Light Lenses And Optical Instruments (1962)

[Music] the motion picture you are now seeing is a photographic record of images of real objects formed by the lens of a camera like this one all the many varieties of cameras represent one kind of optical instrument these are familiar examples of other kinds of optical instruments all these kinds of optical instruments have one thing in common each makes use of one or more lenses and whatever the purpose of the instrument its lens or lenses have one basic function the formation of an image we can use this simple basic camera to demonstrate some preliminary principles by which lenses function opposite the lens at the back of the camera is a ground glass this enables us to see images which would be normally formed on film for example we point the camera at an object an arrow symbol this Arrow can represent any object emitting light or reflecting light light from the object enters the camera through its lens which forms an image a pattern of the lighted object on the ground glass let's see how the image was formed our basic camera has two main components which demonstrate image formation a lens and a screen which receives the image in this camera the image may be formed on the ground glass as we saw or on the film which would be in the same position the word screen can indicate any image receiver in fact our simple camera is very similar to the human eye it too has a lens and a kind of screen called the retina on which visual images are formed our camera lens like that of the eye is thicker at the center than at the edges and is called a convex lens this line is called the principal axis of a lens it passes through the centers of curvature of the lens surfaces we remember that from any point on an object waves of light proceed in all directions in understanding how images are formed let's select two of the many rays which the lens intercepts these two lines indicate the portion of the wavefront we're interested in this ray diagram illustrates two light rays coming from a point on the object Juan Ray passes through the principal x's and is not appreciably refracted the other Ray passing through the edge of the lens is refracted most it intersects the first Ray at a point called I prime where the image is located the principal focus of this image called f is at the intersection of the outer Ray and the principal axis the distance between this principal Focus and the center of the lens is the focal length of the lens notice that the image formed by this lens is inverted and smaller than our object let's see such a lens and the image formation we've been discussing we have a lighted object a convex lens and a screen set in this adjustable Optical bench with the object some distance from the lens we see the kind of image we'd expect from the diagram we saw inverted and smaller than the object we call this kind of image which we are able to project upon a screen a real image now the object is moved closer to the lens we see a somewhat larger image as the screen is adjusted for Sharp Focus moving the object still closer we get a still larger image when the object is moved closer to the lens than its focal length we lose the image on the screen however if we remove the screen and look through the lens we do see an image an image we could not project onto a screen this is a virtual image upright and magnified let's review briefly what we've seen in the first position with the object some distance from the lens the image on the screen was smaller than the object and inverted in this position with the object moved closer to the lens the image was larger but still a real image and inverted in this position and at this particular Point only we lost the image notice the rays are parallel they do not meet to form an image the image is said to be at Infinity when we moved the object closer to the lens than the focal length there was no image on the screen the Rays did not converge here but they do converge here so that when we look through the lens we saw the image an upright virtual image this is one common use of a convex lens as a magnifier or reading glass the image is larger than the object on the table this kind of lens a concave lens forms a virtual image that is smaller than the object concave lenses are thinner toward the center than at the edges using this kind of lens in the optical bench and moving our object toward the lens we cannot find a point at which an image is projected upon the screen however removing the screen and looking through the lens we again see an image upright but now reduced in size it is a virtual image concave lenses form virtual images how can we see an image we cannot project onto a screen as we said a concave lens forms an upright reduced virtual image when we see an image through a lens remember we are actually using two lenses one man-made lens and the eye lens a convex lens which forms real images tracing the formation of an image of our object through the islands we find that it does form a second real image inverted and projected upon the retina the receiving screen of the eye to demonstrate what we've just discussed we'll set up our Optical bench with a concave lens with the lens in place we see that no image is projected upon the screen now we add a convex lens which resembles the eye lens the two lenses show us an inverted real image the kind formed on the retina of the eye we have formed an image by a combination or system of lenses to further illustrate the system of lenses we'll try two convex lenses combined we'll replace the concave lens with a convex lens now with two convex lenses the image we obtain is inverted and magnified a real image forms of both the convex and concave lenses are used in eyeglasses to supplement the lens of the eye in a normal eye parallel light rays are refracted by the lens to converge or Focus exactly on the retina if the lens is thicker than normal in the center light rays converge in front of the retina rather than focusing on it to correct this condition called nearsightedness we need eyeglasses using concave lenses a concave lens will cause light rays to diverge slightly so that they focus farther back on the retina if the lens is thinner than normal at the center light rays will strike the retina before they have converged to a point this condition called farsightedness can be corrected with eyeglasses having convex lenses a convex lens tends to converge light rays so that they focus farther forward on the retina we have at our disposal a wide variety of lenses of differing characteristics four different uses lenses differ according to the shapes or curvatures in which they are ground the greater the curvature of a lens the shorter focal length it has lenses of different focal lengths and curvatures arranged in different lens systems form the specific kinds of images we may desire in an optical instrument this greatly enlarged image is obtained with an arrangement of a thick lens near the object and a thinner lens farther back this Arrangement is similar to that of a compound microscope we'll turn the lens system as in a microscope the short focal length lens gives a real image enlarged and inverted the eyepiece lens of the microscope acting as a common magnifier or reading glass further magnifies this enlarged image which is a virtual image is many times larger than the object the compound microscope is one kind of optical instrument that enables us to magnify images of tiny objects such as these microorganisms hundreds of times more than actual size with a similar system of lenses but now using an objective lens a very long focal length we have set up a model for an astronomical telescope as the eyepiece lens is brought into place we see the image of the distant object in this telescope arrangement of lenses the large objective lens forms a real image this time of a far distant object the eyepiece lens in front of the eye at the extreme right acting as a common magnifier forms a virtual image a second enlarged image that the eye sees in the telescope the astronomical telescope is another kind of optical instrument that enables us to see objects the unaided eye cannot see clearly for instance the moon and other heavenly bodies are brought closer to us visually by means of lenses there is much more to the study of lenses but the principles we've discussed are basic to an understanding of the effect of lenses upon light whether we're dealing with single lenses or with systems of multiple lenses in Optical instruments every lens or lens Arrangement every Optical instrument has one purpose to refract the light emitted by or reflected from some object to form a light pattern of the object which we call an image [Music]