Page 78 - Physics - XII

1 n 2 1 1
f n 1 1 R 1 R 2

Q29. Under what condition a lens does not show the refraction?
Ans. When the refractive index of surrounding medium is equal to the refractive index of the material of

the lens. i.e. the lens maker’s formula is 1 n 2 1 1 1 when n = n

f n 1 R 1 R 2 2 1
1 = P = 1 =
f 0 i.e., f 0

Hence, lens does not show the refraction.

Q30. What type of lens is an air bubble inside water?
Ans. Concave lens

Q31. A dentist uses concave mirror to see the tooth. In what way does it help the dentist? (NCERT)
Ans. A concave mirror gives the dentist a magnifi ed refl ection of the mouth. This means the image in the
mirror is larger, brighter, and, for the dentist easier to see.

Q32. Can you determine the focal length of the concave mirror if u < f ? (NCERT)
Ans. No, because when the object is placed between the pole and the focus, then a virtual image is
formed.
Q33. If focal length of the concave mirror is determined, by using convex lenses of diff erent focal

lengths, do you expect any change in the result? If yes, what type of change? If not, why not?
(NCERT)
Ans. There will be no change in the result because it depends only on the radius of curvature of the
spherical surface.

Q34. How will the result change if a convex lens of diff erent refractive indices were used? (NCERT)
Ans. In this case, we will get two diff erent values of the focal lengths of convex mirror if we shift the
position of the object with respect to the convex lens.

Q35. If the convex lens selected for the experiment has focal length less than that of the convex
mirror, how would this selection limit the experiment? (NCERT)
Ans. This selection limit the experiment as the convex mirror forms virtual and erect image. The focal
length of convex mirror cannot be determined.

Q36. Draw the ray diagram for image formation in case of a convex lens for an object placed at the
infi nity. (NCERT)

Ans.

2F 1 F 1 O F 2 2F 2

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