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Physical
Science: Dying for a Tan?
How much harmful UV radiation reaches yoiur skin? How much protection
is provided by sunblock, sunglasses and clothing?
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Dying for a Tan?: Teacher Notes
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What is UV radiation?
UV (ultraviolet) radiation is light with wavelengths shorter than
that visible by the human eye. The human eye responds to light with
wavelengths from about 790 nm (red) to 430 nm (violet). Light with
shorter wavelengths is called ultraviolet (beyond violet). Energy
is inversely proportional to wavelength. Thus, UV light is very
high in energy. Humans are not designed to be exposed to large amounts
of UV radiation.
UV light is contained in the range of wavelengths produced by the
sun. Most UV light is absorbed by the ozone layer or reflected back
into space, only a small amount reaches the earth's surface.
Scientists divide UV radiation into three categories by wavelengths:
UVA (315 - 400 nm), UVB (290 - 315 nm) and UVC (220-290 nm). UVA
radiation is the largest component to reach the earth's surface.
This radiation contributes to photochemical smog, and the fading
and damage to plastics, paints and fabrics. UVC radiation is totally
absorbed by the ozone and other gases, and does not reach the Earth's
surface. Only 1% of solar radiation is within the UVB band, and
most of this is absorbed by ozone. Nevertheless, UVB wavelengths
are those that can cause the most damage to human skin. Thus, small
doses of this radiation can cause significant harm. In addition,
small changes in the ozone layer can cause significantly more UVB
radiation to reach the earth's surface.
UV
light and humans?
Sunburn is induced mostly by the 300 - 320 nm light. This is mostly
UVB light with the lower wavelength UVA light. Sunburn is evidence
that a person has been exposed to too much UVB too quickly. UV exposure
also causes skin cancer, premature aging of the skin and wrinkling,
and cataracts and other eye damage. Even a tan is just a sign of
sun damage to your skin. Precautions such as sunscreen and sunglasses
must be used to protect oneself from the damaging radiation.
How do we measure UV radiation?
The UV index is a measurement of the amount of skin damaging UV
radiation that is reaching the earth's surface. It ranges from 0
(at night) to 15 or 16 (in the tropics at high elevations under
clear skies). The higher the UV index, the greater the dose rate
of skin and eye damaging UV radiation.
| UV
index number |
Exposure
Risk |
0 to 2 |
Minimal |
3 to 4 |
Low |
5 to 6 |
Moderate |
7 to 9 |
High |
10+ |
Very High |
The UV index indicates your risk of skin damage for that day. The
amount of skin damage an individual will receive depends on your
skin type. But, everyone can receive skin damage on any given day
unless adequate protection is taken.
By taking into account your skin type and the UV index, it is possible
to get an estimate of the time in the sun it takes for sun damage
to occur.
Lab Experiment
For this experiment, you will use a UV meter that measures the UV
index for the current conditions. Using the meter we will do a series
of experiments to see how much protection sunblock, sunglasses and
clothing provide. Some of the questions you will be able to answer
are:
· How waterproof is “waterproof” sunblock?
· Do you need to spend a lot of money on sunglasses to protect
your eyes?
· How much protection does clothing provide?
Your instructor will assign you to a lab group. For each group,
you will need one SunSafe Sensor meter, sunscreen samples, sunglasses,
a data collection sheet, and a pencil.
Thinking ….
How would you design an experiment to see if a given sunscreen
protects you against UV radiation? What are the variables? What
are important conditions that you have no control over?
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