Unit 10 Electromagnetic Radiation And Uv Protection

Unit 10 Electromagnetic radiation and UV Protection

Contents What is electromagnetic radiation? What is duality of a EM radiation? What is a photon and how to calculate its energy? Description of EM spectrum: what are its components? How is UV radiation measured and how to protect from it?

1. Electromagnetic radiation

•a progressive transverse wave •a combination of traveling electric and magnetic fields.

1.1 Electromagnetic spectrum It includes radio waves, optical spectrum (Infrared, visible, ultraviolet), x-rays and gamma rays (γ -rays).

Electromagnetic radiation has two characteristics: •it is more like a wave when it propagates through space; •but is more like a particle when light interacts with matter.

Light as wave: the rainbow

• The speed of light in a prism is different for different wavelength • Each wavelength represents a color

Light as wave: the rainbow • The speed of light in a prism is different for different wavelength • Each wavelength represents a color

Wave equation:

λ = c ⋅T

or

c λ= f

λ the wavelength f the frequency of the light f = 1 / T T the period of the light c=3x108 m/s, the speed of light in free space.

1.2 Wave Properties of electromagnetic radiation 1. All types of electromagnetic radiation travel through free space at 3× 108m/s, i.e. with the speed of light. 2. They obey the wave equation c = fλ

1.3 Particle nature of electromagnetic radiation When an electromagnetic radiation interacts with matters, it behaves as stream of small energy packets called photons. During interaction, individual photon transfers its energy to the atoms. The energy E of each packet or photon can be calculated by its frequency f or wavelength λ :

hc E = hf = λ where c is the speed of light (c=3x108 m/s) in free space and h is a constant called Planck constant, h=6.63x10-34 J-s . The unit of photon energy E is joule (J).

Electromagnetic radiation carries energy from one place to another by photons . When they are absorbed by matter they cause heating and other effects. The higher the frequency and the smaller the wavelength of the radiation, the greater is the energy carried by photons, i.e. gamma rays are more 'energetic' than radio waves.

Electron Volt 1 electron volt is equivalent to the energy of moving 1 electron by an electric potential difference of 1 V. ∆E ∆V = Q

1 eV = ∆ E=∆ V x Q = 1 V x 1.6 x 10-19 C=1.6 x 10-19 J

so 1 eV = 1.6x10-19 Joule.

2. What is UV radiation? The sun emits radiation of different wavelengths. Some of the radiations, such as those making up the colours of raibow, have wavelength to which our eyes respond. Beyond these wavelengths are radiation in the UV and Infrared (IR) which our eye cannot see. UV radiation is of concern to us because unprotected exposure to it can cause skin and eye damage.

2.1 Classification of UV radiation UV-A

UV-B

UV-C

315-400

280-315

100-280

Absorption by ozone(O3) layer

Penetrates the ozone layer

Mostly absorbed Almost all by the ozone absorbed by the layer ozone layer

Amount reaching the Earth’s surface

> 98% of UV radiation is UV-A

< 2% of UV Negligible radiation is UV-B

Wavelength (nanometer, nm)

Effects on humans and Generates Overexposure the environment photochemical smog induces skin ( 煙霧 ) cancer and eye cataracts

None

2.2 Factors that affect UV radiation Sun elevation: the higher the sun in the sky, the higher the UV radiation level. Latitude: the closer to equatorial regions, the higher the UV radiation levels. Cloud cover: Solar UVR can penetrate through light cloud cover, and on lightly overcast days the UV radiation intensity can be similar to that of a cloudfree day. Heavy cloud can reduce the intensity of UV radiation. Scattered cloud has a variable effect on UV radiation levels, which rise and fall as clouds pass in front of the sun.

Factors that affect UV radiation Altitude: at higher altitudes, a thinner atmosphere absorbs less UV radiation. Ozone: ozone absorbs some of the UV radiation that would otherwise reach the Earth's surface. Ground reflection: grass, soil and water reflect less than 10% of UV radiation; fresh snow reflects as much as 80%; dry beach sand about 15% and sea foam about 25%.

2.3 Health risks associated with UV - Skin Short-term exposure to UV radiation causes reddening of the skin, sunburn and swelling ( 脹大 ) which may be very severe. – In some people this sunburn is followed by increased production of melanin, and is recognised as a suntan. – Tanning is a sign that damaged skin is attempting to protect itself from further harm. – A suntan is not an indication of good health and offers only minimal protection against further exposure.

2.3 Health risks associated with UV - Skin long-term effect of UV radiation is the induction of skin cancer. Chronic exposure to solar radiation also causes photoageing of the skin and actinic keratoses ( 光 化學角化癥 ). Photoageing is characterised by a leathery, wrinkled appearance and loss of skin elasticity while actinic keratoses is a known precursor to squamous ( 鱗片狀 ) cell carcinomas ( 癌科 ).

2.4 Health risks associated with UV – Eye snow blindness – Symptoms range from mild irritation to severe pain and possibly irreversible damage. Evidence that chronic exposure to intense levels of solar radiation is a contributory factor in the development of age-related macular degeneration of the retina and cortical ( 皮質的 ) cateracts, both a cause of blindness.

2.5 Health risks associated with UV – Immune system

UV also appears to alter immune response by changing the activity and distribution of the cells responsible for triggering these responses. – UV exposure may enhance the risk of infection and decrease the effectiveness of vaccines in humans. – Additional research is necessary to substantiate this.

