referring now to fig1 and 2 there is seen a preferred embodiment of solar energy concentration apparatus constructed and operative in accordance with an embodiment of the present invention . in accordance with the illustrated preferred embodiment , the collection apparatus comprises first and second elongate solar absorber plates 10 and 12 disposed in generally spaced back to back relationship . solar absorber plates 10 and 12 typically comprise metal strips having a low emissivity coating , such as black paint and are arranged in communication with flowing fluid in a manner not illustrated herein for heat transfer thereto . the exact construction of the heat exchanger which is in thermal communication with the solar absorber plates 10 and 12 may be entirely conventional . disposed adjacent each solar absorber plate is an elongate volume 14 of a material having an index of refraction greater than that of air . according to a preferred embodiment of the invention , volume 14 contains carbon disulfide ( cs 2 ) having an index of refraction of approximately 1 . 65 . volume 14 may be defined by walls of a solar energy transmissive material such as glass or perspex , and has a triangular cross section . disposed adjacent volume 14 is a second elongate volume 16 of a cross section which together with the cross section of volume 14 defines a trapezoid . volume 16 typically comprises a material having an index of refraction less than that of the material contained in volume 14 . according to a preferred embodiment of the invention , volume 16 contains water having an index of refraction of 1 . 33 . volume 16 , similarly to volume 14 may be defined by walls of glass or any other suitable material . alternatively , solid materials may fill volumes 14 and 16 and thus no walls are required . materials such as plastics may be employed . arranged along the underside and bottom surfaces 18 and 20 of volume 16 are reflecting mirrors 22 and 24 having their reflecting surfaces facing towards volume 16 . it will be appreciated by persons skilled in the art that the geometry of volumes 14 and 16 and the indices of refraction thereof are selected in order to maximize the efficiency of the solar energy collector apparatus at a desired output temperature for a desired location and solar radiation conditions . the criteria for such selection will be described and explained hereinafter . it is noted for example that a trapezoidal cross section is employed rather than a simple triangular cross section to provide an increased ratio of collector surface area to absorber surface area . the additional volume , defined by volume 14 , is filled with a material of higher index of refraction than that of volume 16 in order to direct the radiation incident upon volume 14 from the atmosphere and from volume 16 onto the absorber surface , and more particularly to provide total internal reflection for radiation travelling from volume 14 towards volume 16 at the interface between the volumes . it is appreciated that any desired number of volumes having differing indices of refraction may be employed in combination to define the radiation pathway from the atmosphere to the absorber surface . reflecting mirrors 22 and 24 may be separate from volume 16 or may be coatings formed thereon . alternatively reflecting mirrors 22 and 24 may be omitted . considering the geometry of the exemplary solar collector configuration of fig1 and 2 it is appreciated that the broadest surface is oriented such that the perpendicular thereto faces the median position of the sun during the day and the arc described by the sun during the day defines a plane perpendicular to the longitudinal axis 30 of the collector . for the purposes of illustration and description , the surface facing the median location of the sun is identified by reference numeral 32 , its length is termed l and its overall width is 2b . the area of the facing surface is given by : the width of each of solar absorber plates 10 and 12 is termed b and their length is l so that the total area of the absorber surfaces is given by : the solar radiation intensity on the absorber surface is given by : where i is the global radiation including both direct radiation and diffused radiation ; η r is the optical efficiency of the collector , a factor which takes into account reflection losses due to the absence of total internal reflection , and θ is the angle between the position of the sun at any time and the plane parallel to the axis 30 and to the normal to the facing surface . it is desired to maximize the useful energy produced by the collector , which is given by : t liq is the temperature of the liquid adjacent the absorber surface ; it may be appreciated that by maximizing the ratio of b / b , the geometrical concentration ratio of the apparatus is maximized . it will now be shown that there exists a trade - off between the maximization of b / b and the maximization of the amount of radiation which is subject to total internal reflection within volumes 14 and 16 . the critical angle θ for total internal reflection between bodies of different indices of reflection is given by snell &# 39 ; s law : ## equ1 ## where the radiation is travelling from body b to body a and the index of refraction of b is greater than the index of refraction of a . thus in the case where body a is air , whose index of refraction is equal to unity , the greater the index of refraction of body b the larger the critical angle . if α is the acute angle between the facing and underside surfaces 32 and 18 , it may be appreciated that : where θ 2 is the maximum permissible azimuth angle for which total internal reflection will be achieved . thus in order to determine the values of α and θ 2 which provide a maximum q u for a given location , the values for solar radiation intensity for various azimuthal angles through the day for each given location must be considered . such values are known in the literature . thus for example it has been found that for a location in ohio u . s . a . an α of 30 ° provides a maximum amount of useful energy over an entire day . it is appreciated that each given location , having a different intensity pattern for solar radiation as a function of azimuthal angle of the sun &# 39 ; s position , has a different preferred angle α . reference is now made to fig3 and 5 which illustrate in cross - sectional illustration , alternative embodiments of the invention . fig3 shows a collector of circular cross section comprising an absorber surface 40 of circular cross section which is disposed within a volume 42 containing a material such as water which has desired characteristics of total internal reflection , as described hereinabove . the absorber surface 40 may be arranged in the center of the cross section of volume 42 or alternatively , off center , as illustrated . a mirror surface or coating 44 may be provided on a portion of the outer surface of volume 42 for providing desired reflection onto the absorber surface . alternatively this may be omitted . fig4 illustrates an alternative embodiment of the invention in which volume 42 defines facing and back surfaces 46 and 48 of differing cross sectional radii . in this embodiment , a pair of planar absorber surfaces 50 and 52 are employed , and a heat absorbing fluid is circulated in association therewith . fig5 illustrates a further alternative embodiment of the invention in which a geometry substantially similar to that of fig1 and 2 is employed in an upside down orientation , such that the solar energy receiving surface is defined by a plurality of surfaces angled with respect to each other and the bottom surface , which may be provided with a reflective coating is flat . it will be appreciated by persons skilled in the art that the invention is not limited to any particular geometry or arrangement of absorber surfaces or number of volumes of differing index of refraction . the scope of the present invention is defined only by the claims which follow .