{"id":119,"date":"2017-09-04T04:32:39","date_gmt":"2017-09-04T04:32:39","guid":{"rendered":"http:\/\/americanboard.org\/Subjects\/general-science\/?page_id=119"},"modified":"2017-09-20T15:55:42","modified_gmt":"2017-09-20T15:55:42","slug":"optics","status":"publish","type":"page","link":"https:\/\/americanboard.org\/Subjects\/general-science\/optics\/","title":{"rendered":"Optics"},"content":{"rendered":"<div class=\"twelve columns\" style=\"margin-top: 10%;\">\n<div class=\"advance\"><a class=\"button button-primary\" href=\"http:\/\/americanboard.org\/Subjects\/general-science\/magnetism-and-electromagnetism\">\u2b05 Previous Lesson<\/a>\u00a0<a class=\"button\" href=\"http:\/\/americanboard.org\/Subjects\/general-science\/physics\">Workshop Index<\/a>\u00a0<a class=\"button button-primary\" href=\"http:\/\/americanboard.org\/Subjects\/general-science\/further-reading-5\">Further Reading \u27a1<\/a><\/div>\n<p><!-- CONTENT BEGINS HERE --><\/p>\n<h1 id=\"title\">Optics<\/h1>\n<section>\n<h3>Reflection, refraction, and absorption<\/h3>\n<p>When light hits a boundary between different media, three phenomena occur:<\/p>\n<ul>\n<li><abbr title=\"The interaction between a wave and a surface it hits through which it cannot pass.\">Reflection<\/abbr>\u2014Some of the energy bounces back into the original medium.<\/li>\n<li><abbr title=\"A phenomenon that occurs when wave energy transmits into a new medium and bending (changing direction) of a wave when the speed of a wave changes while passing from one medium into another.\">Refraction<\/abbr>\u2014Some of the energy transmits into the new medium.<\/li>\n<li><abbr title=\"A phenomenon that occurs when wave energy transfers to heat energy at a boundary.\">Absorption<\/abbr>\u2014Some of the energy transfers to heat energy at the boundary.<\/li>\n<\/ul>\n<p>Instead of focusing on the crests and troughs, we show the path of light waves with rays. The ray that strikes a boundary is called the incident ray. The ray that bounces back into the original medium is called the reflected ray. Both rays make equal angles with respect to a line perpendicular to the surface (the \u201cnormal line\u201d), as shown in the following figure:<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig39s.jpg\" alt=\"Reflected ray\" \/><\/center>The <abbr title=\"a law that states that the angle of incidence of a light ray is congruent to the angle of reflection. In addition, the incident ray, normal line, and reflected ray are coplanar.\">law of reflection<\/abbr> is stated as follows:<\/p>\n<ul>\n<li>The angle of incidence (i) is equal to the angle of reflection (r)<\/li>\n<li>The incident ray, normal line, and reflected ray exist in the same plane.<\/li>\n<\/ul>\n<p>The law of reflection applies to all surfaces: shiny, curved, or rough. When light rays hit rough surfaces, though, the normal line direction varies for each part of the surface and the light diffuses in all directions.<\/p>\n<p>What happens when light hits an optically transparent medium? Some of the light energy will always reflect back into the original medium. But much of the light and energy usually enters the new medium. Depending on the optical density of that medium the light will &#8220;bled&#8221; or refracts a slight amount from its original path, or it may refract quite a bit from its original path. If the new medium slows down the light we say that the medium has a higher optical density. When this occurs, the light bends to a smaller refraction angle (R) relative to the normal line as shown in the following diagram:<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig40s.jpg\" alt=\"Reflected and refracted rays\" \/><\/center>If the light ray exits a denser medium, the ray will bend back out to a large refraction angle as shown in the following diagram:<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig41s.jpg\" alt=\"Reflected and refraced rays\" \/><\/center><\/p>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>What is the final angle of reflection (r) in the following figure (not to scale)?