A convex lens gives a real, inverted image if the object is outside the focal length of the lens. Mcq questions for class 10 science light reflection and. For this purpose, indirect method is used, as described below. Get the focal length from all measurements by a taking average of calculated f or b from u versus v graph or c from versus graph. To determine the rough focal length of the convex lens. Finding the focal length of a convex lens by plotting graph. The purpose of this lab is to introduce students to some of the properties of thin lenses and mirrors. Quiz if an object is 12 cm away from a convex lens of focal length 4 cm where will the image be. At what distance shouldthe object from the lens be placed so that it forms an image at 10 cmfrom the lens. The length focal length is calculated using the following formula. P rinciple for an object placed at a distance u from the optical centre of a thin convex lens of focal length f, a real and inverted image is formed on. In case, if the focal length of the given thin convex lens is not known then rough value of its focal length f l should be estimated first to ensure that its focal length is less than that of the concave lens. Express your answer in meters, as a fraction or to three significant figures.
Dec 01, 2016 as a concave lens always forms a virtual image, its focal length can not be found directly as for a convex lens. To determine the focal length of a spherical convex lens by lens formula method by lens replacement method. Methods for measuring a lens focal length december 2011 sophie morel opti 521. Introducing a concave lens results in the incoming rays from the convex lens being refracted as shown in the diagram above. For a thin double convex lens,refractionacts to focus all parallel rays to a point referred to as the principal focal point.
Focal length f is positive for converging and negative for diverging lenses. We notice that the image formed by the convex lens can be received on the screen as it is a real image formed as a result of collection of the refracted rays. So just remember, concave has the word cave in it so i always imagine that its kind of caving inwards caving inwards its a little bit of an exaggerated drawing but you get the general idea this is a concave lens right over here and let me draw its principal axis so this is its principal axis shouldnt curve at the beginning just redraw it. Measure the approximate focal length of the mirror by focusing a distance object onto a screen and measuring the distance from the mirror to the screen. Describe the shapes of three typical converging positive thin lenses and three. Measure u and v repeat above step at few more object distances. For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci or focal points of the lens. To determine the focal length of a thin lens and to explore the difference between convex and concave lenses and to determine their focal lengths. A certain point over that distance all the parallel rays will converge, that point is known as focal point. For an appropriate object distance u, find the image distance v. The focal length of a convex lens is positive, while that of a concave lens is negative. Overview of various methods for measuring a lens focal length.
There should not be any hurdle between the rays of light from the object and the convex lens. An important property of a lens is its focal length. If the focal length is in metres, this gives the optical power in dioptres inverse metres. Place the lens on the holder with fixed upright such that the upright is kept at 50 cm mark. An optical bench with uprights for holding the lenses and two needles, a thin concave lens, a convex lens of focal length 15 cm smaller than. Similarly, for concave lens, the focal length is a distance where the. To find the focal length of a convex mirror, using a. Keep the given convex lens of known focal length, f 1, in contact with the concave lens whose focal length to be determined. When beamrays of light fall on highly polished and smooth surface, then all the rays. Ks2 state the lens equation and determine the focal length of lenses using the lens equation. What happens to the focal length of the concave lens in. Type focal length f object distance s image distance s character orientation size concave mirror converging lens f 0 s 2f f 2f real inverted enlarged s 0 s concave lens forms a virtual image that cannot be projected on a screen. Assume the lens is a thin lens and the thin lens formula, equation.
The focal point is the point where parallel light rays meet. Diverging lens to determine the focal length of a diverging lens, the lens must create a measurable, real image as in part 1. Focal length of a concave mirror and a convex lens using u. The principal focal length is the distance between the principal focal point and the centre of the lens. For a converging lens for example a convex lens, the focal length is positive, and is the distance at which a beam of collimated light will be focused to a single spot. The convex lens is thicker in the middle and thinner at the edges and is also known as the converging lens. The bouncing back of light on hitting any mediumsurface is called reflection of light. Ncert class 10 science focal length of concave mirror and. If the object o is close to the front focal point, the beam coming out of the lens is almost collimated and rv is very large. For an object that is infinitely far away, the image distance and the focal. Therefore, for reading small letters, a convex lens of focal length 5 cm will be preferred. Physics mcqs for class 12 with answers chapter 9 ray.
