Why do the extreme ends of the light spectrum have an adverse effect on the body



For example, UV light from the sun is known to cause skin cancer, so what makes it different from the visible spectrum (other than that we can’t see it)? Furthermore, does the spectrum gradually become more harmful, or is it a sudden jump to being harmful?

In: Physics

It’s not both ends of the EM spectrum, only one end. The lowest energy side of the spectrum is radio waves which have no effects on the body. Anyway, the reason is energy. As you move up the spectrum from radio waves, energies increase. By the time you get to hard ultraviolet, the light has enough energy to ionize atoms. We call this ionizing radiation. This damages your DNA and can lead to cancer or, in larger amounts, cause so much damage that your cells are basically shredded and you die within days or weeks.

They don’t – radio waves have no adverse effect. No more than visible light, anyways.

It’s UV and up. These are ionizing radiation. See, for the most part, atoms can only interact with one photon at a time. So, no matter how many photons you have, they only really interact one at a time with atoms.

So, if one photon does not carry enough energy to break molecules apart, then it will not cause harm (except through heating, like in a microwave). However, higher-energy radiation has enough energy to break molecules apart in each individual photon, and so it causes damage on the cellular level, shredding DNA and other important parts of your cells.

As you move from one end of the spectrum to another, there is a gradual energy increase. Basically, red light (low end) has a longer wavelength than blue light (high end), so to make the wave oscillate faster in blue light, you need more energy. The shorter the wavelength is, the more energy is packed in it. This, coupled with intensity, can eventually cause burns or ionizing radiation. Notably, you *could* do that with visible light. The difference is that it often just bounces off our skin or gets dissipated as heat, instead of penetrating our cells. UV light has just enough energy to burn us, and to alter our DNA and cause cancer.

The higher you go, the more energy there is. For example, if you keep going high above UV rays, you eventually find X-rays and gamma rays, both of which can cause serious harm if you’re exposed to them for a long time unprotected. The atmosphere and Earth’s magnetic field both shield us from gamma rays and stuff in that range that is floating about in space. On Earth, this same radiation damage is the reason X-ray technicians (who are working with X-ray machines all day every day) need to go in a separate room while you, only there to get your one X-ray, don’t have to because you’re only exposed for a few seconds. The damage comes from: frequency/wavelength, duration of exposure, intensity of the electromagnetic radiation.

As you move up the light spectrum the wavelength becomes shorter and the individual photons begin to contain more energy, to the point where they start to become “ionizing”. This means they have the ability to knock an electron loose from an atom, changing that atom’s charge and forming an ion.

Chemical bonds are heavily dependent on the charge of the atoms involved, so if you are creating ions in existing molecules then you are likely to be breaking and forming chemical bonds along the way. For living creatures that have the complex code that defines their structure and function stored in the form of a long molecule, randomly changing that molecule is bad news.

The visible spectrum isn’t generally ionizing. There is a sort of vague area of transition into ionizing behavior between 400 nanometers and 125 nanometers, but 125 and lower is what is usually considered ionizing.

Now towards the other end of the spectrum with long wavelengths I don’t think they are generally considered dangerous. Infrared radiation is basically just heat, and everything around us is emitting that to some extent. Barring there being enough to actually cook you it is harmless. Similarly radio waves pass through us without incident.


Light is (sort of) made up of tiny particles called photons. Each photon has a certain amount of energy. The amount of energy depends on (or determines) what type of photon it is. Radio waves are made up of low energy photons. Infrared photons have more energy. Then as we go from red through green to blue light, the energy keeps increasing. Once we get to ultraviolet, the x-ray and finally gamma rays, we have higher and higher energy photons.

Now, the low energy photons are safe. But gradually, the photons begin to be more harmful. A small amount of UV is okay, but too much can be dangerous. A small amount of x-rays or gamma rays can be okay, but it doesn’t take much for it to be bad.

So, why is this the case? Well think of it this way. If you throw a grain of sand at a window, it won’t break. Maybe you throw a whole handful of sand at the window. It’ll still be fine. A bucketful? Probably still okay. This is like a lot of low energy photons. Now, for a high energy photon, let’s say you throw one single brick at a window. It’ll break!