<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig42s.jpg\" alt=\"Reflection question\" \/><\/center><\/p>\n<ol>\n<li>65\u00b0<\/li>\n<li>55\u00b0<\/li>\n<li>45\u00b0<\/li>\n<li>35\u00b0<\/li>\n<\/ol>\n<p><a class=\"button button-primary q-answer\"> Reveal Answer <\/a><\/p>\n<p class=\"q-reveal\">The correct answer is B; r = 55\u00b0. The steps are as follows:<br \/>\n<img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig43s.jpg\" alt=\"Reflection answer\" \/><\/p>\n<\/section>\n<h3>Total internal reflection<\/h3>\n<p>When light rays attempt to exit a medium with high optical density, refraction may not occur. Rather, the energy may be completely reflected back into the denser medium. This phenomenon is called <abbr title=\"a phenomenon that occurs when light energy is completely reflected back into a denser medium.\">total internal reflection (TIR)<\/abbr>. What determines when TIR occurs? It depends on the \u201ccritical angle\u201d of the materials involved. Every pair of transparent materials has a unique critical angle depending on their optical densities. The following table lists some common critical angles:<\/p>\n<table>\n<thead>\n<tr>\n<th>Light starting in\u2026<\/th>\n<th>Light trying to enter\u2026<\/th>\n<th>Critical angle<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Water<\/td>\n<td>Air<\/td>\n<td>49\u00b0<\/td>\n<\/tr>\n<tr>\n<td>Glass<\/td>\n<td>Air<\/td>\n<td>41\u00b0<\/td>\n<\/tr>\n<tr>\n<td>Glass<\/td>\n<td>Water<\/td>\n<td>61\u00b0<\/td>\n<\/tr>\n<tr>\n<td>Diamond<\/td>\n<td>Air<\/td>\n<td>24\u00b0<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Notice that the critical angle is only relevant when light starts in an optically dense material and attempts to move into a less dense material. The critical angle is used with following logic to determine if TIR occurs at a boundary:<\/p>\n<ul>\n<li>When the incident angle is less than the critical angle, partial reflection and partial refraction occur.<\/li>\n<li>When the incident angle equals the critical angle, a tiny amount of light exits at a 90-degree refraction angle while the majority of the energy reflects back into the denser medium.<\/li>\n<li>When the incident angle is greater than the critical angle, total internal reflection occurs.<\/li>\n<\/ul>\n<p>For example, when light attempts to exit a diamond into air at an angle of 20 degrees, it will partially reflect and partially refract because its incident angle is less than the critical angle of 24 degrees. When light tries to exit at 30 degrees however, it will totally reflect because its angle is greater than the critical angle.<\/p>\n<p>Total internal reflection is used in optical fibers to send information efficiently from one location to another. The following diagram shows the total internal reflections that occur through a light pipe:<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig44s.jpg\" alt=\"Total internal reflection inside a light pipe\" \/><\/center>Optical fibers are the main technology behind endoscopes. Light is sent down tiny fibers that illuminate the inside of the human body. The image is then piped back to a video monitor for the surgeon to evaluate.<\/p>\n<p>The phenomena of total internal reflection is also used to reflect light in prisms as follows:<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig45s.jpg\" alt=\"Total internal refraction\" \/><\/center>These prisms are sometimes used in binoculars to reflect and shift light images.<\/p>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>When a light ray attempts to move from glass into air at an incident angle of 45 degrees, which of the following phenomenon occurs? (Note: the critical angle for glass\/air is 41 degrees.)<\/p>\n<ol>\n<li>Total refraction<\/li>\n<li>Total internal reflection<\/li>\n<li>Partial refraction and partial reflection<\/li>\n<li>Total absorption<\/li>\n<\/ol>\n<p><a class=\"button button-primary q-answer\"> Reveal Answer <\/a><\/p>\n<p class=\"q-reveal\">The correct answer is B because the incident angle is greater than the critical angle.<\/p>\n<\/section>\n<h3>Dispersion<\/h3>\n<p>Previously we learned that different colors of light have different frequencies, where red light has the lowest frequency and violet light has the highest frequency. In this lesson, we will study how light of various frequencies refract differently in glass.<\/p>\n<p>When white light hits a glass prism just right, the light exiting the prism will bend into a full rainbow of colors through the process of <abbr title=\"a phenomenon that occurs when white light is split into a rainbow of colors as it moves through a prism.\">dispersion<\/abbr>. This occurs because high-frequency visible light tends to interact with the glass molecules more than the low frequencies. The result looks like this:<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/rainbow.png\" alt=\"Light through a prism\" \/><\/center>In the diagram it is evident that the lower-frequency red light bends less than the higher-frequency violet light.