What do lenses do to light

But the image is closer than the object, a fact that is useful in correcting nearsightedness, as we shall see in a later section. Table 1 summarizes the three types of images formed by single thin lenses. These are referred to as case 1, 2, and 3 images. Convex converging lenses can form either real or virtual images cases 1 and 2, respectively , whereas concave diverging lenses can form only virtual images always case 3.

Real images are always inverted, but they can be either larger or smaller than the object. For example, a slide projector forms an image larger than the slide, whereas a camera makes an image smaller than the object being photographed. Virtual images are always upright and cannot be projected. Virtual images are larger than the object only in case 2, where a convex lens is used.

The virtual image produced by a concave lens is always smaller than the object—a case 3 image. We can see and photograph virtual images only by using an additional lens to form a real image.

In Image Formation by Mirrors , we shall see that mirrors can form exactly the same types of images as lenses. Find several lenses and determine whether they are converging or diverging. In general those that are thicker near the edges are diverging and those that are thicker near the center are converging.

On a bright sunny day take the converging lenses outside and try focusing the sunlight onto a piece of paper. Determine the focal lengths of the lenses. Be careful because the paper may start to burn, depending on the type of lens you have selected.

Step 2. Determine whether ray tracing, the thin lens equations, or both are to be employed. A sketch is very useful even if ray tracing is not specifically required by the problem. Write symbols and values on the sketch. Step 4. Make alist of what is given or can be inferred from the problem as stated identify the knowns. It is helpful to determine whether the situation involves a case 1, 2, or 3 image.

While these are just names for types of images, they have certain characteristics given in Table 1 that can be of great use in solving problems.

Step 5. If ray tracing is required, use the ray tracing rules listed near the beginning of this section. Step 6. Most quantitative problems require the use of the thin lens equations.

These are solved in the usual manner by substituting knowns and solving for unknowns. Several worked examples serve as guides. Step 7. Check to see if the answer is reasonable: Does it make sense?

If you have identified the type of image case 1, 2, or 3 , you should assess whether your answer is consistent with the type of image, magnification, and so on. We do not realize that light rays are coming from every part of the object, passing through every part of the lens, and all can be used to form the final image.

We generally feel the entire lens, or mirror, is needed to form an image. Actually, half a lens will form the same, though a fainter, image. Skip to main content. Geometric Optics. Search for:. Image Formation by Lenses Learning Objectives By the end of this section, you will be able to: List the rules for ray tracking for thin lenses.

Illustrate the formation of images using the technique of ray tracking. Determine power of a lens given the focal length. Converging or Convex Lens The lens in which light rays that enter it parallel to its axis cross one another at a single point on the opposite side with a converging effect is called converging lens. Focal Point F The point at which the light rays cross is called the focal point F of the lens. Focal Length f The distance from the center of the lens to its focal point is called focal length f.

Power P The power P of a lens is defined to be the inverse of its focal length. Example 1. What is the Power of a Common Magnifying Glass? Strategy The situation here is the same as those shown in Figure 1 and Figure 2. Solution The focal length of the lens is the distance from the center of the lens to the spot, given to be 8. Discussion This is a relatively powerful lens.

Diverging Lens A lens that causes the light rays to bend away from its axis is called a diverging lens. Thin Lens A thin lens is defined to be one whose thickness allows rays to refract but does not allow properties such as dispersion and aberrations.

Take-Home Experiment: A Visit to the Optician Look through your eyeglasses or those of a friend backward and forward and comment on whether they act like thin lenses. Rules for Ray Tracing A ray entering a converging lens parallel to its axis passes through the focal point F of the lens on the other side. A ray entering a diverging lens parallel to its axis seems to come from the focal point F. A ray passing through the center of either a converging or a diverging lens does not change direction.

A ray entering a converging lens through its focal point exits parallel to its axis. A ray that enters a diverging lens by heading toward the focal point on the opposite side exits parallel to the axis. Real Image The image in which light rays from one point on the object actually cross at the location of the image and can be projected onto a screen, a piece of film, or the retina of an eye is called a real image.

Image Distance The distance of the image from the center of the lens is called image distance. Example 2. Virtual Image An image that is on the same side of the lens as the object and cannot be projected on a screen is called a virtual image.

Example 3. Example 4. Image Produced by a Concave Lens Suppose an object such as a book page is held 7.

Strategy and Concept This example is identical to the preceding one, except that the focal length is negative for a concave or diverging lens. Take-Home Experiment: Concentrating Sunlight Find several lenses and determine whether they are converging or diverging. Problem-Solving Strategies for Lenses Step 1. Examine the situation to determine that image formation by a lens is involved.

Step 3. Identify exactly what needs to be determined in the problem identify the unknowns. Misconception Alert We do not realize that light rays are coming from every part of the object, passing through every part of the lens, and all can be used to form the final image. Conceptual Questions It can be argued that a flat piece of glass, such as in a window, is like a lens with an infinite focal length.

If so, where does it form an image? That is, how are d i and d o related? You can often see a reflection when looking at a sheet of glass, particularly if it is darker on the other side.

Explain why you can often see a double image in such circumstances. When you focus a camera, you adjust the distance of the lens from the film. If the camera lens acts like a thin lens, why can it not be a fixed distance from the film for both near and distant objects?

