Water is also a reflective surface. When the water in a lake or sea is very still, the reflection of the landscape is perfect, because the reflecting surface is very flat. However, if there are ripples or waves in the water, the reflection becomes distorted.
This is because the reflecting surface is no longer flat and may have humps and troughs caused by the wind. It is possible to make mirrors that behave like humps or troughs, and because of the different way they reflect light, they can be very useful.
Concave mirrors are used in certain types of astronomical telescopes called reflecting telescopes. The mirrors condense lots of light from faint sources in space onto a much smaller viewing area and allow the viewer to see far away objects and events in space that would be invisible to the naked eye.
Light rays travel towards the mirror in a straight line and are reflected inwards to meet at a point called the focal point. Concave mirrors are useful for make-up mirrors because they can make things seem larger. This concave shape is also useful for car headlights and satellite dishes. Parallel rays of light strike the mirror and are reflected outwards. If imaginary lines are traced back, they appear to come from a focal point behind the mirror. Convex mirrors are useful for shop security and rear-view mirrors on vehicles because they give a wider field of vision.
Some light is scattered in all directions when it hits very small particles such as gas molecules or much larger particles such as dust or droplets of water.
The amount of scattering depends on how big the particle is compared to the wavelength of light that is hitting it. Smaller wavelengths are scattered more. If the spoon is moved farther away, a demagnified upside-down view of the whole face will be seen. Here the image is inverted because it is formed after the reflected rays have crossed the focal point of the mirror surface.
Another common mirror having a curved-surface, the convex mirror, is often used in automobile rear-view reflector applications where the outward mirror curvature produces a smaller, more panoramic view of events occurring behind the vehicle. When parallel rays strike the surface of a convex mirror, the light waves are reflected outward so that they diverge. When the brain retraces the rays, they appear to come from behind the mirror where they would converge, producing a smaller upright image the image is upright since the virtual image is formed before the rays have crossed the focal point.
Convex mirrors are also used as wide-angle mirrors in hallways and businesses for security and safety. The most amusing applications for curved mirrors are the novelty mirrors found at state fairs, carnivals, and fun houses. These mirrors often incorporate a mixture of concave and convex surfaces, or surfaces that gently change curvature, to produce bizarre, distorted reflections when people observe themselves.
Spoons can be employed to simulate convex and concave mirrors, as illustrated in Figure 4 for the reflection of a young woman standing beside a wooden fence.
When the image of the woman and fence are reflected from the outside bowl surface convex of the spoon, the image is upright, but distorted at the edges where the spoon curvature varies.
In contrast, when the reverse side of the spoon the inside bowl, or concave, surface is utilized to reflect the scene, the image of the woman and fence are inverted. The reflection patterns obtained from both concave and convex mirrors are presented in Figure 5.
The concave mirror has a reflection surface that curves inward, resembling a portion of the interior of a sphere. When light rays that are parallel to the principal or optical axis reflect from the surface of a concave mirror in this case, light rays from the owl's feet , they converge on the focal point red dot in front of the mirror.
The distance from the reflecting surface to the focal point is known as the mirror's focal length. The size of the image depends upon the distance of the object from the mirror and its position with respect to the mirror surface. In this case, the owl is placed away from the center of curvature and the reflected image is upside down and positioned between the mirror's center of curvature and its focal point.
The convex mirror has a reflecting surface that curves outward, resembling a portion of the exterior of a sphere. Light rays parallel to the optical axis are reflected from the surface in a direction that diverges from the focal point, which is behind the mirror Figure 5. Images formed with convex mirrors are always right side up and reduced in size.
These images are also termed virtual images, because they occur where reflected rays appear to diverge from a focal point behind the mirror. The manner in which gemstones are cut is one of the more aesthetically important and pleasing applications of the principles of light reflection. Particularly in the case of diamonds, the beauty and economic value of an individual stone is largely determined by the geometric relationships of the external faces or facets of the gem.
The facets that are cut into a diamond are planned so that most of the light that falls on the front face of the stone is reflected back toward the observer Figure 6. A portion of the light is reflected directly from the outside upper facets, but some enters the diamond, and after internal reflection, is reflected back out of the stone from the inside surfaces of the lower facets. These internal ray paths and multiple reflections are responsible for a diamond's sparkle, often referred to as its "fire".
