Refraction and lenses

Science, Grade 6


Table Of Contents: Refraction and lenses

1. Lenses
A lens is a curved piece of a transparent material such as glass or plastic. A lens is similar to a mirror except light is refracted, not reflected. A lens has an optical axis which divides the lens in half. The point on the optical axis where distant light rays meet when refracted, is called the focal point. The distance from the focal point to the lens is called the focal length, f. There is a focal point on both sides of the lens.
2. Convex Lenses
A convex lens is thicker in the middle and tapers at the ends. What kind of image will you see when you view an insect through a convex lens? A light ray is drawn from the insect’s head, parallel to the optical axis. It is refracted when it meets the lens and continues through the focal point. Another light ray is drawn through the focal point and is refracted by the lens. The light continues outward, parallel to the optical axis. The two rays meet at a point and form an image. The image is upside down. When the object is beyond the focal point, the image is always real and upside down. If you move the object closer to the lens, between the focal point and lens, the image will be virtual and upright.
3. Concave Lenses
Concave lenses are thinner at the center than at the edges. A concave lens can only produce virtual images because light passing through the lens will bend away from the optical axis. The image of the insect through the concave lens is virtual and upright. It will seem to appear on the same side of the lens as the object.
4. Light and the Human Eye
Light enters your eye through the cornea and then the pupil. A lens behind the pupil forms an upside down image on the lining of the back of your eyeball, called the retina. The retina is made of two types of specialized cells, rods and cones. Rods respond to light, while cones respond to color. These cells send the image information along the optic nerve to the brain. The brain interprets the information and you see the image. If your eyeball is too long or short, the image on the retina is out of focus. Glasses and contact lenses correct these types of vision problems.
5. Uses of light in technology
Telescopes use lenses and mirrors to collect and focus light from distant objects such as planets and stars. There are many types of telescopes that allow us to see into outer space. The simplest telescope, a refracting telescope, uses two lenses. The first lens, the objective, gathers and focuses the light. The second lens, the eyepiece, enlarges the image. A reflecting telescope includes a mirror to help gather light. Microscopes also use a combination of lenses to magnify objects. An objective lens and an eyepiece lens are used to magnify an object to produce a real and enlarged image. Optical fibers are long thin pieces of glass or plastic and are used to transmit light. Light travels along the fiber and only leaves at the other end. Optical fibers are used to send information between telephones or computers.