An excellent question! The main reason is due to inefficiencies of the light. In general, inefficiency emerges as heat in any system that isn't perfect (e.g., your car engine is also really inefficient). What exactly this inefficiency is depends on the type of lightbulb you are talking about.

Lighting has evolved a lot in the past few hundred years ( see timeline here ). You are probably most familiar with lighting technologies from the incandescent light onward, so we will talk mostly about those.

An incandescent lamp works through running a very high current through a metal filament (usually tungsten due to its high melting temperature). Essentially, you heat up the metal filament so hot that it glows. Heat comes from the fact that the filament has a finite resistance. It's for similar reasons your computer gets hot. You can imagine this is not a very efficient lightbulb.

A fluorescent tube light works a little differently. Inside is mercury vapor. When electrical current is run through the tube light, it excites electrons in the mercury vapor that leads to fluorescence, i.e., it will glow. The inefficiency comes from the fact that the light you see doesn't come directly from the mercury inside. In fact the light from the mercury vapor happens in the UV (the same for what you use sunscreen for) and needs to be converted to a lower energy of wavelength of light that we humans can see. This is done with the use of a material called a phosphor. The energy difference between the UV light that is absorbed by the phosphor and the emitted visible light is what causes the light bulb to heat up. These are still more efficient than incandescent lightbulbs, but mercury is a very toxic chemical, so proper disposal is challenging.

The next generation (and commercially available!) is known as solid-state lighting where solid-state semiconductors are used in the lightbulb, instead of gases. The most well-known is the known as a light-emitting diode, or LED. LEDs are made of the same materials that your computer (or any other electrical circuit) is made out of. The LED is a form of solid-state lighting, which means that there are no gases or fluids like there is in older forms of lighting. Unlike an incandescent light or fluorescent light, and LED lightbulb will be much cooler to the touch. However, there are still inefficiencies that many researchers here at UCSB are working hard to improve!

Fun fact: UCSB is a world leader in research for solid-state lighting. The 2014 Nobel Prize in Physics went to our own Prof. Shuji Nakamura for his groundbreaking work on the blue LED! Hope this helps,

If you use light to heat something, for example, the sun heating the Earth, this is called "radiation," which is one type of energy transfer.

Radiation means that light hits an object and gets absorbed in the object's atoms, which are the tiny building blocks that make up the object. These atoms get excited when they absorb light. Atoms don't like to be excited, so they relax by releasing their energy in the form of heat.

Getting heat from lights is a problem, because we only want to get light from lights, and the heat is wasted energy, unless it's cold, and you want the heat, too.

New light bulbs like the curly Compact Fluorescent Lights and LED lights don't give much heat, compared with ordinary old incandescent light bulbs.

The amount of heat and light all depends on something called the wavelength of the energy that comes out of the light bulb. The wavelengths we see are red, orange, yellow, green, blue, and violet. The wavelengths shorter than violet are called UV - ultraviolet - and they give us sunburn. The wavelengths longer than red are called IR - infrared - and they give us heat.

Ordinary old light bulbs have too many of the wavelengths that give us heat. New kinds of light bulbs give us mostly the wavelengths we see, which we call 'Light' [red, orange, yellow, green, blue, violet].

Lights make light because they are so hot inside that the heat has to come out as light.

Light carries energy, so when it hits something, it gives some energy to that thing. When light hits an object, some of that energy will raise the temperature of that object, which causes heat. The energy of light can do more than just heat things though. When light hits a solar panel, that energy gets turned into electricity. And when light hits the leaf of a plant, the energy from the light gets used to make food for the plant. One of the reasons we get heat from lightbulbs is that they aren’t perfect at turning electricity to light, and some of the electricity gets turned into heat.