<\/p>\n<p>Dispersion is responsible for rainbows. If sunlight hits water droplets in the air just right, the droplets behave like little prisms and disperse and reflect a rainbow of light toward the observer.<\/p>\n<p>Light can also be dispersed using prism spectrometers (a.k.a. spectroscopes). These devices analyze visible light so the observer can identify the colors (frequencies) that are present in a particular light source.<\/p>\n<h3>Polarization of light<\/h3>\n<p>Previously we learned that transverse waves vibrate the medium at right angles to the motion of the wave energy. Longitudinal waves, on the other hand, vibrate the medium parallel to the direction of wave motion through compressing the medium. Since light radiation consists of electric and magnetic vibrations that are perpendicular to the direction of wave motion, light is classified as a transverse wave. In this lesson, we will study the phenomenon of light polarization that provides evidence that light is a transverse wave.<\/p>\n<p>The vibrations that produce sunlight are in random directions. Likewise, the electron vibrations in candle flames and the filaments of light bulbs are also in all directions. These sources produce <abbr title=\"light in which the transverse electromagnetic vibrations are in many directions.\">unpolarized light<\/abbr> because the transverse vibrations that produce the light are in many directions. Imagine looking at a beam of light coming straight toward you. The following diagram represents unpolarized transverse light waves vibrating horizontally, vertically, and at angles:<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig46s.jpg\" alt=\"Unpolarized light\" \/><\/center>When unpolarized light is sent through a polarizing filter, all the components of vibration that are not aligned with the filter are absorbed. The light that emerges from the polarizing filter is called <abbr title=\"light in which the transverse electromagnetic vibrations are in a along a single direction.\">polarized light<\/abbr>.<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig47s.jpg\" alt=\"Polarizing filter effect\" \/><\/center>The intensity of light that exits a filter in this case is half of the original intensity.<\/p>\n<p>When vertically polarized light attempts to get through a horizontal filter, the intensity drops to zero:<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig48s.jpg\" alt=\"Horizontal polarizing filter effect\" \/><\/center>When unpolarized light hits horizontal surfaces at large angles to the normal line, the reflected light is horizontally polarized. Vertically polarized sunglasses are effective at eliminating this glare off horizontal surfaces. They also reduce the intensity of all the other light by one-half.<\/p>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>A beam of light moves through a horizontal and then a vertical polarizing filter. The intensity of the light emerging from the combination is<\/p>\n<ol>\n<li>zero.<\/li>\n<li>one-quarter of the original intensity.<\/li>\n<li>half the original intensity.<\/li>\n<li>the same as the original intensity.<\/li>\n<\/ol>\n<p><a class=\"button button-primary q-answer\"> Reveal Answer <\/a><\/p>\n<p class=\"q-reveal\">The correct answer is A. Only horizontally polarized light is able to get through the first filter. Since the second filter is vertical, the horizontal light is completely absorbed.<\/p>\n<\/section>\n<h3>Interference of light<\/h3>\n<p>Previously we have seen that waves are energy. When a wave meets a wave, the waves pass right through each other. When they occupy the same space, they interfere with one another. If identical parts of the waves interfere (e.g. a crest meets a crest), the disturbance grows due to constructive interference. If opposite parts of the wave interfere, the disturbance is reduced through destructive interference. In this lesson, we will study different ways that light waves interfere and how this provides evidence that light behaves like a wave.