Class 10 Physics Experiment : To determine the focal length of a concave mirror by obtaining the image of a distant Object .
Experiment/ Aim :
To determine the focal length of a concave mirror by obtaining the image of a distant Object .
Apparatus:
A concave mirror , an optical bench , three uprights , two needles , a knitting needle and meter scale.
Theory :
A concave mirror is a spherical mirror with inner surface as reflecting surface and is silvered on the outer side. When object lies in between the principal focus F and the centre of curvature C of a concave mirror , a real , inverted and magnified image is formed beyond centre of curvature C as figure.
The relation between the distance of the object (u) , the distance of the image (v) and focal length (f) of the concave mirror given by the mirror formula , i.e
1/f = 1/u + 1/v or f = uv/u+v
As per new Cartesian sign conventions the distances u , v and f for concave mirrors are taken as negative.
Observations:
Rough focal length of concave mirror = ...... cm
Distance between concave mirror and the wall = ...... cm
Length of knitting needle (x) = ..... cm
Observed distance between object needle AB and concave mirror (z) = ...... cm
Observed distance between image needle AB and concave mirror (Z) = ...... cm
Index correction for u , (x-y) = ......cm
Index correction for v , (x-y) = .......cm
Result :
The focal length of given concave mirror , f = ...... cm
Precautions :
1. The uprights should be vertical.
2. The tips of both the needles should be in line with the pole of the concave mirror.
3. Parallax between the object's image and the imag needle should be removed tip to tip.
4. Adequate light should be available for sharp image of distant Objects.
5. The base of the stands of the concave mirror and screen should be parallel to the measuring scale.
Source of Error :
1. Concave mirror should be thin and of good quality polished surface.
2. The eye should be placed at a distance of 25 cm from the image formed by the concave mirror on the screen.
3. The aperture of the concave mirror should be small.