Lenses are mainly made of glass clear plastics. They are widely used in spectacles, cameras, telescopes and many other optical instruments.Our human eye has two crystalline lenses which enable us to form images.
Lenses There are two main types of lenses:• convex lenses—these curve outwards and are fatter in the middle• concave lenses—these curve inwards (a little like a cave) and are thinner in the middle. Convex lenses Concave Lenses
Lenses – An application ofrefraction There are 2 basic types of lensesA converging lens (Convex) A diverging lens (concave) takestakes light rays and bring light rays and spreads themthem to a point. outward.
Convex Mirrors Curves outward Reduces images Virtual images Use: Rear view mirrors, store security…CAUTION! Objects are closer than they appear!
Thicker in the center than edges. Lens that converges (brings together) light The Magnifier rays. Forms real images and virtual images depending on position of the object
A ray thru theCenter of the lenses Parallel ray Passes thru The focus 2f focus (f) focus (f) 2f x x x x Remains unbent Refracts parallel To principal axis
MOVEMENT OF LIGHT THROUGH LENSES CONVEX LENS In a convex lens, an incoming ray parallel to the principal axis is refracted through the principal focus (F).
Movement of Light through a Lens The distance from the centre line (plane) of the lens to the principal focus is called the focal length of the lens. A ray passing through the centre of either type of lens is unaffected. As with all images, rays of light that come from a part of the object come together again at that same part of the image.
Finding the focal length Rays coming into a lens from a distant object are almost parallel and form an image very close to the focus. We can then measure the distance from lens to image to determine the focal length of the lens.
The greater the curvature of a lens, the more it bends light and hence the shorter the focal length.
Convex lenses produce two different types of images, depending on where the object is located. If the object is at a distance greater than the focal length of the lens, a real image is formed.
If the object is at a distance less than the focal length of the lens, a virtual image is formed.
REAL AND VIRTUAL IMAGESReal Image – Image is made from “real” light rays that converge at a real focal point so the image is REAL Can be projected onto a screen because light actually passes through the point where the image appears Always invertedVirtual Image– “Not Real” because it cannot be
Image is: Real Inverted Reduced Parallel ray Appears between f and 2f 2f focus (f) focus (f) 2f x x x xObject beyond 2f
Image is: Real Inverted Same size Appears between f and 2f Parallel ray 2f focus (f) focus (f) 2f x x x xObject at 2f
Image is: Real Inverted Enlarged Appears beyond 2f Parallel ray 2f focus (f) focus (f) 2f x x x xObject between f and 2f
Image is: Virtual Apparent Erect Convergence Enlarged Of rays Appears on same Side as Object2f focus (f) focus (f) 2fx x x x Object Inside focus
OPTICAL INSTRUMENTS Telescopes Telescopes make small, far objects appear larger. Two lenses are used. The objective lens produces a real, inverted image just inside the focus of a second lens, called the eyepiece lens. The image produced by the first lens now acts as the object for the second lens. Because the first image is inside the focus of the second lens, the second image (the one seen by the telescope user) is virtual and enlarged compared to the first one
CAMERAS, IN BRIEFobject pinhole image at film plane In a pinhole camera, the hole is so small that light hitting any particular point on the film plane must have come from a particular direction outside the cameraobject image at film plane lens In a camera with a lens, the same applies: that a point on the film plane more-or-less corresponds to a direction outside the camera. Lenses have the important advantage of collecting more light than the pinhole admits