Question

Find the focal length of a convex mirror using a convex lens​

Answer 1

Answer:

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

Explanation:

Procedure

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.

Answer 2

Answer:

To determine focal length of a convex lens

Place the holder with the screen on the bench. The position of the screen should be such that the sharp image of the distant object is obtained on it. The difference between the position of the lens and the screen is equal to the focal length of the given convex lens.