<\/p>\n<p>Historically, there was much debate about whether light behaves like a wave or like a particle. In fact, the scientific community generally believed that light behaved as a particle up until 1801. In that year, Thomas Young performed an experiment by sending monochromatic light (one frequency) through two tiny openings. When the light hits a screen, it showed multiple bright areas of constructive interference as well as dark areas of destructive interference. <abbr title=\"A phenomenon first demonstrated by Thomas Young in 1801 in which monochromatic light is sent through two tiny openings. When the light hits a screen, it shows multiple bright areas of constructive interference as well as dark areas of destructive interference.\">Two-slit interference<\/abbr> is shown in the following diagram:<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/2slit.PNG\" alt=\"2-slit interference\" \/><\/center>As you can see in the diagram, the light has equal distances to move to the center of the screen, so the waves arrive in phase (trough on trough). This results in constructive interference and a bright spot is observed. On either side of the central bright spot, the waves arrive out of phase (crest on trough), resulting in a dark area due to destructive interference. The pattern continues to alternate between light and dark bands as you go out on the screen.<\/p>\n<p>When light hits soap bubbles or a film of gasoline on water, a beautiful rainbow of colors often results. Why does this occur? To explain this, we must realize that the light is actually reflecting from two surfaces. In the gasoline example, the light reflects from the surface of the gasoline as well as the water. The reflected rays may interfere constructively or destructively depending on their frequency (color). As the thickness of the film varies, certain colors interfere constructively and these are the ones that we see. This phenomenon is called <abbr title=\"a phenomenon in which thin films (like soap bubbles) reflect a rainbow of colors. This is due to variations in the thickness of the film and the interference between reflected rays.\">thin film iridescence<\/abbr>.<\/p>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>Young\u2019s famous two-slit experiment demonstrates which of the following properties of light?<\/p>\n<ol>\n<li>Polarization<\/li>\n<li>Reflection<\/li>\n<li>Its wave-like nature<\/li>\n<li>Its particle-like nature<\/li>\n<\/ol>\n<p><a class=\"button button-primary q-answer\"> Reveal Answer <\/a><\/p>\n<p class=\"q-reveal\">The correct answer is C. The two-slit experiment shows interference patterns from light. Interference is a phenomenon associated with waves.<\/p>\n<\/section>\n<h3>Diffraction of light<\/h3>\n<p>When waves hit an opening, they tend to fan out in many directions. <abbr title=\"the bending of waves around obstacles.\">Diffraction<\/abbr> is the bending of waves around obstacles. The amount of diffraction depends on the size of the opening as in the following figure:<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig49s.jpg\" alt=\"Wave diffraction\" \/><\/center>The amount of diffraction also depends on the wavelength of the disturbance:<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig50s.jpg\" alt=\"Wave diffraction\" \/><\/center>To summarize, the amount of bending due to diffraction is directly related to the wavelength and inversely related to the width of the opening (or obstacle).<\/p>\n<p>Since light is a wave, it also diffracts. If light goes through a big opening like a window, the light does not diffract significantly. This is because the wavelength of light is too small compared to the width of the opening. If light travels through a tiny slit, it spreads out significantly. So one might expect to only see a bright center pattern fading to the edges. But besides this pattern, the following figure shows interference patterns similar to those seen in Young\u2019s two-slit experiment.<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig51s.jpg\" alt=\"Light diffraction\" \/><\/center>When light diffracts through a small circular aperture, a similar observation may be made. The screen pattern has a bright center fading to the edges along with concentric circles of interference, as shown in the following picture:<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig52s.jpg\" alt=\"Circular light diffraction\" \/><\/center><\/p>\n<p class=\"figcaption\">Laser light diffraction pattern through circular aperture<\/p>\n<p>Light diffraction distorts images of very tiny objects like those on microscope slides. Tiny objects have dimensions that are similar to the wavelength of light. This results in significant bending around the objects and their images are not clearly seen. In order to see tiny objects clearly, they must be illuminated with wavelengths that are significantly shorter than light. Electron beams, like those used in electron microscopes, have tinier wavelengths than light and enable scientist to observe very small objects. For each source of illumination, diffraction sets a limit on the resolution of images.