What Happens to Light When It Enters a Lens

Refraction by Lenses

Nosotros have already learned that a lens is a carefully ground or molded slice of transparent material that refracts light rays in such a way every bit to form an paradigm. Lenses serve to refract light at each purlieus. As a ray of light enters a lens, it is refracted; and every bit the same ray of light exits the lens, it is refracted over again. The net result of the refraction of light at these two boundaries is that the light ray has changed directions. Because of the special geometric shape of a lens, the low-cal rays are refracted such that they form images. Earlier we approach the topic of image formation, nosotros will investigate the refractive power of converging and diverging lenses.

How a Lens Refracts Calorie-free

Kickoff lets consider a double convex lens. Suppose that several rays of light approach the lens; and suppose that these rays of calorie-free are traveling parallel to the principal axis. Upon reaching the forepart face of the lens, each ray of low-cal will refract towards the normal to the surface. At this purlieus, the light ray is passing from air into a more dumbo medium (usually plastic or drinking glass). Since the light ray is passing from a medium in which information technology travels fast (less optically dense) into a medium in which it travels relatively slow (more optically dense), it will curve towards the normal line. This is the FST principle of refraction. This is shown for two incident rays on the diagram below. Once the low-cal ray refracts across the boundary and enters the lens, it travels in a direct line until it reaches the dorsum face of the lens. At this boundary, each ray of light will refract away from the normal to the surface. Since the light ray is passing from a medium in which it travels slow (more optically dumbo) to a medium in which it travels fast (less optically dense), it will bend away from the normal line; this is the SFA principle of refraction.

The above diagram shows the behavior of two incident rays approaching parallel to the primary axis. Annotation that the two rays converge at a bespeak; this point is known equally the focal indicate of the lens. The beginning generalization that can be made for the refraction of low-cal past a double convex lens is as follows:

Refraction Rule for a Converging Lens

Any incident ray traveling parallel to the primary axis of a converging lens will refract through the lens and travel through the focal signal on the opposite side of the lens.

Now suppose that the rays of light are traveling through the focal signal on the way to the lens. These rays of low-cal will refract when they enter the lens and refract when they leave the lens. Equally the lite rays enter into the more dense lens material, they refract towards the normal; and as they exit into the less dense air, they refract away from the normal. These specific rays volition get out the lens traveling parallel to the master centrality.

The above diagram shows the behavior of ii incident rays traveling through the focal betoken on the way to the lens. Note that the two rays refract parallel to the main axis. A second generalization for the refraction of low-cal by a double convex lens can be added to the first generalization.

Refraction Rules for a Converging Lens

• Whatsoever incident ray traveling parallel to the primary axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens.
• Whatsoever incident ray traveling through the focal point on the mode to the lens volition refract through the lens and travel parallel to the chief axis.

The Thin Lens Approximation

These two "rules" will profoundly simplify the task of determining the image location for objects placed in front of converging lenses. This topic will be discussed in the side by side part of Lesson 5. For now, internalize the meaning of the rules and exist prepared to use them. Equally the rules are practical in the construction of ray diagrams, do not forget the fact that Snells' Law of refraction of lite holds for each of these rays. It but so happens that geometrically, when Snell'due south Law is applied for rays that strike the lens in the manner described above, they will refract in close approximation with these two rules. The tendency of incident light rays to follow these rules is increased for lenses that are thin. For such sparse lenses, the path of the light through the lens itself contributes very little to the overall alter in the management of the light rays. We will utilise this so-chosen thin-lens approximation in this unit. Furthermore, to simplify the structure of ray diagrams, nosotros will avoid refracting each light ray twice - upon entering and emerging from the lens. Instead, we will continue the incident ray to the vertical axis of the lens and refract the light at that point. For sparse lenses, this simplification will produce the same issue equally if nosotros were refracting the low-cal twice.

Rules of Refraction for Diverging Lenses

Now let'due south investigate the refraction of low-cal by double concave lens. Suppose that several rays of light approach the lens; and suppose that these rays of light are traveling parallel to the principal axis. Upon reaching the front confront of the lens, each ray of light will refract towards the normal to the surface. At this boundary, the light ray is passing from air into a more than dense medium (usually plastic or drinking glass). Since the calorie-free ray is passing from a medium in which it travels relatively fast (less optically dense) into a medium in which information technology travels relatively deadening (more optically dense), information technology will bend towards the normal line. This is the FST principle of refraction. This is shown for two incident rays on the diagram below. In one case the calorie-free ray refracts across the boundary and enters the lens, information technology travels in a straight line until it reaches the dorsum face of the lens. At this purlieus, each ray of calorie-free volition refract abroad from the normal to the surface. Since the lite ray is passing from a medium in which it travels relatively slow (more optically dumbo) to a medium in which it travels fast (less optically dense), it will bend away from the normal line. This is the SFA principle of refraction. These principles of refraction are identical to what was observed for the double convex lens to a higher place.

