146 



Reflection and Refraction of Light. 



2- When a ray enters a new medium of the same refractive density, it 

 continues in the same straight line, without sensible diminution of intensity. 



3- When the new medium has a different refractive density, and a smooth 

 surface, part of the ray is reflected into the same medium, in a distinct ray; 

 and another part, generally more than half, proceeds as a distinct ray in 

 the new medium, but, if oblique, in a different rectilineal course. 



4. Another portion of the light is scattered in radiations from the point 

 of incidence, and a certain portion is absorbed. 



5- The angles of incidence and reflection are equal, and in the same 

 plane. 



6. The angles of incidence and refraction are in the same plane, and the 

 sines of these angles, for liglit of a given sort, are in a constant ratio. 



7- Rays of different sorts of light are unequally refracted, tiie red least, 

 and violet most : and these differences do not observe a given ratio in dif- 

 ferent media. 



8- At angles of greater incidence more light is reflected, and this in- 

 crease follows a different rate in different media, but generally less than 

 half is reflected. 



S- If the new medium is less dense than the other, the light is totally 

 reflected, when the angle of incidence is greater than Ihat which gives the 

 angle of refraction equal to a right angle. 



From the postulates required in the Newtonian Theory, art. 5, it follows, 

 that a particle of light is not impeded, or turned from its direct path, while 

 traversing media in wiiich the forces are equal in all directions ; and this 

 satisfies the first and second of the preceding facts. 



But while it traverses obliquely the confines of two media differing in 

 refractive power, its path will be curvilineal. 



Let A B, fig. l,be the 

 boundary of two media, 

 the spaces between the 

 parallel lines marked a, 

 those in which the forces 

 are directed towards the 

 new medium, which may 

 be called attractive ; and 

 those marked r, opposite 

 or repulsive. It is mani- 

 fest that the curvilineal course of the ray will be convex towards the axis, 

 as mn, in the spaces a, and concave in r ; since by the sixth postulate, 

 art. 5, the resultant of the several forces will be perpendicular to the 

 surface A B. Now if the repulsive forces be sufficient to overcome the 

 perpendicular motion, the ray will be reflected, describing two similar and 

 equal branches of tlie curve whose axis is mn ,• this satisfies the fifth fact. 



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