Sunscreen products with a higher SPF value are able to block a larger amount of UV radiation from the sun, and are thus more effective.

Ultraviolet radiation

UV radiation is a component of the electromagnetic spectrum.  It is located at the blue end of the electromagnetic spectrum, just below visible light, and is therefore undetectable by the naked human eye. Compared to visible light (400 nm to 700 nm), UV radiation has a relatively shorter wavelength (180 nm to 400 nm) and hence a relatively higher energy level, since energy levels are inversely proportional to wavelength. UV light can be categorized into three sub types, namely UVA (315 nm to 400 nm), UVB (280 nm to 315 nm) and UVC (240 nm to 280 nm). The ozone layer in the atmosphere blocks all UVC radiation, most of UVB radiation but none of the UVA radiation. All three components have different effects on the human body.

As UVA has the longest wavelength compared to the others, it is able to penetrate into our skin and is therefore responsible for causing skin to lose its elasticity. Exposure to large amounts of UVA will lead to premature aging of the skin. On the other hand, UVB does not penetrate our skin due to its shorter wavelength. UVB will only reach the surface of our skin and is responsible for sunburn.  UVC has the shortest wavelength among the others, and therefore has the highest energy level. This makes it dangerous for all living organisms on Earth. Fortunately the ozone layer protects us by blocking UVC.

Using sunscreen is one way to protect our skin from UV damage such as sunburn. There are many brands of sunscreen lotions available in the market with different SPF values. The SPF values define the ability of the sunscreen to block UV radiation, especially UVB,  that causes sunburns.

One of the simplest ways of measuring UV radiation is to use the UV index. Each unit of the UV index corresponds to about 25mW per square meter of UV radiation. The UV index range is briefly summarized in the table below.

Less than 3    Moderate 
Between 3 to 6    High
Between 7 to 9    Very high
More than 9    Extremely high
 

The hypothesis that sunscreen lotion with higher SPF values will block a larger amount of UV radiation , is proven to be true for SPF 15 and SPF 30 sunscreen. However, there was no difference in UV levels when sunscreen products containing SPF 30 and SPF 50 were used.

UV light is responsible for causing the skin to age faster and increasing the risk of skin cancer. The use of proper sunscreen can help to protect our skin from the damage caused by UV light. An SPF 15 lotion  blocks 94% of the UV light while  SPF 30 lotion blocks 97% of UV light, and SPF 45 blocks 98% of the UV light.

What would happen if this  science fair project were to be repeated at different times of the day?

The experiment can be also repeated indoors by using a UV lamp.
How about our computer monitors? or televisions? Do they emit UV radiation? Compare the difference in UV radiation emitted, if any, by old CRT (cathode ray television) screens and LCD (liquid crystal display) screens.