The to a higher place diagram shows the beliefs of ii incident rays approaching parallel to the principal axis of the double concave lens. Just similar the double convex lens above, calorie-free bends towards the normal when entering and abroad from the normal when exiting the lens. Yet, because of the unlike shape of the double concave lens, these incident rays are not converged to a point upon refraction through the lens. Rather, these incident rays diverge upon refracting through the lens. For this reason, a double concave lens can never produce a real prototype. Double concave lenses produce images that are virtual. This volition be discussed in more particular in the adjacent part of Lesson 5. If the refracted rays are extended backwards behind the lens, an of import ascertainment is fabricated. The extension of the refracted rays will intersect at a bespeak. This point is known as the focal indicate. Notice that a diverging lens such as this double concave lens does non really focus the incident light rays that are parallel to the main axis; rather, it diverges these lite rays. For this reason, a diverging lens is said to have a negative focal length.

The starting time generalization can now exist fabricated for the refraction of low-cal past a double concave lens:

Refraction Rule for a Diverging Lens

Whatsoever incident ray traveling parallel to the principal centrality of a diverging lens will refract through the lens and travel in line with the focal point (i.e., in a management such that its extension will pass through the focal point).

 Now suppose that the rays of light are traveling towards the focal bespeak on the manner to the lens. Because of the negative focal length for double concave lenses, the light rays will caput towards the focal betoken on the opposite side of the lens. These rays will actually accomplish the lens before they reach the focal point. These rays of light will refract when they enter the lens and refract when they leave the lens. Every bit the lite rays enter into the more than dense lens material, they refract towards the normal; and every bit they exit into the less dumbo air, they refract away from the normal. These specific rays volition exit the lens traveling parallel to the chief axis.

The higher up diagram shows the behavior of two incident rays traveling towards the focal betoken on the way to the lens. Note that the two rays refract parallel to the master axis. A second generalization for the refraction of light by a double concave lens tin can be added to the first generalization.

Refraction Rules for a Diverging Lens

• Any incident ray traveling parallel to the principal axis of a diverging lens will refract through the lens and travel in line with the focal point (i.e., in a direction such that its extension will laissez passer through the focal point).
• Whatever incident ray traveling towards the focal signal on the way to the lens volition refract through the lens and travel parallel to the main axis.

A 3rd Rule of Refraction for Lenses

The above give-and-take focuses on the manner in which converging and diverging lenses refract incident rays that are traveling parallel to the principal axis or are traveling through (or towards) the focal betoken. Simply these are not the but 2 possible incident rays. There are a multitude of incident rays that strike the lens and refract in a variety of ways. Nevertheless, there are three specific rays that behave in a very predictable manner. The third ray that nosotros will investigate is the ray that passes through the precise center of the lens - through the point where the principal axis and the vertical axis intersect. This ray will refract as information technology enters and refract equally it exits the lens, merely the net upshot of this dual refraction is that the path of the light ray is not changed. For a thin lens, the refracted ray is traveling in the same management as the incident ray and is approximately in line with it. The behavior of this third incident ray is depicted in the diagram below.

Now we have three incident rays whose refractive behavior is easily predicted. These three rays pb to our three rules of refraction for converging and diverging lenses. These three rules are summarized beneath.

Refraction Rules for a Converging Lens

• Any incident ray traveling parallel to the master axis of a converging lens will refract through the lens and travel through the focal point on the contrary side of the lens.
• Whatsoever incident ray traveling through the focal point on the style to the lens will refract through the lens and travel parallel to the principal axis.
• An incident ray that passes through the center of the lens will in event continue in the same direction that information technology had when it entered the lens.

Refraction Rules for a Diverging Lens

• Whatever incident ray traveling parallel to the principal axis of a diverging lens volition refract through the lens and travel in line with the focal point (i.e., in a management such that its extension volition pass through the focal signal).
• Whatsoever incident ray traveling towards the focal betoken on the way to the lens will refract through the lens and travel parallel to the principal axis.
• An incident ray that passes through the center of the lens will in effect keep in the same direction that it had when it entered the lens.

These three rules of refraction for converging and diverging lenses will exist practical through the remainder of this lesson. The rules merely describe the behavior of three specific incident rays. While there is a multitude of light rays beingness captured and refracted by a lens, only two rays are needed in order to determine the epitome location. So as we continue with this lesson, pick your favorite two rules (ordinarily, the ones that are easiest to remember) and use them to the construction of ray diagrams and the determination of the image location and characteristics.

We Would Like to Suggest ...

Why just read well-nigh it and when you lot could be interacting with it? Collaborate - that's exactly what you lot do when you use one of The Physics Classroom'south Interactives. We would similar to propose that you combine the reading of this page with the employ of our Eyes Demote Interactive. You lot tin detect this in the Physics Interactives section of our website. The Optics Bench Interactive provides the learner an interactive enivronment for exploring the formation of images by lenses and mirrors. Its like having a complete eyes toolkit on your screen.

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Source: https://www.physicsclassroom.com/class/refrn/Lesson-5/Refraction-by-Lenses#:~:text=Lenses%20serve%20to%20refract%20light,light%20ray%20has%20changed%20directions.

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