Measuring Ultraviolet Radiation Underwater: A Practical Application of the Beer-Lambert-Bouguer Law for High School Physics

Article excerpt

The measurement of solar UV under water is not a simple process. In the underwater environment the difficulty of obtaining useable data is greatly amplified due to the optically complicated and at times unpredictable nature of water itself. The following practical exercise designed for use in the Year 11 and Year 12 Physics classroom aims to provide students with a practical understanding of how the Beer-Lambert-Bouguer Law can be applied in an underwater environment and how handheld solar UV measurement instrumentation can be employed in conjunction with the Beer-Lambert-Bouguer Law to accurately measure the optical properties of water.

INTRODUCTION

The precise measurement of solar UV on the surface of the Earth is not a simple process. Instruments such as spectroradiometers, spectrometers, radiometers and basic handheld UV meters must be adequately calibrated to appropriate standards, maintained fastidiously and employed correctly within their operational limitations in order to obtain accurate data that is of the quality necessary for scientific research. Other factors, including atmospheric parameters such as column ozone, trace gases and aerosols and their influence upon instrument response must also be taken into account during measurements, or critical errors may become apparent in the measured data. These issues also affect solar UV measurements made underwater, except that in the underwater environment the difficulty of obtaining useable data is greatly amplified due to the optically complex and at times unpredictable nature of water itself. The instrumentation employed to take the solar UV measurements must be calibrated to proper standards, prepared for changes in the dynamic atmosphere, completely sealed and waterproofed in readiness for the harsh surrounds of the underwater environment and also be corrected for the optical phenomenon known as the 'immersion effect'.

Taking solar UV irradiance measurements in real-world underwater environments is hampered by cumbersome spectroradiometric and radiometric measurement equipment, poor instrument response with changes in depth, and unpredictable changes in water quality and transparency as a result of both natural and anthropogenic activity. Shading by nearby trees and plants and the presence of surface waves have made recording reliable estimates for the optical properties of water bodies an extremely difficult proposition (Morrow & Booth, 1997). Morrow and Booth have also noted that underwater UV measurements can be complicated by the fact that total UV becomes a very small signal that has to be measured alongside the much more pronounced visible waveband, as well as by unpredictable features at the surface of the water, including wave-focusing effects such as those reported by Deckert & Michael (2006) and changes in water line elevation caused by evaporation and tidal changes.

From an optical perspective, three main factors influence underwater UV measurements made with all types of UV-measuring instrumentation. The first is that the field of view as seen by any sensor is reduced and hence a smaller amount of radiation is intercepted by the sensor. The second is the change in the local refractive index between the open-air environment in which the sensor was calibrated and the underwater environment where it is employed (Ohde & Siegel, 2003). Thirdly, during a water-based measurement, more light is scattered out of the sensor than in a similar air-based measurement. This is caused by the difference between the refractive indices for air and water at the sensor interface (Hooker & Zibordi, 2005, Zibordi, 2005, Zibordi et al, 2004). This is known as the 'immersion effect'. In order for reliable solar UV measurements to be made underwater, wavelength-dependent immersion factors must be calculated and applied using a strict methodology. However, for the practical exercise presented in this article, it is not necessary to calculate the immersion factor for the recommended UV meter, as only relative measurements will need to be obtained and analysed in order to investigate the Beer-Lambert-Bouguer Law. …