<\/p>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>In which of the following cases is the amount of light diffraction the most significant?<\/p>\n<ol>\n<li>Long wavelength of light and tiny opening<\/li>\n<li>Long wavelength of light and large opening<\/li>\n<li>Short wavelength of light and tiny opening<\/li>\n<li>Short wavelength of light and large opening<\/li>\n<\/ol>\n<p><a class=\"button button-primary q-answer\"> Reveal Answer <\/a><\/p>\n<p class=\"q-reveal\">The correct answer is A. The amount of wave diffraction depends on the ratio of wavelength to the size of the opening.<\/p>\n<\/section>\n<h3>Lens ray diagrams<\/h3>\n<p>Previously you have learned that when light hits a medium with a different optical density, the beam refracts and bends to different angles. In this lesson, we will study the images produced when light refracts through lenses.<\/p>\n<p>Lenses are used in a variety of applications, including glasses, microscopes, telescopes, and the human eye. There are two types of lenses: convex or concave. A <abbr title=\"a lens that is thicker in the middle and takes parallel light rays and focuses them to a common point (also known as a converging lens).\">convex lens<\/abbr> (or converging lens) is thicker in the middle and takes parallel light rays and focuses them to a common point, called the focal point (F)<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig53s.jpg\" alt=\"Convex lens\" \/><\/center>It is for this reason that they are sometimes call converging lenses.<\/p>\n<p>A <abbr title=\"a lens that is thinner in the middle and takes parallel rays of light and spreads them apart (also known as a diverging lens).\">concave lens<\/abbr> (or diverging lens) is thinner in the middle and takes parallel rays of light and spreads them apart. The diverging rays appear to originate from the focal point (F), sometimes called a virtual focal point because the rays don\u2019t actually go through this point.<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig54s.jpg\" alt=\"Concave lens\" \/><\/center>When light reflects off an object, it typically diffuses out in all directions. As this reflected light hits a lens, the rays are bent to form images of the object. Images are classified in the following manner:<\/p>\n<ul>\n<li>Type\u2014Real images formed by converging light rays or virtual images that are an illusion formed by diverging light rays.<\/li>\n<li>Orientation\u2014Upright (erect) or inverted with respect to the object.<\/li>\n<li>Size\u2014Larger, smaller, or same size as compared to the object.<\/li>\n<\/ul>\n<p>An image\u2019s characteristics depend on the type of lens used as well as the location of the object with respect to the lens.<\/p>\n<p>To determine the location of images, lens ray diagrams are used. The following terms are used in the ray diagram rules:<\/p>\n<ul>\n<li>Optical axis\u2014A line along the center axis of the lens where the rays are shown to bend at.<\/li>\n<li>Principal Axis (PA): A line that goes though the middle of the lens, striking the optical axis at a 90-degree angle<\/li>\n<li>Focal point (F)\u2014The point where rays parallel to the PA converge to (or appear to diverge from). This point is one focal length from the lens.<\/li>\n<li>Optical Center (O)\u2014The very center of the lens.<\/li>\n<li>Secondary focal point (F\u2019)\u2014A point on the PA that is one focal length from the lens but on the other side of the lens from F.<\/li>\n<\/ul>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig55s.jpg\" alt=\"Lens ray diagram\" \/><\/center>The following three rules are used for constructing ray diagrams for convex (converging) lenses:<\/p>\n<ul>\n<li>Any ray parallel to the principal axis refracts through the focal point.<\/li>\n<li>Any ray going through the secondary focal point refracts parallel to the principal axis.<\/li>\n<li>Any ray going through the optical center goes straight through the lens (if it\u2019s a thin lens).<\/li>\n<\/ul>\n<p>Let\u2019s begin by looking at objects that are located beyond two focal lengths from the lens and apply the three rules above.