This document is highly rated by jee students and has been viewed 1907 times. Optic virtual image always produced by concave lens cannot be viewed on screen since rays are diverging on the right of the lens however can be viewed with the eye since the eye converges the rays onto the retina. If a convex lens of focal length 80 cm and a concave lens of focal length 50 cm are combined together, what will be their resulting power. Determination of focal length of concave mirror and convex lens. Calculate the position of the images formed by the following concave lenses. Conversely, the concave lenses are thinner at the centre and thicker at its edges, in structure. Use the lensmakers equation to determine the focal length of a thin lens. Focal length of convex lensb11 lab report objectives of the experiment. The uv method to find focal length of a given concave mirror or convex lens consists of following steps.
Light shows the property of reflection and refraction. Experimentally determine the focal length and magnification of a concave diverging lens. If the concave lens was not there and the object was at a distance greater that its focal length the convex lens would form a real image. It will give a virtual upright image if the object is within the focal length. Measure the approximate focal length of the lens by focusing a distant object onto a screen and measuring the distance from the lens to the screen using the metre stick.
Focal point and focal length ray diagram for lenses. Ncert cbse solutions for class 10 science chapter 10. The lens used above is convex and the focal length is positive. The lens should be placed in such a way that its surface is vertical and perpendicular to the length of the optical bench. If the object is placed very far away from the lens d o. Determination of focal length of a converging lens and mirror. Jun 08, 2016 focal length of a concave mirror experiment class 12 cbse c.
Now a lens is placed in the path of the convergent beam 12 cm from p. Since light can pass through a lens in either direction, there are two focal points. Focal length of a concave mirror and a convex lens using uv. To find the focal length of a concave lens using a convex. The structure of convex lens is like, thicker at the centre and thinner at the edges. Measure the focal length from the center of the convex lens to the focal point. In addition, for spherical mirrors the focal length. In this experiment, we have chosen two methods to find out the focal length of a spherical lens. To understand ray diagrams it is important to know something about images. Calculating the focal length of a lens requires knowing the distance from an object to the lens and the distance from the lens to the image. As a concave lens always forms a virtual image, its focal length can not be found directly as for a convex lens.
Tutorial introduction the focal length is one of the most important parameter of a lens. The procedure to find focal length of a concave mirror by uv method is given below. Also, magnification is more for convex lenses having shorter focal length. The concave lens l 2 is placed between convex lens l 1 and image needle i. Thus, for convex lens, the focal length is a distance over which all the parallel rays will converge. An optical bench with four uprights two fixed uprights in middle, two outer uprights with lateral movement, convex lens 20 cm focal length, convex mirror, a lens holder, a mirror holder, two optical needles, one thin, one thick a knitting needle, and a half metre scale. Concave lenses video geometric optics khan academy. Its focal length in air and water differ by a 15 b 20 c 30 d 0.
The primary goals are to understand the relationship between. Use raytracing techniques tracing techniques to construct images formed by converging and diverging lenses. Convex lenses, geometrical optics alevel physics tutor. The focal lengths of a biconvex lens and a concave mirror are shown in figures 1 and 2, respectively. Jan 17, 2020 cbse class 10 science lab manual focal length of concave mirror and convex lens. Cs2 explain with diagrams the various properties of the types of lenses. A concave lens forms a virtual image that cannot be projected on a screen. Use lens equation to find the final image m i 2o i 2i 1 i 1o m 1 x m 2. Part c what is the magnitude of the radius of curvature of this mirror. Focal length of concave lens with the help of convex lens duration. Since focal length of concave lens is negative focal length f. We notice that the image formed by the convex lens can be received on the screen as it is a real image formed as. Arrange the screen on the table so that the image of the distant object is obtained on it.
