• Question: How do Sunglasses work, is it to do with the UV rays and the sunglasses have something that blocks them?

    Asked by lavamonster to Rachael, Hywel, Joseph, Patience, Poonam on 24 Jun 2010 in Categories: .
    • Photo: Joseph Cook

      Joseph Cook answered on 23 Jun 2010:

      Most glass and plastic reflect or absorb most of the light in the ultraviolet range, so just plain glasses protect your eyes from UV. They are transparent to visible light, but UV light can’t pass through.

      Sunglasses also remove a lot of visible light, making them appear dark. They usually do this with polarisers, which absorb all of the light apart from that which is oriented in a particular direction, so the intensity is significantly reduced.

    • Photo: Poonam Kaushik

      Poonam Kaushik answered on 24 Jun 2010:

      Sunglasses offer protection against excessive exposure to light, including its visible and invisible components.
      The most widespread protection is against ultraviolet radiation (UV), which can cause short-term and long-term ocular problems such as photokeratitis, snow blindness, cataracts, pterygium, and various forms of eye cancer. Medical experts advise the public on the importance of wearing sunglasses to protect the eyes from UV; for adequate protection, experts recommend sunglasses that reflect or filter out 99-100 % of UVA and UVB light, with wavelengths up to 400 nanometers (nm). Sunglasses which meet this requirement are often labeled as “UV 400.” This is slightly more protection than the widely used standard of the European Union (see below), which requires that 95 % of the radiation up to only 380 nm must be reflected or filtered out. Sunglasses are not sufficient to protect the eyes against permanent harm from looking directly at the sun, even during a solar eclipse.
      More recently, high-energy visible light (HEV) has been implicated as a cause of age-related macular degeneration; before, debates had already existed as to whether “blue blocking” or amber tinted lenses may have a protective effect. Some manufacturers already design to block blue light; the insurance company Suva, which covers most Swiss employees, asked eye experts around Charlotte Remé (ETH Zürich) to develop norms for blue blocking, leading to a recommended minimum of 95% of the blue light. Sunglasses are especially important for children, as their ocular lenses are thought to transmit far more HEV light than adults (lenses “yellow” with age).
      There has been some speculation that sunglasses actually promote skin cancer. This is due to the eyes being tricked into producing less melanocyte-stimulating hormone in the body.

    • Photo: Dr Hywel Jones

      Dr Hywel Jones answered on 24 Jun 2010:

      Sunglasses work by selectively filtering out certain wavelengths of the electro-magnetic spectrum. Light is only a small part of the electro-magnetic spectrum, ultraviolet (the stuff we are trying to stop), infrared (IR, heat) and microwaves are also part of the same spectrum.
      For light sunglasses absorb some, but not all, of the incident radiation, hence why they make things darker, but for certain parts of the ultraviolet (UV) section of the spectrum they stop a lot more (think of sunglasses as really dark sunglasses if we could see in UV). It is the UV part that can cause damage to our eyes, the bright sunlight might be uncomfortable but in itself is not harmful. This is alos what sun creams are stopping.
      Certain molecules can absorb different wavelengths of light, UV, IR etc. and sunglasses manufacturers (and sun screen manufacturers) include the right molecules in their lenses so that these molecules absorb and block much of the frequency that we want to stop. In sun creams the main material used is titanium dioxide (TiO2) but I’m not really sure what is used in the lenses of sunglasses. Normal glass itself stops most of the UV spectra, hence why we don’t get sunburnt through glass.

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