<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig56s.jpg\" alt=\"Lens ray diagram beyond two focal lengths\" \/><\/center>The characteristics of this image are real, inverted, and smaller. Real, because actual light rays converge to a point; inverted, because the rays intersect below the principal axis; and smaller, because the image arrow is smaller than the object.<\/p>\n<p>Here are some other diagrams with the object at different locations:<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig57s.jpg\" alt=\"Lens ray diagram\" \/><\/center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig58s.jpg\" alt=\"Lens ray diagram\" \/><\/p>\n<p><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig59s.jpg\" alt=\"Lens ray diagram\" \/><\/p>\n<p><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig60s.jpg\" alt=\"Lens ray diagram\" \/><\/p>\n<p>The last ray diagram above shows how in the formation of a virtual image the real light rays do not intersect. To see the image, it must be viewed by looking back through the lens, giving the illusion of a magnified image. This is how a simple magnifying lens works, but only if the object is within one focal length of the lens. An example of convex lenses would be the ones used as reading glasses to correct farsightedness.<\/p>\n<p>The ray rules are almost identical for concave (diverging) lenses:<\/p>\n<ul>\n<li>Any ray parallel to the principal axis diverges as it comes from the virtual focal point.<\/li>\n<li>Any ray going toward the secondary focal point refracts parallel to the principal axis.<\/li>\n<li>Any ray going through the optical center goes straight through the lens (if it\u2019s a thin lens).<\/li>\n<\/ul>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig61s.jpg\" alt=\"Lens ray diagram\" \/><\/center>Concave lenses always produce virtual, smaller, and upright images. Concave lenses are used as distance glasses to correct myopia (nearsightedness).<\/p>\n<p>The table below summarizes the images formed by lenses:<\/p>\n<table>\n<thead>\n<tr>\n<th>Lens Type<\/th>\n<th>Object Distance from Lens<\/th>\n<th>Image Type<\/th>\n<th>Image Orientation<\/th>\n<th>Image Size<\/th>\n<th>Possible Applications<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Convex<\/td>\n<td>Beyond two focal lengths<\/td>\n<td>Real<\/td>\n<td>Inverted<\/td>\n<td>Smaller<\/td>\n<td>Camera, Human eye<\/td>\n<\/tr>\n<tr>\n<td>Convex<\/td>\n<td>At two focal lengths<\/td>\n<td>Real<\/td>\n<td>Inverted<\/td>\n<td>Same Size<\/td>\n<td>Copy machine (100%)<\/td>\n<\/tr>\n<tr>\n<td>Convex<\/td>\n<td>Between 1 &amp; 2 focal lengths<\/td>\n<td>Real<\/td>\n<td>Inverted<\/td>\n<td>Larger<\/td>\n<td>Movie Projector<\/td>\n<\/tr>\n<tr>\n<td>Convex<\/td>\n<td>At one focal length<\/td>\n<td>None<\/td>\n<td>None<\/td>\n<td>None<\/td>\n<td>Spot lights<\/td>\n<\/tr>\n<tr>\n<td>Convex<\/td>\n<td>Between lens and focal point<\/td>\n<td>Virtual<\/td>\n<td>Upright<\/td>\n<td>Larger<\/td>\n<td>Reading Glasses<\/td>\n<\/tr>\n<tr>\n<td>Convex<\/td>\n<td>Anywhere<\/td>\n<td>Real<\/td>\n<td>Upright<\/td>\n<td>Smaller<\/td>\n<td>Distance Glasses<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>A copy machine is used to make an image that is 50% of the original. What type of lens may be used to project the right size image to copy?<\/p>\n<ol>\n<li>Convex<\/li>\n<li>Concave<\/li>\n<li>Diffraction<\/li>\n<li>Myopic<\/li>\n<\/ol>\n<p><a class=\"button button-primary q-answer\"> Reveal Answer <\/a><\/p>\n<p class=\"q-reveal\">The correct answer is A. Real light rays must be focused to a point in order to burn an image onto the copy. Only convex lenses are capable of producing real images.<\/p>\n<\/section>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>A copy machine is used to make an image that is 50% the size of the original. How far from the lens should the original be placed?<\/p>\n<ol>\n<li>Less than 1 focal length<\/li>\n<li>1 focal length<\/li>\n<li>2 focal lengths<\/li>\n<li>More than 2 focal lengths<\/li>\n<\/ol>\n<p><a class=\"button button-primary q-answer\"> Reveal Answer <\/a><\/p>\n<p class=\"q-reveal\">The correct answer is D. In order to get a smaller image, the original must be beyond two focal lengths from the lens.