Sunlight has a lot of energy. The highest energy waves are called ultraviolet or UV. UV light can pass right through lots of materials, including skin. Unprotected skin cells can be damaged by UV light. It can actually cause mutation in the DNA of cells on the surface of the skin. Our bodies respond to all sorts of damage with something called the inflammatory response, which sends blood to an area that is being damaged. The swelling puts pressure on nerves and receptors, causing pain. Sunburn is painful, but the pain usually starts after the damage is done because the swelling takes some time.

A person with light skin will have red sunburn because of the extra blood going to the area. If a person’s skin is dark, it may look darker. The skin cells may die and peel off later. A more severe sunburn may actually cause blisters. If all goes well, the skin will recover after a few days or weeks. Unfortunately, some of the cells with mutated DNA may become cancerous. That does not mean that sunburn always causes cancer, but more exposure to the sun increases the risk of skin cancer, which may take years to show up.

The skin cells can protect themselves by making a pigment called melanin. The amount of melanin that cells make is responsible for the beautiful diversity of skin color we see in humans. In addition to each person’s normal level of melanin, cells can make more melanin if they are exposed to the sun. When lighter-skinned people get a tan, that is an effort by skin cells to protect themselves. Even people with dark skin can have skin cell damage from too much sun, it just takes more time and/or more intensity of the light.

So why don’t we all have dark skin? Well, people who lived in places where there wasn’t much sun (especially during the winter) did better if they had light skin. Our bones and teeth need calcium to be strong. We usually absorb calcium from food, but we need vitamin D to do that. Our skin cells make vitamin D when they are exposed to the sun.

No sun, no vitamin D, no calcium absorbed, means weak bones. So people living in places closer to the North Pole than the Equator did better if they had less melanin. Anyone who had a random mutation that gave them lighter skin probably had stronger bones and was more likely to survive and have kids. Over time, the genes for lighter skin got more common.

So there’s a trade-off between the benefits of sunlight and the dangers of it. As with a lot of things, a little is good, but too much is bad. There’s a really interesting video on skin color at: this site. It’s made for high school students, but I think you’ll understand it.

Why do you think they put vitamin D in milk?

Sunburns are the damage caused by UV light from the sun. When the UV light enters your skin, it can harm the skin cells and cause them to become inflamed, causing a burn on your skin. The UV light can also signal to your cells to produce a chemical called melanin, which causes the skin to darken Melanin blocks the UV light, and is also responsible for your skin becoming tan. The tan skin blocks more of the UV light, and protects the skin cells from damage. A lot of us get burned without sunscreen because we don't spend all of our time outside and in the sun. If we spent a lot of time outside, our skin's natural defense of melanin would help protect us from burns.

Sunburn is a type of radiation burn caused by ultraviolet light. As a result, arc welders can get sunburn even without being exposed to sunlight. UV can directly damage the DNA in a cell and cause inflammation.

The same UV that causes sunburn is also needed for the body to synthesize vitamin D. With enough vitamin D intake, people have been living near or in the arctic circle where the night is half a year long.

We need sunlight to help us make vitamin D, which is essential for our survival However, when we're in the sun for too long without protection, a type of ultraviolet light (UV) in sunlight that has high energy can go through our cells and damage our DNA. When the DNA in our skin cells is damaged, our bodies try to get rid of this damage by triggering the inflammatory response that we see as sunburn. When our skin turns red and painful, it's our body's way of clearing out the damaged cells and repairing our tissues. The fact that sunlight both helps and hurts us is not any different from anything else we need, even water -- water can be toxic to us, too, if we have too much! In other words, as long as we're not overexposed to the sun, we will get what we need without being burned.

Sunlight comprises light having a range of wavelengths and sunburns are a response to exposure to the ultraviolet (UV) components.

Ultraviolet light is another name for the light with wavelengths in the range of 10 to 400 nm. In electromagnetic radiation, energy is inversely proportional to wavelength (that is, energy increases as wavelength decreases). Compared to the rest of the wavelengths making up the sunlight, UV light has the highest energy. This light has sufficient energy to damage DNA, either by directly breaking bonds in the DNA or by changing other molecules that then interact with and alter DNA strands. If a cell's DNA is damaged and cannot be repaired, the cell typically undergoes a process called apoptosis, which is basically cellular suicide. Cells do this to minimize potential further damage to the organism as a whole.

The damaged DNA could contain a mutation which would be harmful to the organism if it were to be replicated. However, the rest of the body recognizes that cells are dying, which triggers a response to try to help. Extra blood and specialized cells are sent to the sunburned area to help with healing, producing the redness and swelling or inflammation associated with sunburns. Skin cell death happens constantly, but generally the dead cells fall off unnoticed as tiny specks because not many cells near each other die at once. Sunburn causes cells to die in large patches though, and these then peel off together, another characteristic of sunburns.

Regarding the second question, UV light, such as that in sunlight, is used by humans to produce vitamin D. Although UV light is essential in this regard, getting too much is possible. This is not unique to sunlight though - food is obviously necessary to survive, but consuming more calories than are used leads to unhealthy weight gain and conditions such as diabetes and high blood pressure; exercise helps to maintain your cardiovascular system, but too much can lead to overuse injuries and mental problems; medicines help to deal with illness, but too large a dose can kill.

The sun emits light in colors that we can't see as well as colors that we can see. Violet light has more energy than red light, and colors in-between red and violet have energy in-between the energy of violet and red. We can't see light that has even more energy than violet light, but the high energy of the light can cause chemical reactions in our skin when it hits it that are similar to the chemical reactions that happen when something gets really hot. These chemical reactions are what we call sunburn.

Ultraviolet light - that is what we call this very high-energy light - is good for us, in small quantities. It helps us make vitamin D. However, too much of a good thing can cause damage. This is true of everything else, too. For example, we need water to live as well, but have too much water and you can drown in it. Too much heat will burn you, and too little will freeze you. Ultraviolet light is similar.

Sunlight is like most things. We need salt to survive, but too much salt is not good for us. We need food to survive, but too much food is bad for us. The same is true for the sun. It's like a fireplace or a camp fire. The heat feels good, but being too close to the heat for too long will burn us.