Fig.4.2. Formation of a virtual image by a plane mirror.

Figure 4.2 shows how an image is formed by a plane mirror (that is flat), according to the ray model. We are viewing the mirror, on edge, in the diagram of Fig.4.2, and the rays are shown reflecting from the front surface. (Good mirrors are generally made by putting a highly reflective metallic coating on one surface of a very flat piece of glass.) Rays from two different points on an object are shown in Fig.4.2: rays leaving from a point on the top of the bottle, and from a point on the bottom. Rays leave each point on the object going in many directions, but only those that enclose the bundle of rays that reach the eye from the two points are shown. The diverging rays that enter the eye appear to come from behind the mirror as shown by the dashed lines. That is, our eyes and brain interpret any rays that enter an eye as having traveled a straight-line path. The point from which each bundle of rays seems to come is one point on the image. For each point on the object, there is a corresponding image point. Let us concentrate on the two rays that leave point A on the object and strike the mirror at points B and B'. The angles ADB and CDB are right angles; and angles ABD and CBD are equal because of the law of reflection. Therefore, the two triangles ABD and CBD are congruent, and the length AD = CD. Thus the image appears as far behind the mirror as the object is in front: theimage distance, (distance from mirror to image, Fig.4.2), equals theobject distance, . From the geometry, we also see that the height of the image is the same as that of the object.

The light rays do not actually pass through the image location itself. It merely seems as though the light is coming from the image because our brains interpret any light entering our eyes as having come in a straight line path from in front of us. Because the rays do not actually pass through the image, the image would not appear on paper or film placed at the location of the image. Therefore, it is called avirtual image. This is to distinguish it from areal image in which the light does pass through the image and which therefore could appear on paper or film placed at the image position. We will see that curved mirrors and lenses can form real images. A movie projector lens, for example, produces a real image that is visible on the screen.

4.4. FACTS AND THEORIES ABOUT LIGHT

Light sources, other than those which shine by reflected light, are generally hot; and in the case of solids and liquids (such as filament lamp or the surface of molten iron) the nature of the light emitted depends largely on the t° of the sources - the lower the t° of the source, the greater the preponderance of red light in the emission. The following is an approximate guide to colour t° for surfaces which are good radiators:

very dull red. 500-550° С vellow 1050-1150°C

dark red. 650-750° С yellow-white 1250-1350° С

bright red. 850-950° С white 1450-1550° С

Light of all types travels through space at the rate of 186,000 miles/sec, or 3·10¹⁰ cm./sec.

This was first discovered by a Danish astronomer, Roemer, who in 1674 found that light from a celestial source (he observed the eclipse of the light from one of Jupiter's moons) took 1000 seconds to cross the Earth's orbit of 186,000,000 miles. The speed is the same for light of all colours. It is also the same for all types of thermal radiation and for radio waves whether of long or short wave-length. The common speed of such widely different types of radiation is part of the evidence which suggests that they are different aspects of similar phenomena. The name electro-magnetic radiation has been given to all these radiations. Present day theory assumes that light is a disturbance, propagated in space, similar to that sent out from a radio aerial, but whereas the electrical oscillations in an aerial have a frequency of the order of a million per second, the electrical oscillations set up by the electrons in the atoms of a light-source have a frequency of the order of a thousand million per second.

About the beginning of the 18th century there was much speculation as to whether light consists of minute weightless particles, shot out by source (Corpuscular Theory) or a disturbance spreading out from the source as a wave motion (Wave Theory).

Huyghens (1629-1695) developed the Wave Theory. Newfon (1642-1727) recognizing, that both theories could explain all that was then known about light, somewhat favoured the Corpuscular Theory. He was influenced in this by the fact that water ripples and sound waves can bend round obstacles, a phenomenon known as diffraction. Point sources of light cast sharp shadows with no diffraction. Though Newton had observed peculiarities at the edges of shadows he did not consider that they were examples of diffraction. It was not until the beginning of the 19th century that Young (1773-1829) and Fresnel (1788-1927) showed that diffraction of light does occur, and that the apparently straight-line travel of light is the result of its very short wave-length.

The refraction of light passing from air into water was explained by the Corpuscular Theory as due to an increase in speed of the waves.

In 1851 Foucault showed that the speed of light in water is less than in air.

4.5. WAVE-LENGHTS AND FREQUENCIES OF ELECTRO-MAGNETIC RADIATION

Measurement of wave-length of light in vacuo shows that it lies between 0.70 and 0.35 of a micron (millionth of a metre); the larger value is for red light and the smaller for violet light. To produce such a small wave-length, the frequency of the oscillation which gives rise to it must be very great. It can be calculated from the general formula for wave motion.

Date: 2015-12-11; view: 844