(U-V)विधिद्वारा अवतल दर्पणकीफोकस दूरी ज्ञा
to trace the path of light ray • To trace the path of t...
गोलीय दर्पण की फोकस दूरी को परिभाषित कीजिए । Answer: दर्पण के फ़ोकस F तथा ध्रुव P के बीच की दूरी दर्पण की फ़ोकस दूरी कहलाती है तथा इसे f द्वारा निर्दिष्ट किया जाता है। अब हम यह दर्शाते हैं कि f= R/2, यहाँ R दर्पण की वक्रता त्रिज्या है।
English and hindi medium
To determine the focal length of a concave mirror by the one pin method
guddan tumse na ho payega
mirror , optical bench, गोलीय दर्पण की फोकस दूरी को परिभाषित कीजिए । Answer: दर्पण के फ़ोकस F तथा ध्रुव P के बीच की दूरी दर्पण की फ़ोकस दूरी कहलाती है तथा इसे f द्वारा निर्दिष्ट किया जाता है। अब हम यह दर्शाते हैं कि f= R/2, यहाँ R दर्पण की वक्रता त्रिज्या है।
Aim
To find the value of v for different values of u in case of a concave mirror and to find the focal length.
Materials Required
An optical bench with three uprightsA concave mirrorA mirror holderTwo optical needlesA knitting needleA half-metre scale
Procedure
To determine the rough focal length
Place the concave mirror in the mirror holder.Take this mirror holder outside and face the mirror towards a distant building.The image of the building is obtained on a wall which is painted white. Move the mirror forward and backwards to obtain a sharp image on the wall.The rough focal length of the mirror is equal to the distance between the mirror and the wall.
To set the mirror
The placement of the fixed upright should be near zero ends of the optical bench while the other two uprights must be near the other ends.Attach the mirror to the holder at the fixed upright such that the mirror faces towards the other end of the bench and surface where it is perpendicular to the length.To make the mirror surface vertical, use levelling screws.At the base of the mirror upright, note the position of the index mark.
To set the object needle
Let O be the thin optical needle which is mounted on the second upright.The object needle along with the clamp is moved 1.5 times to obtain a rough focal length of the mirror.To make the object needle and pole of the mirror horizontal, adjust the height of the object needle.Close left eye, open right eye to see an inverted and enlarged image of the needle such that the tip of the image is at the centre of the mirror.Record the index mark position at the base of the object needle.
To set the image needle
Attach the thick optical needle on the third upright and move it to the other end of the optical bench.To make the tip of the image needle and tip of the image inline, adjust the height of the image needle by closing the left eye and opening the right eye.To check if parallax exists, move the eye towards the right and see that the tips get separated.Remove the parallax tip to tip.At the base of the image needle upright, record the index mark.To determine the index corrections, all the index marks are to be recorded as observation 2.Determine the index correction for the distance between the pole of the mirror, the tip of the object needle and the image needle.To get observations 2 and 1, move the object needle upright towards the mirror by 1 cm.To get observations 4, 5, and 6, move the object needle away from the mirror by 1 cm.Note all the observations.
Result
The focal length of the given concave mirror is determined
Precautions
The uprights should be vertical.The principal axis of the mirror should be perpendicular while the central line of the optical bench should be parallel.The eye should be at least 30 cm away from the needle to locate the position of the image.Tip to tip parallax should be removed between the needle and the image obtained from the needle.The tips of object and image should be at the same height when compared to the pole of the concave mirror.Index correction should be applied for u and v.
Sources of Error
Parallax removed may not be perfect.The uprights may not be vertical.
Viva Questions
Q1. Define lens formula.
Ans: Lens formula is defined as the equation which explains the relation between object distance (u), image distance (v), and focal length of the lens (f).
Q2. What is the optical centre of a lens?
Ans: Optical centre of a lens is defined as the fixed point inside the lens on the principal axis through which light ray passes.
Q3. What is the principal axis of a lens?
Ans: Principal axis of a lens is the straight line passing through the centre of curvature of the lens.
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