LENS

REFRACTION

• bending of light as it travels from one medium to another medium

• each medium has a different density from the other

• air is less dense than water, but denser than vacuum

• water is denser than air but less dense than solid

• speed of light in a medium is greatly affected by the medium’s optical density

• when light passes from a less dense to denser medium, it slows down. the ray bends towards the normal line and angle of refraction (∠r) is less than the angle of incidence (∠i)

• the path of light is reversible. thus, when light passes from water to air, it speeds up. the refracted ray bends away from the normal line and angle of incidence (∠i) is less than the angle of refraction (∠r)

• If light travels from Less dense to More dense material, then the bending of light is Towards the normal line. Thus, LeMoTo. If light travels from More dense to Less dense medium, then the bending of light is Away from the normal line. Thus, MoLeA.

• We know that stars are basically a ball of fire located so far away from earth that they only appear as pinpoints. As their light enters from outer space (which is vacuum) into the earth’s atmosphere (which is made up of different gasses), their light rays bend and are refracted. Thus in our eyes, they seem to twinkle.

LENS

• optical device made of glass or plastic that focuses or disperses a light beam by means of refraction

• transparent material made of plastic or glass

• forms images by refraction

• divided into two types

CONVEX LENS

• thicker in the center than edges

• forms real images and virtual images depending on position of the object

• also called Converging Lens because the light that passes through it tends to converge at a particular point called the focal point

CONCAVE LENS

• thicker at the edges and thinner in the center

• forms upright and reduced images

• also called Diverging Lens because the light that passes through it tends to diverge at a particular point called the focal point

LENS TERMINOLOGY

IMAGES FORMED BY LENSES

• In locating the image formed in lenses graphically, two important points are considered. The following important points are enumerated below.

• Vertex, V – The geometric center of the lens also referred to as the optical center, O.

• Focal point / Focus, F – point where light rays converge (or appear to converge) when parallel light rays pass through a lens. Its distance from the vertex is called the focal length.

RAY DIAGRAMS

• used to determine the location, size and kind of image formed by the plane and curved mirrors.

THE “THREE MOST USEFUL RAYS” IN LENSES

To graphically determine the position and kind of the image formed, the ray diagram can be used. Consider the following steps using the three major rays described above:

• From the object, draw the first ray (P–F ray). From the same point on the object, draw the second (F–P ray), and third (V ray) rays.

• The intersection of the rays is the image point corresponding to the object point. For example, if you started diagramming from the tip of the arrow-shaped object, the intersection of the refracted rays is also the tip of the arrow-shaped image. Thus, you can determine completely the position and characteristics of the image.

• For a concave lens, light rays diverge from a virtual focus; but the procedure for locating images is the same as for convex lenses.

USES OF LENSES

• Like the mirror, the lens is usually made of glass. The only thing that makes it different is that the lens can also be made of other transparent materials like plastics that allow light to pass through, instead of bouncing the light rays off. Lenses come also in two types, namely: convex (converging) and concave (diverging) lenses. When these lenses Uses of Lenses Lesson 2 10 are combined, the produced lens is called meniscus lens. This type has one concave and one convex lens on opposite sides.

USES OF CONVEX LENSES

• Sometimes called a positive lens, a convex lens is characterized by its bulging surfaces that are directed outward making it thicker at the center than on its edges. It is also known as converging lens because as light passes through it, its rays bend inward and converge at a spot located beyond the lens known as the focal point causing the object behind the lens to magnify or to appear bigger than its actual size. Image produced by convex lenses can be a virtual or real image.

• If both sides of the lens are curved inward, then that type of convex lens is called biconvex typically seen in magnifying glasses. If only one side of the lens is curved inward and the other has a flat surface, it is called plano-convex.

• Among the applications in which convex lenses are used is found in compound microscopes. A microscope is an instrument that is used in viewing small objects and microorganisms that are impossible for our unaided eyes to see. It uses lenses that can magnify objects 100x or even more than a thousand times.

• There are many types of microscopes, among this is the most commonly used in school laboratories; light microscope. This type of microscope uses at least two convex lenses, one on the eyepiece near the observer’s eye and the other one is the objective lens located near the object being observed.

• As illustrated in Figure 9.1, DSLR cameras use convex lenses where light rays pass through. These light rays are directed towards the slanted mirror located near the camera shutter, and then reflected from a translucent screen projected to another mirror until it reaches the viewer’s eye. Through this, the photographer will be able to see what he is capturing.

