Page 392 - Schaum's Outline of Theory and Problems of Applied Physics

CHAP. 30] LIGHT 377

SOLVED PROBLEM 30.9
A lamp is suspended 6 ft over a large table. How much greater is the illumination directly under the lamp
as compared with the illumination at a point on the table 4 ft to one side?
Directly under the lamp the illumination is
I
E 1 = 2
R 1
and at a distance d away on the table it is
I cos θ
E 2 = 2
R 2
With the help of Fig. 30-4 we have

2 2 2 2
R 2 = R + d = (6ft) + (4ft) = 7.2ft
1
6ft
R 1
and cos θ = = = 0.83
R 2 7.2ft
Hence,
E 1 I/R 2 1 R 2 2 (7.2ft) 2
= 2 = 2 = = 1.7
2
E 2 (I cos θ)/R 2 R cos θ (6ft) (0.83)
1
The illumination is 1.7 times greater directly under the lamp.

Fig. 30-4

REFLECTION OF LIGHT
When a beam of light is reflected from a smooth, plane surface, the angle of reflection equals the angle of
incidence (Fig. 30-5). The image of an object in a plane mirror has the same size and shape as the object but with
left and right reversed; the image is the same distance behind the mirror as the object is in front of it.

Angle of Angle of
incidence reflection

Fig. 30-5