Page 77 - Physics - XII

Q15. Why is it advised to use small aperture of mirrors for the experiments?
Ans. Because the image formed may not suff er from various defects.

Q16. Which method is more accurate for the determination of focal length of a concave mirror—
1 1
(i) u versus v or (ii) versus graphs?
u v
1
1
Ans. 1 versus method because the graph between 1 and is a straight line while the graph
u v u v
between u and v is a curve.

Q17. What is a lens?
Ans. It is a portion of refracting medium bounded by two surfaces, one or both of these may by curved.

Q18. Why is a convex lens called a converging lens?
Ans. A convex lens converges the parallel beam of light incident on it, hence, it is called a
converging lens.
Q19. Why is a concave lens called a diverging lens?
Ans. When a parallel beam of light passing through a concave lens, appears to diverge out from a point,
it is called a diverging lens.

Q20. Can you fi nd the focal length of a concave lens directly?
Ans. No, because in case of concave lens, the image formed is virtual.

Q21. What do you mean by a virtual image?
Ans. If the rays after refraction do not meet actually, but only appear to diverge from a point, the image
is said to be virtual.
Q22. What is a real image?
Ans. Image formed is real if the rays actually meet at a point.

Q23. Which lens is used as magnifying glass?
Ans. A convex lens. When placed at a distance less than its focal length from an object, a virtual, magnifi ed,
and erect image is formed.

Q24. Give the various practical uses of lenses.
Ans. Lenses are used in (i) spectacles (ii) telescopes, microscopes, and other optical instruments
(iii) lenses are also used in photographic camera, etc.

Q25. Which convex lens has more focal length—thick or thin?
Ans. A thin convex lens has more focal length.

Q26. What is the type of the eye lens?
Ans. The eye lens is convex.

Q27. Is virtual image ever inverted?
Ans. No, when the object is erect, a virtual image is always erect and a real image is always inverted.
Q28. What is the lens maker formula?
Ans. It is the relation between focal length (f), radii of curvature (R & R ), refractive index of material
2
1
of lens (n ), and refractive index of the surrounding (n ).
1
2

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