An interesting consequence of a perfectly cut stone is that it will show brilliant reflection when viewed from the front, but will look darker or dull from the back, as illustrated in Figure 6. Light rays are reflected from mirrors at all angles from which they arrive.
In certain other situations, however, light may only be reflected from some angles and not others, leading to a phenomenon known as total internal reflection. This can be illustrated by a situation in which a diver working below the surface of perfectly calm water shines a bright flashlight directly upward at the surface. If the light strikes the surface at right angles it continues directly out of the water as a vertical beam projected into the air. If the light's beam is directed at a slight angle to the surface, so that it impacts the surface at an oblique angle, the beam will emerge from the water, but will be bent by refraction toward the plane of the surface.
The angle between the emerging beam and the surface of the water will be smaller than the angle between the light beam and the surface below the water.
If the diver continues to angle the light at more of a glancing angle to the surface, the beam rising out of the water will get closer and closer to the surface, until at some point it will be parallel to the surface.
Because of light bending due to refraction, the emerging beam will become parallel to the surface before the light below the water has reached the same angle. The point at which the emerging beam becomes parallel to the surface occurs at the critical angle for water. If the light is angled still further, none of it will emerge. Instead of being refracted, all of the light will reflect at the water's surface back into the water just as it would at the surface of a mirror.
The principle of total internal reflection is the basis for fiber optic light transmission that makes possible medical procedures such as endoscopy, telephone voice transmissions encoded as light pulses, and devices such as fiber optic illuminators that are widely used in microscopy and other tasks requiring precision lighting effects.
The prisms employed in binoculars and in single-lens reflex cameras also utilize total internal reflection to direct images through several degree angles and into the user's eye. In the case of fiber optic transmission, light entering one end of the fiber is reflected internally numerous times from the wall of the fiber as it zigzags toward the other end, with none of the light escaping through the thin fiber walls.
This method of "piping" light can be maintained for long distances and with numerous turns along the path of the fiber. Total internal reflection is only possible under certain conditions. The light is required to travel in a medium that has relatively high refractive index, and this value must be higher than that of the surrounding medium.
Water, glass, and many plastics are therefore suitable for use when they are surrounded by air. If the materials are chosen appropriately, reflections of the light inside the fiber or light pipe will occur at a shallow angle to the inner surface see Figure 7 , and all light will be totally contained within the pipe until it exits at the far end.
At the entrance to the optic fiber, however, the light must strike the end at a high incidence angle in order to travel across the boundary and into the fiber. The principles of reflection are exploited to great benefit in many optical instruments and devices, and this often includes the application of various mechanisms to reduce reflections from surfaces that take part in image formation. The concept behind antireflection technology is to control the light used in an optical device in such a manner that the light rays reflect from surfaces where it is intended and beneficial, and do not reflect away from surfaces where this would have a deleterious effect on the image being observed.
One of the most significant advances made in modern lens design, whether for microscopes, cameras, or other optical devices, is the improvement in antireflection coating technology. Thin coatings of certain materials, when applied to lens surfaces, can help reduce unwanted reflections from the surfaces that can occur when light passes through a lens system.
Modern lenses that are highly corrected for optical aberrations generally have multiple individual lenses, or lens elements, which are mechanically held together in a barrel or lens tube, and are more properly referred to as a lens or optical system. In Figure 1 we use a single line to illustrate a light ray reflected from the surface. The law of reflection requires that two rays are at identical angles but on opposite sides of the normal which is an imaginary line dashed in Fig.
We show in Fig. The dashed line normal is perpendicular to the surface. All reflected light obeys the relationship that the angle of incidence equals the angle of reflection. Just as images are reflected from the surface of a mirror, light reflected from a smooth water surface also produced a clear image.
We call the reflection from a smooth, mirror-like surface specular as shown in Figure 2a. When the surface of water is wind-blown and irregular, the rays of light are reflected in many directions. The law of reflection is still obeyed, but the incident rays Fig. Consequently, the outgoing rays are reflected at many different angles and the image is disrupted.
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