DATE: PAGE NO:
EXPERIMENT 6
CONCAVE MIRROR
Aim:
To find the value of v for different values of u in case of a concave mirror and to find
the focal length.
Apparatus:
Concave mirror, a stand for mounting the mirror, illuminated wire gauge, white
screen, scale etc.
Theory:
1. u-v method:
If u is the distance between the object and the mirror and v is the distance
between the mirror and the image, then the focal length of the mirror is given
by the relation
1 1 1 𝑢𝑣
= + Or f =
f 𝑢 𝑣 𝑢+𝑣
In case of concave mirror, for the real image both u and v are
negative as per the new Cartesian sign convention. The focal length
f is negative.
2. u-v graph:
Focal length of the concave mirror is given by, f= 0𝐴+0𝐵
4
Procedure:
1) distant object method:
Fix the given concave mirror on the stand facing towards a distant
object.
Arrange the screen such that its clear image is obtained on it.
Measure the distance between mirror and screen using a meter scale. It
can be taken as the focal length (f) of the mirror.
Repeat the experiment and calculate the mean value of f.
2) u-v method:
Fix the mirror at a distance u from the object (wire gauge)
Place a screen to obtain a clear reflected image.
Measure the distance between mirror and the image (v)
Calculate the focal length using the relation, f = uv
u+v
Repeat the experiment for different values of v
Calculate the focal length in each case and find its mean value.
3) u-v graph:
taking u along X axis and v along Y axis
focal length is calculated from, f= 0𝐴+0𝐵
4
Result:
Focal length of the given concave mirror
by distant object method =------------------ m
by u-v method =------------------ m
by u-v graph = -------------------m
Precautions:
For the distant object method, the object should be sufficiently far away to
approximate parallel rays. This ensures that the light rays hitting the mirror are
nearly parallel, simplifying the measurement of the focal length.
Ensure that both the mirror and screen are clean and free from dust and
smudges, as these can affect the quality of the reflected image and lead to
measurement errors.
Sources of error:
If the screen is not positioned at the point where the image is sharply focused,
the image distance measurement will be incorrect. This can lead to errors in
determining the focal length.
Variations in the quality of the concave mirror, such as defects or uneven
curvature, can affect the focal length measurement. Use a high-quality mirror
with minimal defects for accurate result.
On the left hand side of the record
a) Distant object method
Sl.no Distance between screen and mirror Mean (f)
1
2
3
4
b) u-v method:
Sl. no Distance between mirror and uv
f =u+v
Object (u) Image (v)
cm cm cm
1
2
3
4
5
6
Mean f = cm
= m
c) u-v graph:
OA = cm
OB = ------------- cm
0𝐴+0𝐵
f =
4
= cm
= m
