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Lesson 3.4
Lesson 3.1 |
Lesson 3.4 Rays & Shadows
Overview This lesson deals with the paths followed by rays of light and the behavior of light beams when obstacles are placed in their path. We introduce the concept of diffraction. On completion of this lesson, you should be able to explain the difference between umbra and penumbra. You should be able to explain why waves bend when they pass an obstruction. You should also be able to draw simple ray diagrams and illustrate eclipses and the operation of a pinhole camera.
Light and Lasers Light spreads out – laser light does not. Light waves occur in three dimensions. Normal light is chaotic in that the waves oscillate in an infinite variety of planes. Polarizing filters can limit the waves to a narrow band of planes but the individual rays within any plane are still out of phase. The "crowded" waves interfere with each other, collide with each other and move apart whenever possible. As rays move further from the source, there is usually more room for the waves to spread into and the waves radiate outwards. Lased radiation is not chaotic. Lasers produce waves with the same frequency that oscillate in the same plane and are in phase with each other. As a result, the waves in laser beams do not interfere with each other or spread out as they move away from the source.
Rays Thin beams of light are often referred to as rays. Rays can theoretically be as thin as one wave but are generally considered to be small bundles of waves. Light rays travel in straight lines from their source and spread out as they move away from the source. Electromagnetic waves emitted from a source are generally chaotic. The waves oscillate in different planes and are out of phase with each other. Peaks and troughs of adjacent waves collide or interact. This causes emitted radiation to travel in all possible directions and to spread out as it moves away from the emitter. Rays travel in straight lines because they are prevented from deviating by adjacent rays.
Beams and Shadows Light travels in straight lines: All waves travel directly and outwards from their source unless they interact with material in a particular way or are influenced by a force. Radiation beams travel in straight lines in any homogeneous medium. A shadow is an area that light cannot reach due the presence of an opaque object in its path. Wave fronts spread out
Eclipses and Pinholes Extended light sources cause partial shadows. Sources of light vary considerably in size. Stars are the largest. The nature of a shadow is governed to a large extent by the difference in size between the light source and the object blocking the light path and the distance between them. The shadow created by an object in the path of a small, point source of light will be dark with sharp edges. This complete shadow is referred to as an umbra. A slightly larger source of light will create regions around the edge of the shadow where some, but not all of the light from the source has been blocked from reaching. This gray area around the edge of the shadow is known as the penumbra. Larger light sources create larger areas of penumbra. Eclipses: A source of light is eclipsed when an object prevents all or part of the destined light from the source from reaching a particular point. For example, a solar eclipse occurs in a particular region of the earth’s surface when the moon blocks rays from the sun from reaching that part of the earth’s surface. The nature of the shadow will depend on the relative distances of the earth, sun and moon from each other. In some cases there will be a region of umbra, in other cases the shadow eill be entirely penumbra and the eclipse will be known as an annular eclipse.
Pinhole camera The effect of the size of the light source is evident in the operation of the pinhole camera. The amount of light that can enter the camera depends on the lource of light and the size of the pinhole. If the hole is small, light from a very small part of the object will strike the screen at athe back of the camera at a particular point. Light from other parts of the object will strike the screen at other points and a relatively sharp image will be formed on the screen. The image will not be very bright because the small pinhole will limit the light considerably. If the hole is made larger, the image will be brighter but fuzzy around the edges. This is because light from a number of points (close together) on the object can strike the same point on the screen. light from a single point on the object can also strike a number of points that are close together on the screen.
Wave Fronts As light radiates from a source, the wave fronts spread out in 3 dimensions. Wave fronts are lines (or planes in the case of 3-D waves) that join the parts of waves that are in phase. Diffraction The edges of a shadow are never perfectly sharp. This is primarily due to the fact that all sources of light have a finite size but also due to a phenomenon known as diffraction. Waves tend to radiate or spread out in order to attain a higher degree of dispersion of their energy. With the exception of laser beams, bundles of electromagnetic waves interact with each other in such a way that they tend to move apart. This lateral movement of a wave is limited by the presence of similar waves on all sides. However, when part of a beam is blocked by an object, space is created. This space allows waves to spread out in to the area where waves no longer exist. Part of the beam appears to bend in the direction of the shadow created by the obstruction. Part of the beam has been diffracted and results in a fuzziness of the shadow created by the obstruction in the path of the light beam. This is particularly evident in waves moving along the surface of a liquid such as water. Waves that are more or less straight and moving along the coastline will change direction as they spread into bays in the coast. Diffraction is the bending effect which occurs when a wave meets an obstacle in its path or passes between obstacles in the form of an aperture. The amount that the wave bends depends on the wavelength of the wave compared to the size of the obstacle or aperture. When a straight wave travels along the surface of a liquid, particles of the liquid near the surface move in a circular path in a vertical plane in the direction of the wave. Observed from above, there is no lateral component to the movement of the particle. The particle does not move from side to side at all. As a wave front approaches a particle, it gains potential energy as it is dragged upwards by the approaching wave. It then loses potential energy under the influence of gravity and passes this energy on to particles further down the path and in the direction of the wave’s movement. If however part of the wave has been removed to the left or right of the direction of travel, the particle is free to move sideways as it returns to a lower state of potential energy. This causes some migration of material sideways and a resultant slowing down of the wave in the region in which this occurs.
Review Questions
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