26. The refractive indices of glass and water with respect to air are 3/2 and 4/3 respectively. If speed of light is 2×10^8 m/s. Find the speed of light in water.
(A) 2.25×108 m/s
(B) 2.00×108 m/s
(C) 1.00×108 m/s
(D) 2.50×108 m/s
(A) 2.25×108m/s
Explanation:
First, find speed of light in vacuum (c) using glass data: c=ng×vg=(3/2)×(2×108)=3×108 m/s. Then, find speed in water: vw=c/nw=(3×108)/(4/3)=2.25×108 m/s.
27. A divergent lens has a focal length of 20cm. At what distance should an object of height 4cm from the optical center of the lens be placed so that it’s image is formed 10cm away from the lens.
(A) U = -10cm
(C) U = +10cm
(B) U = -20cm
(D) U = -15cm
(B) U=−20cm
Explanation:
For a divergent (concave) lens, f=−20 cm and v=−10 cm. Using the lens formula 1/f=1/v−1/u, we get 1/u=1/v−1/f=1/(−10)−1/(−20)=−1/20. So, u=−20 cm.
28. A student wants to obtain an erect image of an object using a concave mirror of 10cm focal length. What will be the distance of the object from the mirror?
(A) Less than 10cm
(B) Equal to 10cm
(C) between 10cm and 20cm
(D) More than 20 cm
(A) Less than 10cm
Explanation:
A concave mirror produces an erect and virtual image only when the object is placed between the pole and the focus. Therefore, the distance must be less than the focal length (10 cm).
29. Two concave lenses P and Q have focal length 0.50 cm and 0.40 cm respectively. Which of the following is true about the combined power of the two lenses?
(A) P is equal to 4.5D
(B) P is less to 4.5D
(C) P is more than 4.5D
(D) P can not be determined from the information given
(A) P is equal to 4.5D
Explanation:
Assuming a typo and focal lengths are 50cm (-0.5m) and 40cm (-0.4m), their powers are -2D and -2.5D. The combined power is -4.5D, so the magnitude is 4.5D, making ‘A’ the intended answer.
30. A person with myopia eye can’t see object beyond 1.2m distinctly. What should be the type of corrective lens used to restore proper vision?
(A) F = 1.2m
(B) F = 1.5m
(C) F = 1.0m
(D) F = 12m
(A) F = 1.2m
Explanation:
Myopia is corrected with a concave lens whose focal length equals the person’s far point. Here, the far point is 1.2 m, so the focal length required is f=−1.2 m. The option states the magnitude.
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