Page 76 - Physics - XII

Q8. Give the mirror formula for the spherical mirrors.
1 1 1
Ans.
f u v
Q9. Is this mirror formula applicable for plane mirror?
Ans. Yes, it is applicable for plane mirror. We have mirror formula

1 1 1
f u v

For a plane mirror, R = ∞, i.e. f R
2 2
1
and hence, = 0
f

1 1 1
We have,
f u v
1 1
We have, 0 = +
u v
1 1
i.e. − =
u v
i.e. –u = v
It means image is virtual (v is positive as u is negative) and it is as much behind the mirror as the
object is in front of it.

Q10. How can we distinguish between a plane, concave, and a convex mirror?
Ans. We see our face in the mirror from a close distance. In all cases, virtual image will be formed. Now,
judge the magnifi cation of the image:
(i) If the magnifi cation is one, i.e. size of image is same then the mirror is plane.
(ii) If the magnifi cation is greater than one, i.e. enlarged image is formed, the mirror is concave.
(iii) If the magnifi cation is less than one, i .e. diminished image is formed, the mirror is convex.

Q11. Why does a ray of light incident normally on a mirror retrace its path?
Ans. For a ray of light incident normally on a mirror, i.e. ∠i = ∠r = 0°. Hence, the ray retraces its path.
Q12. What do you mean by parallax?
Ans. The relative or apparent shift between the two objects placed at diff erent distances from the eye,
when the eye is moved sideways is called parallax.

Q13. At what distance from the needle should the eye be placed while removing parallax?
Ans. Eye should be kept at least distance of the distinct vision (25 cm).

Q14. What is the index correction?
Ans. In an experiment in which optical bench is used, we have to measure the object and image distances
from the pole of the mirror. The distances between the pole of the mirror and tip of the needle are
the actual distances. Observed distance is the diff erence of positions of the index marks on the base
of the mirror uprights. The actual distance may not be equal to observed distance, hence an error
is introduced in the measurement of the distances. This error is called index correction or bench
correction.
Index correction = Actual distance – Observed distance

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