Objectives describe the action of thin lenses both

Objectives • describe the action of thin lenses (both converging and diverging) on a beam of light. • define the term focal length. • *draw ray diagrams to illustrate the formation of real and virtual images of an object by a converging lens, and the formation of a virtual image by a diverging lens. • define the term linear magnification and *draw scale diagrams to determine the focal length needed for particular values of magnification (converging lens only). • describe the use of a single lens as a magnifying glass and in a camera, projector and photographic enlarger and draw ray diagrams to show each forms an image. • draw ray diagrams to show the formation of images in the normal eye, a short-sighted eye and a long-sighted eye. • describe the correction of short-sight and long-sight.

Action of thin lenses on a beam of light Convex lens

Action of thin lenses on a beam of light Concave lens

Focal length Convex lens Concave lens

Focal length

Focal length In Experiment 1, you will use the convex lens to produce a real image of a distant object and capture the image on a screen.

Focal length In Experiment 2, you will locate the focus of the convex lens using no-parallax method with the help of a plane mirror.

Focal length In Experiment 3, you will use graphical method to determine the focal length of the convex lens

Focal length In Experiment 4, you will use a paper and sunlight to determine the focal length of the convex lens with the help of a convex lens.

Focal length If you slowly pull the magnifier away from the face, you will see that the magnification steadily increases until the image begins to blur. Pulling the magnifier even farther away produces an inverted image as seen in the next slide. The distance at which the image blurs, and beyond which it inverts, is the focal length of the lens. To use a convex lens as a magnifier, the object must be closer to the converging lens than its focal length. Think different and act different

Focal length Think different and act different

Focal length In Experiment 1, you will measure the focal length of the concave lens using the reversibility of light.

Focal length In Experiment 2, you will use graphical method to determine the focal length of the concave lens with the help of a convex lens.

Convex Lens Ray Diagram When an object is placed in front of a lens, light rays coming from the object fall on the lens and get refracted. The refracted rays produce an image at a point where they intersect or appear to intersect each other. The formation of images by lenses is usually shown by a ray diagram. To construct a ray diagram we need at least two rays whose path after refraction through the lens is known. Any two of the following rays are usually considered for constructing ray diagrams The nature of images formed by a convex lens depends upon the distance of the object from the Optical Center of the lens. Let us now see how the image is formed by a convex lens for various positions of the object

Convex Lens Ray Diagram An incident ray parallel to the principal axis after refraction passes through the focus.

Convex Lens Ray Diagram A ray of light passing through the Optical Center of the lens travels straight without suffering any deviation. This holds good only in the case of a thin lens.

Convex Lens Ray Diagram An incident ray passing through the focus of a lens emerge parallel to the principal axis after refraction.

Convex Lens Ray Diagram An incident ray parallel to the principal axis after refraction passes through the focus. A ray of light passing through the Optical Center of the lens travels straight without suffering any deviation. This holds good only in the case of a thin lens. An incident ray passing through the focus of a lens emerge parallel to the principal axis after refraction. Refraction at the surface of the lens in ignored

When the Object is Placed between F and O: The image is - ü Formed on the same side of the lens ü Virtual ü Erect ü Magnified

When the Object is Placed between F and O: Magnifying Glass The image is - ü Formed on the same side of the lens ü Virtual ü Erect ü Magnified

When the Object is Placed at F The image is - ü Formed at infinity ü real ü Inverted ü Magnified

When the Object is Placed at F The image is - ü Formed at infinity Spot light ü real ü Inverted ü Magnified

When the Object is Placed between F and 2 F The image is - ü Formed beyond 2 F ü Real ü Inverted ü Magnified

When the Object is Placed between F and 2 F Projector The image is - ü Formed beyond 2 F ü Real ü Inverted ü Magnified

When the Object is Placed at 2 F The image is - ü Formed at 2 F 2 ü Real ü Inverted ü Same size as the object

When the Object is Placed at 2 F The image is - ü Formed at 2 F 2 ü Real Photocopier ü Inverted ü Same size as the object

When the Object is Placed beyond 2 F The image is - ü Formed between F and 2 F ü Real ü Inverted ü Diminished

When the Object is Placed between F and 2 F Camera & Eye The image is - ü Formed between F and 2 F ü Real ü Inverted ü Diminished

When the Object is Placed at Infinity The image is - ü Formed at F 2 ü Inverted ü Real ü Highly diminished

When the Object is Placed at Infinity Astronomical Telescope The image is - ü Formed at F 2 ü Inverted ü Real ü Highly diminished

When the Object is Placed at Infinity Binocular is a device which is used the far off objects clearly. Its principle of working is exactly same as that of a telescope. The only difference is that objects on finite distances on earth are viewed by binoculars. Telescopes are used to study objects that are at infinite distances. Mostly they are used studying astronomical objects The image is - ü Formed at F 2 ü Inverted ü Real ü Highly diminished

Image formed by a Convex lens – Summary

Magnification or Linear Magnification is the ratio of the image size to the object size or the image distance to the object distance. It can be showed as follows Image Distance Image Size Magnification = Object Size or Object Distance Magnification or Linear Magnification does not carry any unit it is always given with a “X” sign.

Image formed by a Concave lens The image is always ü Upright ü Diminished ü Virtual ü Formed between F and O

Image formed by a Concave lens The image is always ü Upright ü Diminished ü Virtual ü Formed between F and O

Concave lens as Door Eye

Human Eye An image is formed on the retina with light rays converging most at the cornea and upon entering and exiting the lens. Rays from the top and bottom of the object are traced and produce an inverted real image on the retina. The distance to the object is drawn smaller than scale

Human Eye Since light rays from a nearby object can diverge and still enter the eye, the lens must be more converging (more powerful) for close vision than for distant vision. To be more converging, the lens is made thicker by the action of the ciliary muscle surrounding it. The eye is most relaxed when viewing distant objects

Human Eye Short sight Long sight

Correction of Long sightedness A person who is long sighted can focus clearly on distant objects but cannot focus on near objects. This is because the eyeball is too short. Light from near objects is focused at a point behind the retina resulting in a blurred image. This defect can be corrected by wearing a convex (converging) spectacle lens. The rays of light from a near object are converged before entering the eye so that the cornea and eye lens can direct the focal point onto the retina

Human Eye A person who is short sighted can focus clearly on near objects but cannot focus on distant objects. This is because the eyeball is too long. Light from distant objects is focused at a point in front of the retina resulting in a blurred image. This defect can be corrected by wearing a concave (diverging) spectacle lens. The rays of light from a near object are diverged before entering the eye so that the cornea and eye lens can direct the focal point onto the retina

Ray diagrams for the correction of eye sight Short sighted eye and the Long sighted eye and the correction of short sightedness correction of long sightedness by a concave lens by a convex lens