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in Physics
A lens has a focal length of 15 cm. What is its power?
A+6.67 D
B-6.67 D
C+2.25 D
D-2.25 D
Correct Answer:
A. +6.67 D
Explanation:
Power P = 1/f (in meters). P = 1/0.15 = 6.67 Diopters (positive for converging lens)
An object is placed 30 cm in front of a concave mirror with focal length 10 cm. Where is the image formed?
A15 cm behind the mirror
B15 cm in front of the mirror
C20 cm in front of the mirror
D20 cm behind the mirror
Correct Answer:
A. 15 cm behind the mirror
Explanation:
Using mirror formula 1/f = 1/u + 1/v, where f = -10 cm, u = -30 cm, we get v = -15 cm (behind mirror)
Which phenomenon explains the formation of a rainbow?
ARefraction and dispersion
BReflection only
CDiffraction only
DInterference only
Correct Answer:
A. Refraction and dispersion
Explanation:
Rainbows form due to refraction of sunlight entering water droplets and dispersion of different wavelengths
A transformer has 500 turns in primary and 100 turns in secondary coil. If primary voltage is 220V, what is the secondary voltage?
A44 V
B1100 V
C22 V
D110 V
Explanation:
Using transformer equation Vs/Vp = Ns/Np, we get Vs = 220 × (100/500) = 44 V
What is the angle of incidence when light ray falls on a plane mirror and the angle of reflection is 35°?
Explanation:
According to law of reflection, angle of incidence equals angle of reflection
A current-carrying conductor is placed in a magnetic field perpendicular to it. What force acts on it?
AGravitational force
BLorentz force
CElectrostatic force
DFrictional force
Correct Answer:
B. Lorentz force
Explanation:
The force on a current-carrying conductor in a magnetic field is called the Lorentz force, given by F = BIL
Which of the following has the lowest refractive index?
AWater
BGlass
CAir
DDiamond
Explanation:
Air has a refractive index of approximately 1, which is the lowest among common materials
A radioactive element has a half-life of 10 years. After 30 years, what fraction of the original sample remains?
Explanation:
After 3 half-lives (30 years), the fraction remaining = (1/2)³ = 1/8
Which statement about elastic collisions is correct?
AMomentum is not conserved
BKinetic energy is conserved
CObjects always stick together
DMaximum energy is lost
Correct Answer:
B. Kinetic energy is conserved
Explanation:
In elastic collisions, both momentum and kinetic energy are conserved
What is the young's modulus of a material?
AStress/Strain
BStrain/Stress
CForce × Area
DForce/Volume
Correct Answer:
A. Stress/Strain
Explanation:
Young's modulus Y = Stress/Strain = (F/A)/(ΔL/L), a measure of material stiffness