The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

Answer explanation

R=2f

f=10cm

The refractive indices of glass and water with respect to air are 3/2 and 4/3 respectively. If speed of light in glass is 2 × 10⁸ m/s, find the speed of light in water.

Answer explanation

2.25*10^8 m/s

In the following given circuits, the heat produced in the resistor or combination of resistors connected to a 12 V battery will be

• a. Same in all the cases

• b. Minimum in case (i)

• c. Maximum in case (ii)

• d. Maximum in case (iii)

Answer explanation

Two conducting wires of the same material and of equal lengths and equal diameters are first connected in series and then parallel in a circuit across the same potential difference. The ratio of heat produced in series and parallel combinations would be:

Answer explanation

1:4

An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be

Answer explanation

25W

The refractive index of a medium V with respect to a medium ‘y’ is 2/3 and the refractive index of medium ‘y’ with respect to medium ‘z’ is 4/3. Find the refractive index of medium ‘z with respect to medium V. If the speed of light in medium ‘x’ is 3 × 10⁸ m s^-1, calculate the speed of light in medium ‘y’.

Answer explanation

Given, refractive index of medium x with respect to y,
^yµx = 2/3
Refractive index of medium y with respect to z,
^zµy = 4/3
∴ Refractive index of medium x with respect to z,
^zµx = ^yµx . ^zµy = 2/3 × 4/3 = 8/9
∴ Refractive index of medium z with respect to x,
^xµy = 1𝑧µ𝑥 = 9/8
v_y = 23 × 3 × 10⁸ = 2 × 10⁸ m/s

When a 4 V battery is connected across an unknown resistor there is a current of 100 mA in the circuit. The value of the resistance of the resistor is:

Answer explanation

40 ohm

A boy records that 4000 joules of work is required to transfer 10 coulombs of charge between two points of a resistor of 50 Ω. The current passing through it is _____.

Answer explanation

8A

A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in close contact with each other. Calculate the lens power of this combination.

Answer explanation

Power of convex lens of focal length 25 cm is
p₁ = 100/25(𝑖𝑛𝑚) = 4 D
Power of concave lens of focal length 10 cm is 100
p₂ = 100/−10(𝑖𝑛𝑚) = -10 D
∴ Power of the combination = P = P₁ + P₂
∴ P = 4 – 10 = -6D

An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focussed image can be obtained ? Find the size and the nature of the image.

Answer explanation

v=-54cm

hi=-14cm