A thin lens has two focal points, one on either side, at equal distances from its center, and should behave the same for light entering from either side. Look through your eyeglasses or those of a friend backward and forward and comment on whether they are thin lenses. Will the focal length of a lens change when it is submerged in water? What is its range of powers? What is the focal length of 1. You note that your prescription for new eyeglasses is —4.

What will their focal length be? How far from the lens must the film in a camera be, if the lens has a Explicitly show how you follow the steps in the Problem-Solving Strategy for lenses.

A certain slide projector has a mm focal length lens. Explicitly show how you follow the steps in the Problem-Solving Strategy for Lenses above. A doctor examines a mole with a A camera with a A camera lens used for taking close-up photographs has a focal length of The farthest it can be placed from the film is Suppose your What magnification will be produced by a lens of power —4.

In Example 3, the magnification of a book held 7. Suppose a mm focal length telephoto lens is being used to photograph mountains A camera with a mm focal length lens is used to photograph the sun and moon. What is the height of the image of the sun on the film, given the sun is 1.

Licenses and Attributions. CC licensed content, Shared previously. The primary mirror of the Subaru telescope, built by Japan's National Astronomical Observatory, has a diameter of 8. This is good enough resolution to be able to make out a small coin placed on the tip of Mt.

Fuji from as far away as Tokyo. Moreover, the Subaru telescope is about million times more sensitive to light than the human eye. Even the largest telescopes until Subaru were unable to observe stars more than about one billion light years away, but Subaru can pick up light from galaxies lying 15 billion light years away.

Light from 15 billion light years away and beyond is, in fact, thought to be light produced by the "big bang" that supposedly gave birth to the universe. Subaru's primary focus camera boasts a very wide field of view of 30 minutes, which is equivalent to the diameter of the full moon as seen from earth, enabling Subaru to make not only very precise, but also speedy observations of the heavens.

The only telescope in the world equipped with a glass primary mirror of 8 m in diameter, Subaru is a powerful aid to research on the birth of galaxies and the structure of the universe. Previously, structural considerations prevented heavy optical systems from being placed on top of the primary focus of large reflecting telescopes. This problem was overcome by the development of a smaller and lighter prime focus corrector lens optical system, comprising seven large lens elements in five groups.

With a diameter of 52 cm and total weight of kg, this high-precision lens unit is the fruit of Canon's lens design and manufacturing technologies. Stellar light picked up by the world's largest mirror and passed through this unit is focused on a giant CCD unit consisting of ten 4, x 2, pixel CCDs, producing images of 80 megapixels.

Chapter 1: The Mysteries of Light. Why Is the Sky Blue? How Do Rainbows Form? Why Light Fades in the Bathroom? Why Do Water Surfaces Shine? Why Do Comets Have Tails? Chapter 2: Making Light. Chapter 3: Applications of Light. Chapter 4: Light and Its Future.

This site requires a JavaScript enabled browser. Canon Science Lab Lenses The word "lens" owes its origin to the Latin word for lentils, the tiny beans that have from ancient times been an important ingredient in the cuisine of the Mediterranean region.

Convex and Concave Lenses Used in Eyeglasses Lenses may be divided broadly into two main types: convex and concave. Concave Lenses Are for the Nearsighted, Convex for the Farsighted Concave lenses are used in eyeglasses that correct nearsightedness. Telephoto Lenses Are Combinations of Convex and Concave Lenses Most optical devices make use of not just one lens, but of a combination of convex and concave lenses.

Lenses that Correct the Blurring of Colors The focused image through a single convex lens is actually very slightly distorted or blurred in a phenomenon known as lens aberration. These rays of light will refract when they enter the lens and refract when they leave the lens.

As the light 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 will exit the lens traveling parallel to the principal axis. The above diagram shows the behavior of two incident rays traveling through the focal point on the way to the lens.

Note that the two rays refract parallel to the principal axis. A second generalization for the refraction of light by a double convex lens can be added to the first generalization. These two "rules" will greatly simplify the task of determining the image location for objects placed in front of converging lenses. This topic will be discussed in the next part of Lesson 5. For now, internalize the meaning of the rules and be prepared to use them.

As the rules are applied in the construction of ray diagrams, do not forget the fact that Snells' Law of refraction of light holds for each of these rays. It just so happens that geometrically, when Snell's 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 thin lenses, the path of the light through the lens itself contributes very little to the overall change in the direction of the light rays. We will use this so-called thin-lens approximation in this unit. Furthermore, to simplify the construction of ray diagrams, we 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 thin lenses, this simplification will produce the same result as if we were refracting the light twice. Now let's investigate the refraction of light by double concave lens. Since the light ray is passing from a medium in which it travels relatively fast less optically dense into a medium in which it travels relatively slow more optically dense , it will bend towards the normal line.

Since the light ray is passing from a medium in which it travels relatively slow more optically dense 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 above.

The above diagram shows the behavior of two incident rays approaching parallel to the principal axis of the double concave lens.

Just like the double convex lens above , light bends towards the normal when entering and away from the normal when exiting the lens. Yet, because of the different 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 image. Double concave lenses produce images that are virtual. This will be discussed in more detail in the next part of Lesson 5. If the refracted rays are extended backwards behind the lens, an important observation is made. The extension of the refracted rays will intersect at a point.