If focal length of convex lens is 9 cm and that of a concave lens is -18 cm, then focal length of combination is

• + 9 cm

• - 18 cm

• - 9 cm

• + 18 cm

Two mirror are put at an angle of 60°C, then number of image of an object placed between the mirrors is

• 3

• 4

• 5

• 6

A convex lens makes a real image of 4 cm long on a screen when the lens is shifted to a new position without disturbing the object or the screen, we get a real image on the screen, which is 16 cm long, then the length of the object is

• 3/4 cm

• 8 cm

• 20 cm

• 2 cm

An astronomical telescope has an objective of focal length 100 cm and magnifying power of the distance between the two lenses in normal adjustment will be

• 106 cm

• 102 cm

• 92 cm

• 78 cm

Which of the following is not correct regarding the radio telescope?

• It cannot work at night

• It can detect a very faint radio signal

• It can be operated even in cloudly weather

• It is much cheaper than optical telescope

The distance between two point objects P and Q is 32 cm. A convex lens of focal length 15 cm is placed between them, so that the images of both the objects are formed at the same place. The distance of P from the lens could be

• 20 cm

• 12 cm

• 18 cm

• 28 cm

167.

The radii of curvature of the two surfaces of a lens are 20 cm and 30 cm and the repeative index of the material of the lens is 1.5. If the lens is concave-convex, then the focal length of the lens is

• 24 cm

• 10 cm

• 15 cm

• 120 cm

A tank is filled with water to a height of 12.4 cm. The apparent depth of a needle lying at the bottom of the tank is measured by a microscope to be 9.3 cm. If water is replaced by a liquid of refractive index 1.6 upto the same height, by what distance would the microscope have to be moved to focus on the needle again

• 4 cm

• 3 cm

• 2.18 cm

• 1.55 cm

A ray of light falls on a transparent right-angled isosceles prism made from a glass of refractive index $\sqrt{2}$. The incident ray falls normally on one of the equal sides of this prism. The ray will suffer

• total internal reflection

• Interference

• refraction with angle of refraction 90°

• None of the above

The magnifying power of a telescope in its normal adjustment is 20. If the length of the telescope is 105 cm in this adjustment, the focal length of the objective lens is

• 100 cm

• 5 cm

• 80 cm

• 95 cm