Reflection of light at an opaque surface

When the surface light falls on is rough and opaque (Fig. 3) the incident light is scattered in all directions. On the other hand, if the surface is so smooth that the distance between the successive elevations on the surface is less than about one-quarter wave length of light the light will scarcely be scattered at all and the surface is said to be polished.

Thus, it is evident that while a surface may be considered to be polished for light of long wave length it does not necessarily follow that we may consider it to be polished for light of short wave length. A polished surface reflecting light with but little scattering is called a mirror. The rays of the invisible part of the spectrum are also reflected.

Fig. 3 Irregular reflection. The light is scattered or diffused.

Fig. 4 Spherical mirror. The angle of incidence is equal to the angle of reflection.

Selective reflection.

The proportion (fraction) of the light which is reflected depends on the wave length of the light, the angle of incidence, and the medium surrounding the body. No substance has been found so far which reflects light of one wave length only. Consequently, no substance shows a pure color when viewed by reflecting light. A substance completely reflecting light of all wave lengths is white when illuminated by white light. Substances which reflect wave lengths unequally appear to be colored in white light.

Concave Spherical Mirrors.

A concave spherical mirror is part of a spherical shell with its inner surface polished. The center of the sphere the mirror was taken from is called the center of curvature of the mirror. The radius of the sphere is called the radius of the curvature of the mirror. The middle point M of the mirror (Fig. 4) is said to be the vertex of the mirror and its center of curvature is the principle axis of the mirror. Evidently, it follows from the property of the mirror that any straight line from C, the center of curvature, to the mirror is perpendicular to the surface of the mirror at the point it is drawn to: Such a line is the radius of the sphere the mirror is a part of, and the radius is perpendicular to the sphere.

Suppose a luminous point to be placed on the principle axis and to send a number of rays to the mirror. Consider two of the rays, UM and UP. The first of these rays, after passing through the center of curvature, strikes the mirror normally and is reflected back along its former path. The other ray UP will be reflected by the mirror in such a way as to make the angle of incidence equal to that of reflection. Hence, the incident ray UP and the reflected ray PV will make equal angles with the radius of curvature CP. The reflected ray crosses the principle axes at some point V. The ray UM is also reflected through V. The point V is image of U, and all the rays from U will pass through it provided the aperture of the minor is «small. This relation holds for all possible positions of the point U.

I. Translatete and memorize:

1. Success, to succeed (in), succeeding, successful, successive, successively, successfully.

a) Lebedev, the great scientist, succeeded in proving the pressure of light. b) Our first experiment was quite a success. c) Their last examination was rather successful. d) The succeeding findings confirmed the results of the first experiments. e) A number of successive data made it dear the experimentalists were right in their hypothesis.

2. while

He played the piano while they were working. Mary is a blond, while Anne is a brunette. While Petrov is not a good chess-player, he won three successive games while playing with Sidorov.

3. but

a) He likes the cinema, but he has no time to see all the new pictures he is interested in. b) He has seen but few new pictures this month. c) He liked all of them but two. d) At the last but one lesson we will listen to different answers to the questions the lecturer wanted us to consider. e) All of us will have to make some reports at the next but one meeting.

II. Put 3-4 questions to the text Reflection of Light at an Opaque Surface.