<\/p>\n<\/section>\n<h3>Applications of lenses<\/h3>\n<p>Lenses (as well as mirrors) are used in combination to produce significant magnifications. The compound microscope, for example, takes the rays coming from a microscope slide and refracts them through a convex lens (the objective lens) to produce a real, inverted, larger image. This image is viewed through a second convex lens (the eyepiece lens) that magnifies it even more. <abbr title=\"a device with 2 or more lenses that magnify tiny objects that are close to the system.\">Compound microscopes<\/abbr> have 2 or more lenses, depending on the resolution needed.<\/p>\n<p><abbr title=\"takes the nearly parallel light from distant objects and projects an image that the observer can see. It typically uses two convex lenses.\">Astronomical refracting telescopes<\/abbr> usually use two convex lenses in combination. The light from distant planets is refracted to a real, smaller, inverted image through the first lens (the objective lens). The second lens (the eyepiece lens) magnifies this image. The image coming through the two-lens system is inverted relative to the original objects.<\/p>\n<p>Mirrors are also used in combination with lenses to produce images. Sir Isaac Newton designed the first reflecting telescope in 1668. The <abbr title=\"a device that uses mirrors and lenses to focus the light from distant stars.\">Newtonian reflecting telescope<\/abbr> uses a concave mirror to focus the light from distant starts to a diagonal plane mirror as shown in the diagram below. The plane mirror reflects the rays though a hole in the side of the telescope into an eyepiece lens. Because large diameter mirrors can be supported better than lenses, these telescopes gather more light than astronomical telescopes.<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig62s.jpg\" alt=\"Newtonian telescope\" \/><\/center><\/p>\n<p class=\"figcaption\">Newtonian telescope diagram<\/p>\n<p>The human eye can also be visualized as a two-lens system. The front part of the eye, called the cornea, is where most of the refraction occurs. Significant bending occurs here because light slows down significantly as it enters the cornea. After the light bends through the cornea, it hits the lens. The lens makes fine adjustments in order to focus the light on the retina. The retina sends the image information to the brain for processing.<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/Images\/gensci\/img\/Mod5Fig63s.jpg\" alt=\"Human eye\" \/><\/center><\/p>\n<p class=\"figcaption\">Human eye lens<\/p>\n<\/section>\n<p><!-- CONTENT ENDS HERE --><\/p>\n<div class=\"advance\"><a class=\"button button-primary\" href=\"http:\/\/americanboard.org\/Subjects\/general-science\/magnetism-and-electromagnetism\">\u2b05 Previous Lesson<\/a>\u00a0<a class=\"button\" href=\"http:\/\/americanboard.org\/Subjects\/general-science\/physics\">Workshop Index<\/a>\u00a0<a class=\"button button-primary\" href=\"http:\/\/americanboard.org\/Subjects\/general-science\/further-reading-5\">Further Reading \u27a1<\/a><\/div>\n<p><a class=\"backtotop\" href=\"#title\">Back to Top<\/a><\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"<p>\u2b05 Previous Lesson\u00a0Workshop Index\u00a0Further Reading \u27a1 Optics Reflection, refraction, and absorption When light hits a boundary between different media, three phenomena occur: Reflection\u2014Some of the energy bounces back into the original medium. Refraction\u2014Some of the energy transmits into the new medium. Absorption\u2014Some of the energy transfers to heat energy at the boundary. Instead of focusing [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-119","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/americanboard.org\/Subjects\/general-science\/wp-json\/wp\/v2\/pages\/119","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/americanboard.org\/Subjects\/general-science\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/americanboard.org\/Subjects\/general-science\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/americanboard.org\/Subjects\/general-science\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/americanboard.org\/Subjects\/general-science\/wp-json\/wp\/v2\/comments?post=119"}],"version-history":[{"count":4,"href":"https:\/\/americanboard.org\/Subjects\/general-science\/wp-json\/wp\/v2\/pages\/119\/revisions"}],"predecessor-version":[{"id":419,"href":"https:\/\/americanboard.org\/Subjects\/general-science\/wp-json\/wp\/v2\/pages\/119\/revisions\/419"}],"wp:attachment":[{"href":"https:\/\/americanboard.org\/Subjects\/general-science\/wp-json\/wp\/v2\/media?parent=119"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}