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    To Find Focal Length of a Convex Mirror Using a Convex Mirror

    A convex mirror is positioned in the way of the light rays between the image and lens such that the light rays, after refraction through the lens, normally strike on the mirror's surface.

    Aim

    To find the focal length of a convex mirror, using a convex lens.

    Apparatus and Materials Required

    An optical bench A convex mirror Three needles A half-metre scale A convex lens

    Four uprights with at least two having lateral motion

    A lens holder

    Theory

    A convex lens generates a real image of a subject. A convex mirror is positioned in the way of the light rays between the image and lens such that the light rays, after refraction through the lens, normally strike on the mirror’s surface. The light rays are then reflected back, retracing their trajectory and generating an image of the object.

    The focal length of the mirror is calculated as,

    f = R/2,

    where f is the focal length mirror and R is the radius of curvature.

    Diagram

    Procedure

    Calculate the probable focal length of the convex lens by focussing the image of a distant subject (example: a tree) on a clear screen and calculating the length between the image and the lens.

    Fix the object needle, the mirror, and the lens on the optical bench as shown in the above figure. Then adjust their height in a way that the needle’s tip, the pole of the mirror, and the lens’s optical centre lie at the same horizontal alignment.

    Place the object pin between 2F and F. Tune the mirror and the needle in such a way that there is no significant parallax between the object needle’s tip and its inverted image produced at O.

    4. Measure the positions of the lens (L), the mirror (M), and the object (O).

    5. Take out the mirror and place another needle, C, on the same side of the mirror. Tune the needle C in such a way as to remove the parallax between the needle C and the image of object O. Note down the position of C.

    6. Repeat procedures 3 to 5 to note down at least five various locations of the lens and the object.

    7. Calculate the index correction between the imaging needle and the mirror as described previously.

    Observation

    The focal length of the convex lens, F = _____ cm.

    The actual length of the index needle, L = _____ cm.

    Observed length of the index needle = Position of mirror upright – Position of pin upright on the scale = _____ cm

    4. Index correction, e = Actual length – observed length = … cm.

    Upright Position of S. NoObject pin O(cm)Convex Lens L (cm)Convex MirrorP(cm)Image pin C(cm)R = PC(cm) Focal Lengthf125

    Calculations

    Determine the mean value of the radius of curvature of the convex mirror, R, and determine its focal length using the following relation

    f = R/2 = ____ cm

    Sources of Error

    1. Parallax cannot be corrected completely.

    2. The uprights are not ideally vertical.

    3. The pole of the mirror, the tip of the object needle and the optical centre are not placed in a line.

    Result

    The focal length of the given convex mirror is …… cm. Here f is the mean value of the focal length.

    Viva Voce

    1. What are the main types of mirrors? Answer: There are two main types of mirrors, and they are as follows:

    Plane Mirror Spherical Mirror

    2. Is silvering in mirrors generated by applying a silver coating or some other substances?Answer: In inexpensive mirrors, silvering is generated by the application of mercuric oxide, while the silver coating in excellent quality mirrors is generated by the application of silver nitrate.3. What kind of mirror is typically used for the dressing desk?Answer: A plane mirror is employed in a dressing desk as it provides a virtual image of the exact dimension of the object positioned in front of it.4. What is meant by the index error?Answer: The variation between the realdistance between the point object and the mirror’s pole and the recorded distance calculated on the optical bench is known as the index error. It is also called the bench error.

    Index Correction = Actual Distance – Observed Distance

    Index Error = Observed Distances – Actual Distance

    5. What is meant by focal length?Answer: Focal length is the length between the optical centre and the principal focus of a lens.6. Is a convex mirror a transparent or opaque optical tool?Answer: A convex mirror is an opaque optical tool.7. What happens to the incident light ray when it falls on a convex lens?Answer: A convex lens coincides with the incident light rays approaching the principal axis.8. Which lens is called a diverging lens?Answer: A concave lens is called a diverging lens.9. When a convex lens is paired with a concave lens, what will the image’s resolution be?

    स्रोत : byjus.com

    Physics : Online Labs for schools

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    Physics Class 9

    Experiments for Class IX in physical science that facilitate and improve the learning experience with a set of simulators, practical content and assessment. They are aligned to the CBSE syllabus and describe the working of real world experimentation concepts. The experiments cover various areas such as: sound, understanding physical properties of matter, etc.

