EM Waves — Set 2
Physics · विद्युत चुंबकीय तरंगें · Questions 11–20 of 50
The energy of a photon is given by?
Correct Answer: B. E = hf
Photon energy is E = hf, where h is Planck’s constant. Higher frequency means higher photon energy. For a given medium, frequency stays the same during refraction.
Electromagnetic waves can travel through vacuum because they?
Correct Answer: C. Do not require a material medium
Electromagnetic waves do not need a material medium. They consist of changing electric and magnetic fields. That is why sunlight reaches Earth through space.
The direction of the Poynting vector represents the direction of?
Correct Answer: A. Energy flow in the EM wave
The Poynting vector points in the direction of energy transport. It is perpendicular to both E and B in a plane wave. Its magnitude relates to the intensity of the wave.
An EM wave has frequency 100 MHz. Its wavelength in vacuum is closest to?
Correct Answer: A. 3 m
Use λ = c/f with c = 3.0 × 10^8 m/s and f = 1.0 × 10^8 Hz. This gives λ = 3.0 m. Lower frequency waves always have longer wavelengths in vacuum.
Which of the following is NOT an electromagnetic wave?
Correct Answer: A. Sound waves
Sound waves are mechanical waves that need a medium. Electromagnetic waves can travel in vacuum. Sound in air involves pressure variations, not electric and magnetic fields.
The approximate wavelength range of visible light is?
Correct Answer: B. 400–700 nm
Visible light generally spans about 400 nm to 700 nm. Violet light has shorter wavelength than red light. Wavelength and frequency are inversely related when speed is fixed.
Which electromagnetic radiation is widely used for medical imaging of bones?
Correct Answer: B. X-rays
X-rays are used to image bones because they penetrate soft tissue better than bone. Bones absorb more X-rays, creating contrast on a detector. Proper shielding is used because X-rays are ionizing.
In a plane electromagnetic wave, the electric and magnetic fields are generally?
Correct Answer: B. In phase with each other
In a plane EM wave, E and B reach maxima and minima at the same time. This means they are in phase. Their directions are perpendicular, but their oscillations are synchronized.
The existence of electromagnetic waves was predicted theoretically by?
Correct Answer: D. Maxwell
Maxwell’s equations predict waves of coupled electric and magnetic fields. These waves travel with speed 1/√(μ0ε0) in vacuum. This prediction linked light to electromagnetism.
Electromagnetic waves were experimentally demonstrated by?
Correct Answer: C. Hertz
Hertz produced and detected radio waves in the laboratory. His experiments supported Maxwell’s theoretical prediction. The unit hertz (Hz) is named in his honor.