• The image formed by the passing of light rays, from the object through the lens directing to the film of the camera, is affected by the angle of the light entry. The closer the lens to the object, the farther the beams converge. The farther the object from the lens, the shorter the distance the light beams converge. That is how the camera works. Though it seems complicated, the entire process is just seconds-long.

• Our eyes are our natural cameras. Some of the basic and fundamental parts of a camera functions just like how the different parts of our eyes work. Among these are the lens, the aperture, iris diaphragm, shutter, and photographic film.

• The lens, just like the cornea of our eyes, gathers and bends light rays inwardly to the camera. The iris diaphragm controls the size of the aperture (opening) to regulate the light that enters. This duo resembles the iris and the pupil of our eyes. The shutter opens and shuts the aperture to avoid too much light exposure thus helping the photographic film capture a beautiful image similar to the work done by the eyelids and the retina.

USES OF CONCAVE LENSES

• Contrary to concave lens, convex lens has thicker edges curving towards the center, causing light to diverge, hence it is called the diverging lens. It spreads out light, producing virtual images, making objects appear smaller and farther than the way they actually are.

• Although convex lenses can magnify objects, they still cannot transfer light accurately. That's why an object would appear blurry, so makers of binoculars and telescopes use concave lenses to these instruments in order to focus objects that are too far for our eyes to see.

• Concave lenses, such as eyeglasses and contact lenses are used in correcting myopia (nearsightedness). Myopia is a condition in which light rays focus in front of the eye’s retina instead of on the retina. The result of this condition is that distant objects appear blurry, while near objects appear normal.

• Hyperopia is the eye condition that enables someone to see distant objects clearly but makes nearby objects seem blurry. This condition is due to the inability of one's eyes to focus on objects closer to the eyes.

• A person with hyperopia has a shortened eyeball, in which the retina lies closer than usual to the cornea and lens. This disorder leads to the formation of the image of a nearby object to form beyond the retina.

• Hyperopia is the opposite of myopia, another vision problem that needs correction.

• To correct hyperopia, converging meniscus converging lenses are used. The converging meniscus lens converges light before it enters the eye, eventually reducing the image distance.

THE HUMAN EYE

• one of the most complicated optical systems

• almost spherical shape (2.5 cm in diameter)

1. Iris

– controls the opening of the pupil and regulates the amount of light that enters the eye

2. Cornea and the Lens

– together act as a converging lens which refracts light as it enters the eye

– the converging lens focuses light to form a real, inverted image on the retina

– the rods structures on the retina enables us to see in the dark

– when the eye loses its ability to change the shape of its lens, vision becomes poor and defective

FARSIGHTED PERSON OR HYPEROPE

• can see very far objects clearly but has difficulty focusing on near objects

• a convex lens can be used to correct farsightedness

NEARSIGHTED PERSON OR MYOPE

• can see near objects clearly but has difficulty focusing on far objects

• a concave lens can be used to remedy nearsightedness

THE CAMERA

• boxlike device for taking pictures modelled after the eye

• the opening is covered by a lens or a combination of lenses

1. Shutter

– which opens and closes corresponds to the eyelid of the eye

2. Film

– corresponds to the retina where the image is formed

3. Diaphragm

– regulates the amount of light that enters the camera through its aperture, just as the iris permits the proper amount of light that enters the eye through the pupil

THE MAGNIFYING GLASS

• a single convex lens that will add convergence to the visual system

• gives an upright image of an object placed within its focal length

MICROSCOPE

• made up of two converging (convex) lenses

1. Objective Lens

– short focal length; it forms the real magnified image of the object

2. Eyepiece

– the image of the objective lens becomes the objects of the eyepiece lens; basically a magnifying glass that looks at and enlarges the image created by the objective lens

TELESCOPE

• device used to see very far or distant objects clearly

• it uses two lenses

• the eyepiece magnifies the image formed by the objective lens

1. Refracting Telescope – use a lens to collect and focus rays of light

2. Reflecting Telescope – use a large concave mirror instead of a lens for the same purpose

BINOCULARS

• the image experiences four inversions by the reflection of the beam of light through a series of four totally reflecting prisms

PERISCOPE

• made up of a tube complete with prisms or mirrors arranged in series

• enables things in front to be seen over an obstacle producing an erect image

• used in submerged submarines for scanning surfaces above water

PROJECTOR

• has a concave mirror that reflects light from an intense source back into a pair of condenser lenses

DIFFERENCE BETWEEN CONVEX & CONCAVE LENS