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    स्रोत : www.olabs.edu.in

    To Find the Focal Length of a Convex Mirror, Using a Convex Lens

    To Find the Focal Length of a Convex Mirror, Using a Convex Lens Aim To find the focal length of a convex mirror, using a convex lens. Apparatus 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 […]

    To Find the Focal Length of a Convex Mirror, Using a Convex Lens

    November 30, 2016 by Sastry CBSE

    To Find the Focal Length of a Convex Mirror, Using a Convex Lens

    Aim

    To find the focal length of a convex mirror, using a convex lens.

    Apparatus

    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.

    A Short Description about the Arrangement

    As a convex mirror always forms a virtual image, its focal length cannot be found directly as for a concave mirror. For this purpose, indirect method is used, as described below.

    An auxiliary convex lens L is introduced between the convex mirror M and object needle O as shown in ray diagram (a). Keeping the object needle at distance about 1.5 times rough focal length of convex lens, the position of convex mirror behind convex lens is so adjusted that a real and inverted image of object needle O, is formed at O itself. Under such condition, the light rays are incident normally over the convex mirror to retrace their path. In the absence of convex mirror, these rays would have met at centre of curvature C of the convex mirror. The distance PC gives the radius of curvature R of the mirror.

    To locate the position of C, convex mirror is removed (without disturbing the object needle O and convex lens L). An image needle I is put behind the convex lens and moved to a position at which there is no parallax between tip of inverted image of O needle and tip of I needle. Position of image needle I gives position of centre of curvature C of the mirror M ray diagram (b)]

    TheoryRay Diagram

    ProcedureTo determine rough focal length of convex lens

    Follow steps 1 to 4 of Experiment 1 (Section B).

    To set the lens

    Clamp the holder with lens in a fixed upright and keep the upright at 50 cm mark.

    Adjust the lens such that its surface is vertical and perpendicular to the length of the optical bench.

    Keep the upright fixed in this position throughout.

    To set the object needle

    Take the thin optical needle as object needle (O). Mount it in outer laterally move-able upright near zero end.

    Move the object needle upright and clamp it at a distance (in full cm) nearly 1.5 times the obtained rough focal length of the lens.

    Adjust height of the object needle to make its tip lie on horizontal line through the optical centre of the lens.

    Note the position of the index mark on the base of the object needle upright.

    To set the convex mirror

    Clamp the holder with convex mirror in second fixed upright near the lens upright, keeping reflecting surface of the mirror towards lens.

    Adjust the height of the mirror to make its pole lie on horizontal line through the optical centre of the lens. /

    Make the mirror surface vertical and perpendicular to the length of the optical bench (the principal axes of mirror and lens must coincide.)

    Move towards zero end of the optical bench (where object needle is mounted).

    Closing left eye, keep open right eye about 30 cm away from the tip of the object needle.

    See the inverted image of the object needle (formed by reflection from the convex mirror).

    Keep the eye in a position at which the tips of the inverted image and the object needle are seen simultaneously.

    Adjust the height of the needle so that the two tips are seen in one line with right open eye.

    Move the eye towards right. The tips will get separated. The tips have parallax.

    Move the convex mirror back and forth till tip to tip parallax is removed.

    Note the position of the index mark on the base of the convex mirror upright.

    To set the image needle

    Remove the convex mirror, keeping upright in its position.

    With left eye closed, see with the right open eye from the other end of the optical bench. An inverted and enlarged image of the object needle will be seen. Tip of the image must lie in the middle of the lens.

    Mount the thick optical needle (image needle) in the fourth upright near the other end of the optical bench.

    Adjust the height of the image needle so that its tip is seen in line with the tip of the image when seen with right open eye.

    Move the eye towards right. The tips will get separated. The image tip and the image needle tip have parallax.

    Remove the parallax tip to tip.

    Note the position of the index mark on base of the image needle upright.

    Record the position of the index marks in the table against observation 2.

    To determine index correction

    Find index correction for distance between pole of convex mirror and tip of the image needle as described.

    To get more observations

    Move object needle upright towards lens by 2 cm to get observation 1. Repeat the experiment.

    Move object needle upright away from lens (from position of observation 2) by 2 cm to get observation 3. Repeat the experiment.

    Record all the observations as given ahead.

    स्रोत : www.learncbse.in

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