Optics - Lenses and Images

Question 1

lenses and images

Answer 1

• In the previous unit, we learned that light rays can be bent, either by reflecting off a mirror, or by refracting in a transparent medium like glass. Knowing that, we can design objects like cameras or binoculars that manipulate light rays to form “images.”
• Unless it is pitch black, objects have light rays reflecting off them in all directions. Say we have an Object “O” with light rays coming off it. Let’s pick some point “P” on the object. The rays leaving point P will spread out, or “diverge”, as they move away from the object. However, we can intercept some of the rays, and use refraction (or reflection) to bend them back together, so that they “converge” on some point. If we put a screen or film there, the rays come together to create a bright spot at that point. That point is called the “Image” of the point P on the Object. Each point on the object creates its own image point. The result is that a picture or image of the object will appear on the screen or film. That’s how a basic camera works, or the human eye.
• So what kind of device will bend the rays to form an image? If we use refraction, it has to be a curved piece of some transparent material, like glass or plastic. That is called a “Lens.”
• There are two basic shapes for lenses. A lens that is fatter in the middle is called a “Convex” lens. Convex lens bend the light rays together, trying to make them converge; it is also sometimes called a “converging lens.” The other basic shape is for the lens to be skinnier in the middle than it is on the edges, which is called a “Concave” lens. Concave lens cause rays to spread out and diverge, so it is sometimes called a “Diverging Lens.”