Q: Three observes A, B, C measure speed of light from a source in vacuum to be vA , vB , vC respectively . If A was moving towards source, B was moving away from source with same speed and C was at rest, then
(a) vA < vB < vC
(b) vA < vB < vC
(c) vC = (vB + vA)/2
(d) vA = vB = vC
Click to See Answer :
Ans: (c) , (d)
Sol: Speed of light in vacuum is an absolute constant. It does not depend upon velocity of observer.
As vA = vB = vC , therefore, vC = (vB + vA)/2
Q: Photographs of the ground are taken from an aircraft flying at an altitude of 3 km by a camera with a lens of focal length 50 cm. The size of the film in the camera is 18 cm × 18 cm. What is the area of the ground that can be photographed by this camera ?
(a) 720 m × 720 m2
(b) 240 m × 240 m2
(c) 1080 m × 1080 m2
(d) 100 m × 100 m2
Click to See Answer :
Ans: (c)
Sol: As ground is at very large distance from camera, therefore image of ground can be assumed to be formed at the focus of the camera lens.
v = 50 m = 0.5 m , u =-3 km = -3000 m
If x is length of ground photographed, then
$\displaystyle m = |\frac{h_i}{h_o}| = |\frac{v}{u}|$
Q: A beautiful person with two normal eyes wants to see full width of her face by a plane mirror. The eye to eye and ear to ear distance of her face are 10 cm and 14 cm respectively The minimum width of required mirror is :
Sol: Here, eye to eye distance, a=10 cm ear to ear distance, d = 14 cm
Minimum width of required mirror is
Q: Diameter of a Plano – convex lens is 6 cm and thickness at the center is 3 mm , as shown in figure . If speed of light in material of lens is 2 × 108 m/s, the focal length of the lens is :
(a) 15 cm
(b) 20 cm
(c) 30 cm
(d) 10 cm
Click to See Answer :
Ans: (c)
Sol: Here, d = 6 cm, h = 3 mm = 0.3 cm,
v = 2 × 108 m/s , f = ?
If R is radius of curvature of the convex surface of lens, then from
$\displaystyle (\frac{d}{2})^2 = (2R – h)h $
$\displaystyle (\frac{d}{2})^2 = 2 R h $ (Since h << R)
Q: A Plano convex lens fits exactly into a Plano concave lens. Their plane surface are parallel to each other. If lenses are made of different materials of refractive indices μ1 and μ2 and R is the radius of curvature of the curved surface of the lenses, then the focal length of combination is