Page 75 - Physics - XII

Calculations

Put the value of u and v in the formula f uv and calculate f for each set of observations, and compute
the mean value of f. uv

Result
The focal length of given concave lens using convex lens is _____ cm.

Precautions

1. The uprights should be rigid and vertical.
2. While locating I', the convex lens and object needle should not be disturbed.
3. Focal length of the convex lens should be less than the focal length of the concave lens so that the
combination is convex.

4. Parallax should be removed tip to tip.
5. Sometimes a faint, real, and inverted image of needle may be formed due to refraction from concave
lens. It should not be confused with the bold and bright image formed by the lens combination.

Sources of Error

1. Lens may be thick.
2. Uprights may be shaky.

VIVA VOCE

Q1. What are the diff erent types of mirrors.
Ans. Mirrors are of two types: (i) plane mirror (ii) spherical mirror.

Q2. What is a spherical mirror?
Ans. A spherical mirror is a part of hollow sphere with one face silvered so as to refl ect light.
Q3. Defi ne focal length of a spherical mirror.
Ans. It is a distance between the pole and focus of the spherical mirror.

Q4. What are the diff erent types of spherical mirrors.
Ans. There are two types of spherical mirrors: (i) concave mirror (ii) convex mirror

Q5. What is the material used for “silvering” the mirror?
Ans. In cheap mirrors, silvering is done by mercuric oxide whereas in good mirrors, it is done by depositing
silver which is obtained by reducing silver nitrate to silver.
Q6. What is the relation between the radius of curvature and focal length of a spherical mirror?
Ans. Focal length of the mirror (f) is equal to half of the radius of curvature (R).
R
i.e. f =
2
Q7. Give the value of radius of curvature of a plane mirror.
Ans. Infi nity

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