OPTICAL IMAGES. 



to explain the manner in which rays of light are reflected when 

 these fall on a plane surface. 



4. The rays are reflected in this case exactly as an elastic ball 

 is repelled when it encounters a hard and flat surface. Let c, 

 fig. 2, be a point upon a reflecting surface A c, upon which a ray 



of light D c is incident. Draw the 

 line c E perpendicular to the reflect- 



D \ / D ' ing surface at c ; the angle formed 



\ by this perpendicular, and the inci- 



\ / dent ray D c, is called the angle of 



V / incidence. 



V j / From the point c, draw a line c D' 



\i/ in the plane of the angle of incidence 



A. c B D c E, and forming with the per- 



pendicular c E an angle E c D', equal 



to the angle of incidence, but lying on the other side of the per- 

 pendicular. This line c D' will be the direction in which the ray 

 will be reflected from the point c. The angle D' c E is called 

 the angle of reflection. 



The plane of the angles of incidence and reflection which passes 

 through the two rays c B and c D', and through the perpendicular 

 c E, and which is therefore at right angles to the reflecting surface, 

 is called the plane of reflection. 



This law of reflection from perfectly polished surfaces, which is 

 of great importance in the theory of light and vision, is expressed 

 as follows : 



When light is reflected from a perfectly polished surface, the 

 angle of incidence is equal to the angle of reflection, in the same 

 plane with it, and on the opposite side of the perpendicular to the 

 reflecting surface. 



From this law it follows, that if a ray of light fall perpendicu- 

 larly on a reflecting surface, it will be reflected back perpen- 

 dicularly, and will return upon its path ; for in this case, the 

 angle of incidence and the angle of reflection being both nothing, 

 the reflected and incident rays must both coincide with the per- 

 pendicular. If the point c be upon a concave or convex surface, 

 the same conditions will prevail; the line c E, which is per- 

 pendicular to the surface, being then what is called in geometry, 

 the normal. 



5. This law of reflection may be experimentally verified as 

 follows : 



Let c d c', fig. 3, be a graduated semicircle, placed with its dia- 

 meter c c* horizontal. Let a plumb-line b d be suspended from its 

 centre 6, and let the graduated arch be so adjusted that the 

 plumb-line shall intersect it at the zero point of the division, the 

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