2.6 Are there beneficial effects of UV radiation? Some UV radiation is essential to the body as it stimulates the production of vitamin D. – The vitamin D has an important function in increasing calcium and phosphorus absorption from food and plays a crucial role in skeletal development, immune function and blood cell formation. – There is no doubt that a little sunlight is good for you. But 5 to 10 minutes of casual sun exposure of hands face and arms two to three times a week during summer months is sufficient to keep your vitamin D levels high. – Closer to the equator, where UV levels is higher, even shorter periods of exposure suffice.

3. How is UV Radiation measured? Spectrophotometer – the standard equipment for measuring the intensity of UV (ultraviolet) radiation at different wavelengths. – very expensive Broadband UV sensor – a widely used and inexpensive instrument for measuring the intensity of the UV radiation – This type of sensor has a response which approximates the erythemal action spectrum. （红斑作用光谱曲线）

Spectrophotometer (Narrow band) The Brewer spectrophotometer at Dartmouth, Nova Scotia, (Canada) which measures total column ozone and high resolution UV

Broadband UV sensor The Hong Kong Observatory's broadband UV sensor at its King's Park Meteorological Station

3.1 What is the UV Index? The UV Index is a measure of the solar UV intensity at the Earth's surface relevant to the effect on human skin. The skin-damaging by UV radiation is governed by the erythemal action spectrum. This spectrum has been adopted by the International Commission on Illumination (CIE) to represent the average skin response over the solar UV spectrum. 

Erythemal action spectrum

3.2 How is the UV Index calculated? The standard way to calculate the UV Index recommended by the World Meteorological Organization (WMO) and the World Health Organization (WHO) is : measure the intensity of solar UV radiation at different wavelengths up to 400 nanometres (Blue line in Figure 1), multiply these UV intensities by the weighting factors at the corresponding wavelengths in the erythemal action spectrum (Red line in Figure 1) to reflect the human skin's response to each wavelength,

How is the UV Index calculated?

Measured UV intensity across the solar UV spectrum (blue line) and the erythemal action spectrum (Fig. 1) Source : The National Oceanic and Atmospheric Administration

How is the UV Index calculated? sum up the products above to obtain the total erythemally weighted UV intensity in milliWatt/metre2, i.e. area under the red line in Figure 2, multiply the total erythemally weighted UV intensity by 0.04 to obtain the UV Index.

How is the UV Index calculated?

Fig. 2 Total erythemally weighted UV intensity. (Area under the red line)

Source : The National Oceanic and Atmospheric Administration

The UV Index is a measure of the intensity of UV from the sun. The table shows the UV Index values with the corresponding exposure categories set by the World Health Organization. On a day with a higher UV Index rating, the skin is more likely to be damaged. In Hong Kong, the UV Index can often exceed 10 on a sunny summer day. During the period with clouds and rain, the UV Index will be lower.

3.3 Suntan and Sunburn UVA causes sun-tanning – UVA penetrates both our epidermis and dermis skin layers. Since the basal layer (the last layer) of the epidermis and the pricklecell layer (the first layer) of the dermis both contain melanin cells (melanocytes), these two layers of cells are stimulated simultaneously by the UVA rays to produce more melanocytes to prevent the harmful effects of the UV rays against the underlying cells.

– Tanning-beds in beauty salons also emit UVA rays to produce quick tans. – However, the energy in UV-A is weaker than in UV-B. That is why the tan doesn't last long (ususally disppeared after one to three weeks), and that is also why UV-A doesn't cause sun-burn on its own.

Suntan and Sunburn UVB's energy is much greater than UV-A's, and thus, it causes the burning sensation very quickly. – However, the penetration level of UV-B is only upto the epidermis. Thus, UV-B causes only the epidermal layer (that is, only one layer) to multiply the production of its melanin cells. That is why a tan is not produced as instantly as exposure to UV-A rays.

– Due to its high energy, UV-B rays once absorbed into the skin do not deplete quickly, but retained for a long time. UV-B accumulates to cause a slow 'tan' which results in delayed pigmentation of the skin(that is, those unsightly brown blocky patches).

Tan without burn (or less degree of burn) By exposure to a salon's sun-tanning bed (which emits only UV-A rays). By applying a sunscreen that blocks out only the UV-B rays. – Some screens even contain active ingredients to speedify the taning effect, by amplifying the UV-A rays penetration. There are also suntan products which can amplify tanning - and within 10 to 15 minutes - under room lighting, without exposure to the sun. Such products are often named as suntan lotions (milk, or the like).

So do not get mixed up between suntan and sunscreen products

3.4 Common misconceptions A suntan is healthy. – A tan results from your body defending itself against further damage from UV radiation. A tan protects you from the sun. – A dark tan on white skin offers only limited protection equivalent to an SPF of about 4. You can’t get sunburnt on a cloudy day. – Up to 80% of solar UV radiation can penetrate light cloud cover. Haze in the atmosphere can even increase UV radiation exposure.

Common misconceptions You can’t get sunburnt while in the water. – Water offers only minimal protection from UV radiation, and reflections from water can enhance your UV radiation exposure. UV radiation during the winter is not dangerous. – UV radiation is generally lower during the winter months, but snow reflection can double your overall exposure, especially at high altitude. Pay particular attention in early spring when temperatures are low but the sun’s rays are unexpectedly strong.

Common misconceptions Sunscreens protect me so I can sunbathe much longer. – Sunscreens should not be used to increase sun exposure time but to increase protection during unavoidable exposure. The protection they afford depends critically on their correct application. If you take regular breaks during sunbathing you won’t get sunburnt. – UV radiation exposure is cumulative during the day.

Common misconceptions If you don’t feel the hot rays of the sun you won’t get sunburnt. – Sunburn is caused by UV radiation which cannot be felt. The heating effect is caused by the sun’s infrared radiation and not by UV radiation.