Final answer:
The UVB portion of the optical spectrum is most closely associated with adverse human health effects, notably skin cancer and cataracts. Despite its relatively lower energy and proportion reaching Earth compared to other UV types, UVB's damage potential remains significant.
Step-by-step explanation:
The portion of the optical spectrum that has been most closely associated with adverse human health effects is UVB radiation. UVB radiation is responsible for harmful effects on the skin and eyes. It can cause skin cancer and cataracts. Despite very little UVB reaching the Earth's surface, there are significant increases in skin-cancer rates in regions like Australia, indicating the importance of UVB absorption by the Earth's upper atmosphere. Repeated exposure to UVB radiation can lead to the formation of cataracts - a common cause of blindness. It's estimated that as many as 20% of adults will develop skin cancer over their lifetime due to UVB exposure. Eye protection from UV radiation is more effective with plastic sunglasses than those made of glass. To illustrate, the sunlight spectrum comprises not only the visible light that we see, but also UV and infrared (IR) light. UV light has a shorter wavelength and is more energetic than visible light, which explains its potential for cellular damage. Out of the UV spectrum, UVB is a lower energy form than UVC, but higher than UVA. Relative to UVC, a larger proportion of UVB reaches the Earth’s surface, causing the mentioned health hazards.
Learn more about UVB radiation