flame on a gas stove is caused by the combustion of hydrocarbons.

All objects emit radiation, and the wavelength of the radiation depends on the object's temperature. The earth, for example, emits infrared (long wavelength) radiation because it is fairly cool, but the sun emits bright radiation that we can see. Another example is that if you take a piece of metal and heat it up, it will start glowing when it heats up. This phenomenon is described by Wien's displacement law, which states that the peak wavelength of radiation emitted by an object is equal to a constant (~2900 K µm) divided by the object's temperature in Kelvin.

So, if you wanted to find out the temperature of an object that mostly emits red light, for example, you can solve for temperature:

the wavelength of red light is ~0.7 µm
, so you can divide 2900 K µm by 0.7 µm to solve for the temperature in K (Kelvin).

An object that mostly glows red is therefore about 4140 K, or almost 7000 degrees Fahrenheit!

You can find the wavelength of orange, yellow, green, blue, purple, and pink light to calculate the temperature of an object that emits most of its radiation in the wavelength of these colors.

A black object would need to be at a low enough temperature such that it emits almost no radiation in the visible spectrum. To use the metal analogy again, a piece of cast iron at room temperature looks black, and it starts to glow as it is heated to high enough temperatures.

The heat a color gives off depends on the power of the light, not the color.

Blue light is more energetic than red light , so a certain amount of blue light will make more heat than the same amount of red light, but a bright red light will still overpower a faint blue light. Also, most colored lights are just lights with paint over the bulb. The actual color of the light is something else (usually yellow or white) behind the paint. The paint will absorb light of some colors, which will heat up the paint and the bulb, but still the same amount of heat will be produced in the end.

The whole energy carried by a beam of light depends on both its color, and its "brightness".

The color of light is determined by the energy carried by each photon (quantum of light), and the brightness of light is determined by how many photons that beam of light carries. If we just look at a single photon, then we can rank its energy by its color: purple > blue > green > yellow > orange > red. But I do not think there is "black light" or "pink light" though...

Note from ScienceLine moderator:
The symbol ">" means greater than.

All of the visible light are electromagnetic waves and they can also be described as photons with different frequency (or as wavelength/energy).

The energy of a single photon is
E = hc/lambda,
here h is the Planck constant , c is the speed of light, lambda is the wavelength of light.

Since h and c are known, we end up with
E = 2*10^(-19) J/lambda.
Here lambda is still the wavelength but in the unit of micrometer, J is the unit of energy.

Basically it means a photon with a wavelength of 1 micrometer carries the energy of about 2*10^(-19) J. This is a very small amount of energy. But remember the sun or other light source can give a huge amount of photons, which means non-negligible energy in total.

To talk about the heat that the light gives off, you need to know about absorption and energy conversion. Basically when a photon hits an object, it will either get scattered or absorbed. Assuming the photon is fulled absorbed, then the energy is usually turned into heat energy. So a photon with a wavelength of about 0.7 micrometer (red light) can give the maximum heat energy of about 2.9*10^(-19) J.

Note: First, the red light wavelength is not a single number 0.7 micrometer; it has a range of 0.62 to 0.75 micrometer . This is also true to light with different colors. Second, as mentioned before,unless the photon is fully absorbed, otherwise, the heat energy you get from the light will be smaller than the energy of the photon.

You can use the formula to calculate the energy of other photons with different wavelength. It is good to know that shorter wavelength means higher energy.

The last thing I want to mention is black light. There is no such light as black light An object looks black because it absorbs all of the visible light and so it appears to be black.