Power of a lens of focal length 25 cm is
- A.+2.5 Dioptre
- B.+3 Dioptre
- C.+4 Dioptre
- D.+5 Dioptre
Correct answer
Explanation
The correct answer is C, +4 Dioptre. Power is the reciprocal of the focal length in metres, so P = 1 ÷ 0.25 m = +4 D.
The power of a lens tells how strongly it converges or diverges light. It is P = 1/f, with f in metres, and its SI unit is the dioptre (D). A focal length of 25 cm is 0.25 m, and 1 divided by 0.25 is 4. The positive sign shows a convex, or converging, lens; a concave lens has a negative focal length and negative power. When lenses are placed in contact, their powers simply add, which is how opticians build up spectacle prescriptions.
A is wrong because +2.5 D is the power of a lens with a focal length of 40 cm. B is wrong because +3 D would need a focal length of about 33 cm. D is wrong because +5 D belongs to a lens of focal length 20 cm.
Exam tip: P = 1/f (f in metres); convex lens positive, concave lens negative; powers in contact add up.
Practice Questions
View allWhich one of the following statements about sound is NOT correct?
- A.Sound travels at a speed slower than the speed of light
- B.Sound waves are transverse waves
- C.Sound waves are longitudinal waves
- D.Sound travels faster in moist air than in dry air
Show answer
This statement is false, because sound in air travels as longitudinal waves, not transverse waves. In a longitudinal wave, the particles of the medium vibrate back and forth along the direction in which the wave moves, forming compressions and rarefactions.
Read the full explanation →In which one of the following devices, the light energy is converted into the electrical energy?
- A.Light-emitting diode
- B.Laser diode
- C.Solar cell
- D.Transistor
Show answer
A solar cell turns the energy of sunlight directly into electricity through the photovoltaic effect. A solar cell is a large p-n junction, usually made of silicon.
Read the full explanation →Two conducting wires of the same material and of equal lengths and equal diameters are first connected in parallel and then in series in a circuit across the same potential difference. The ratio of heat produced in parallel and series combinations is
- A.2:1
- B.4:1
- C.1:2
- D.1:4
Show answer
With the same voltage, heat varies inversely as resistance, and the parallel resistance is one-fourth of the series resistance. The two wires are identical, so each has the same resistance R. In series the total resistance is R + R = 2R; in parallel it is R/2.
Read the full explanation →The amount of air breathed in and out during a normal respiration by human lungs is referred to as
- A.Vital capacity
- B.Tidal volume
- C.Inspiratory volume
- D.Total lung capacity
Show answer
Tidal volume is the volume of air breathed in or out in one normal, quiet breath, about 500 mL in a healthy adult.
Read the full explanation →Cornea in human eye
- A.is a light sensitive screen.
- B.is a muscular diaphragm.
- C.contains blood vessels.
- D.is composed of proteins and cells.
Show answer
The cornea is a clear layer made of cells and proteins, chiefly collagen fibres laid out in a regular pattern that keeps it transparent.
Read the full explanation →