Magnification, power of a lens and lenses in contact — practice questions

Question 1 · difficulty L1 · recall

The power of a thin lens in dioptres is

• A. P=f in metres
• B. P=1/f²
• C. P=1/f with f in metres
• D. P=f/100

Question 2 · difficulty L1 · understanding

A concave lens in air has

• A. positive power
• B. zero power always
• C. power unrelated to focal length sign
• D. negative power

Question 3 · difficulty L2 · understanding

For two thin lenses in contact, the equivalent power is

• A. P=P1+P2
• B. P=P1P2
• C. P=P1-P2 always
• D. 1/P=P1+P2

Question 4 · difficulty L2 · application

A +4 D lens and a -1 D lens are placed in contact. Their combined power is

• A. +5 D
• B. +3 D
• C. -3 D
• D. +4 D

Question 5 · difficulty L3 · application

The focal length of a +2 D lens is

• A. 2.0 m
• B. 0.20 m
• C. 0.50 m
• D. -0.50 m

Question 6 · difficulty L3 · application

A lens forms a real inverted image three times the object height. Its linear magnification is

• A. +3
• B. -1/3
• C. +1/3
• D. -3

Question 7 · difficulty L4 · analysis

Two +5 D thin lenses in contact have equivalent focal length

• A. 0.10 m
• B. 0.20 m
• C. 0.40 m
• D. 1.0 m

Question 8 · difficulty L4 · analysis

A +6 D lens is combined in contact with a lens so the pair has zero net power. The second lens must be

• A. +6 D
• B. -6 D
• C. -3 D
• D. +3 D

Question 9 · difficulty L5 · analysis

A converging lens has its focal length doubled without changing object placement units. Its optical power

• A. doubles
• B. quadruples
• C. halves
• D. is unchanged

Question 10 · difficulty L5 · analysis

A +10 D lens and -5 D lens in contact form a system that is

• A. diverging with focal length -0.20 m
• B. afocal with infinite focal length
• C. converging with focal length 0.10 m
• D. converging with focal length 0.20 m