In this section of the experiment, we will use two lenses to form a real image of the virtual image from a concave negative focal length lens. The refracted rays from the parallel beam of light converge on the other side of the convex lens. Dec 08, 2018 the focal length of a convex lens is positive, while that of a concave lens is negative. Relating focal length and radius of curvature for spherical mirrors, the focal length is half of the radius of curvature i. Object placed inside focal length of converging lens f.
As a preliminary step to determine approximate focal length of the converging lens, form image of a distant object on a white paper. Cbse class 10 science lab manual focal length of concave. Materials required a concave mirror, a mirror holder, a small screen fixed on a stand, a measuring scale and a distant. An object needle o is placed on one side of a convex lens l 1 and its real inverted image i is located by image needle on the other side as shown in ray diagram. From sign convention, the f obtained from the above formula will be negative as v u and u v is negative. Determine the focal length and position measured from the wall of a lens that will make a half size image of the spider on the wall. Difference between convex and concave lens with figure. Sep 26, 2015 the distance between the focus of the lens and its optical center is called the focal length of the lens f, the focal length f. Sometimes the focal length is slightly different from the datasheet value given by the manufacturer, having an impact on the predicted optical system performances. On the other hand, the focal length of concave lens is negative. Determination of focal length of concave lens using convex. Physics mcqs for class 12 with answers chapter 9 ray optics. Apply the lensmakers equation s equation to find parameters related to lens construction. To accomplish this, a real image from a converging lens will.
Generally, a convex lens forms a real image, but it can also create a virtual image when the object is in the middle of the focus and optical centre. On the contrary, the image formed by the concave lens is erect, virtual and smaller, than the object. To find focal length of a plano concave lens use the lens makers formula. Measure the approximate focal length of the lens by focusing a distant object onto a screen and measuring the distance from the lens. Where is the final image as shown, two converging lenses, each with a focal length of 10cm, are separated by 50cm. Key skills ks ks1 determine the object and image distance. Convex lenses are converging lenses, and concave lenses are diverging lenses. Look at the image of a distant object it could an object outside the window and measure the distance between the image and the lens. Nov 30, 2016 to find the focal length of a convex mirror, using a convex lens.
The relation between u, v and for a convex lens is. To find the focal length of a convex mirror, using a convex lens. Cbse class 10 science lab manual focal length of concave mirror and convex lens experiment 4a aim to determine the focal length of concave mirror by obtaining the image of a distant object. At what point does the beam converge if the lens is a a convex lens of a focal length 20 cm, and b a concave lens of focal length 16 cm.
Lenses have the same focal length when light travels from the back to the front as when light goes from the front to the back. The reciprocal of the focal length, 1 f, is the optical power of the lens. For a diverging lens for example a concave lens, the focal. The distance from the lens to that point is the principal focal length f of the lens. Focal length f f is positive for converging and negative for diverging lenses. Properties of the formed images by convex lens and concave. R 11 and r 2 2 are positive for convex outward surface and negative for concave surface. Determination of focal length of concave mirror and convex. Oct 19, 2019 the procedure to find focal length of a convex lens by uv method is given in ncert book. Lenses 1 of 5 lens combinations two converging lenses. Focal length of a concave mirror experiment class 12 cbse. Determine the focal length focal length of converging and diverging lenses. However, light cannot be focused through a diverging lens to form a real image unless that light was already converging.
We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. A concave mirror of radius 30 cm is placed in water. Following are the ways to obtain the focal length of convex lens. To determine the rough focal length of the lens, measure the distance between the lens and the screen. R1 is radius of curvature of the surface on the object side. Use a light source, concave mirror, and half screen accessory on an optics bench to measure the focal length of the concave mirror. Visualizing a virtual image is possible but dicult. Cognitive skills cs cs1 explain how the focal length of various lenses can be determined. One characteristic feature of all thin lenses and concave mirrors is the focal length, f, and is defined as the image distance of an object that is positioned infinitely far way.