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 hall 
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¬ 
ing surface at c; the angle formed 
by this perpendicular, and the inci¬ 
dent ray D c, is called the angle of 
incidence. 
From the point c, draw a line c d' 
in the plane of the angle of incidence 
D c e, and forming with the per¬ 
pendicular c e an angle E c D', equal 
to the angle of incidence, hut lying on the other side of the per¬ 
pendicular. This line c D' will he 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 d and c d', and through the perpendicular 
c E, and which is therefore at right angles to the reflecting surface, 
is called th q 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 he 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 h 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 
Si 
Fig. 2. 
