56 The Sea-water and its Physical and Chemical Properties 



made, principally by the Oceanographic Institute in Goteborg (Pettersson), since 

 1933 in fiords, in the Skagerrak, in the Kattegat, and in the Baltic have given similar 

 results, but they also show a particularly strong dependence of the reduction in in- 

 tensity of the light near to the thermocline (discontinuity in vertical density distribu- 

 tion). This intensification of the extinction is undoubtedly due to an enrichment of 

 suspended particles at such layers. This enrichment shows considerable local diff'erences 

 and causes strong variations in the extinction coefficient. If the scattering and the 

 absorption due to the suspended particles is removed by filtering the water samples 

 there remains a selective absorption which must be due to strongly absorbing humic 

 material dissolved in the water. This "yellow material" must be an organic metabolic 

 product, either from the land or from the remains of decomposed plankton. The 

 turbidity of the water can now be determined continuously from a moving ship by the 

 self-recording transparency meter (Joseph, 1950, 1952) and the results can be used in 

 suitable cases to determine the origin of a water mass since the extinction value pro- 

 vides a persistent characteristic (Dietrich, 1953; Joseph, 1953; Jerlov, 1953; see 

 also Wyrtki, 1950). The distribution of particles in suspension can be studied with 

 the Tyndall-meter which measures the intensity of the scattered light produced from 

 a parallel beam of light, by comparison with the known intensity of an illuminated 

 glass filter using a Pulfrich photo-meter. This apparatus can also be used for the 

 measurement of the scattering from suspended and dissolved material in especially 

 transparent ocean water, corresponding measurements of this type have been made 

 by Jerlov (1953) in the three oceans during the "Albatross" Expedition. 



{h) Refraction and Reflection of Radiation 



Parallel radiation incident on the surface of the water will be partly reflected and in 

 part will enter the water. The angle of reflection will be the same as the angle of inci- 

 dence but the ratio of the intensities of the incident and the reflected beam will be 

 dependent on the angle of incidence of the original radiation itself. Radiation entering 

 the reflecting medium undergoes a change of direction on passing through the surface, 

 and the angle of this refracted beam is given by the equation 



sin / 



-^ — = n, 



sm r 



where / is the angle of ncidence, r is the angle of refraction and n is known as the 

 refractive index. For air and pure water it is almost exactly 1-333338 or -^4/3. That is, 

 in water which is optically denser the beam is refracted towards the perpendicular 

 (Fig. 31). The refractive index for a ray passing from the water into air is Xjn ~ 0-75. 

 If the angle of incidence of radiation passing from the water into air increases, the 

 angle / will increase faster than the angle r until finally the value of / reaches 90°; 

 the outgoing ray then passes along the surface of the water. This occurs when r = 

 48-5° = R (see Fig. 31). If/- increases still further, radiation cannot enter the air but is 

 reflected entirely within the water; R is known as the critical angle for total reflection. 

 SoRET and Sarasin (1889) have measured the refractive index of mediterranean 

 water (approx. 37%o S) for various wavelengths and compared these values with those 

 for pure water. Table 21 shows the results. The dependence on salinity is, however, 

 suflUciently large for use in the optical determination of salinity (refractometer) ; 



