In a light-emitting diode (LED), the color of emitted light depends primarily on:
Answer: B
The wavelength of emitted light λ = hc/E_g, where E_g is the bandgap. Different materials (GaAs, GaN, AlGaAs) emit different colors based on their bandgap energy.
Q.362Medium
A solar cell operates as a p-n junction under illumination. The maximum power output occurs at:
Answer: C
Maximum power point (MPP) occurs where P = V × I is maximum, typically at ~80% of V_oc and ~80% of I_sc, determined by the fill factor.
Q.363Medium
In a p-n junction diode, the depletion width is primarily determined by which of the following factors?
Answer: A
The depletion width W = √(2εε₀(V_bi + V_R)/(eN_D N_A/(N_D + N_A))) depends on applied voltage, doping concentrations, and built-in potential. Both doping levels and reverse bias voltage are critical factors.
Q.364Medium
A photodiode is reverse-biased to operate in photoconductive mode. What is the primary advantage of this configuration over photovoltaic mode?
Answer: B
In photoconductive (reverse-bias) mode, the depletion region widens significantly, reducing junction capacitance and transit time, resulting in faster response (MHz to GHz range) and shot noise is suppressed due to the reverse field.
Q.365Medium
A sound wave with frequency 500 Hz travels through air (speed = 330 m/s) and then enters water (speed = 1500 m/s). What happens to the wavelength?
Answer: B
Frequency remains constant during refraction. λ = v/f. Since speed increases from 330 to 1500 m/s, wavelength increases proportionally
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Q.366Medium
A stationary observer hears a sound from an approaching source. The observed frequency is:
Answer: B
Doppler effect: When source approaches observer, f' = f(v)/(v-vs), which is greater than f
Q.367Medium
When two coherent light sources interfere, the condition for destructive interference is:
Answer: B
For destructive interference, the path difference must be odd multiples of λ/2: (2n+1)λ/2 or (n+21)λ
Q.368Medium
A wave equation is given as y = 5sin(2πx/4 - 2πt/2). The amplitude and frequency are:
Answer: A
From y = Asin(2πx/λ - 2πt/T), A = 5 m, and f = 1/T = 21π × 2π = 1 Hz
Q.369Medium
The intensity of a sound wave is I. If the amplitude is doubled and frequency is halved, the new intensity will be:
Answer: B
Intensity ∝ A²f². New intensity = (2A)² × (f/2)² × I/(A²f²) = 4 × (41) × I = 2I
Q.370Medium
A pipe closed at one end and open at the other has a length of 0.5 m. The fundamental frequency is (speed of sound = 340 m/s):
Two sources of sound have intensities I₁ and I₂. The ratio of their amplitudes is:
Answer: A
Since I ∝ A², the ratio A₁/A₂ = √(I₁/I₂)
Q.372Medium
When a wave reflects from a denser medium, the phase change is:
Answer: C
Upon reflection from a denser medium (fixed end), there is a phase change of π radians or 180°
Q.373Medium
In Young's double slit experiment, if the distance between slits is d, distance to screen is D, and wavelength is λ, the fringe width is:
Answer: B
Fringe width β = λD/d, where D is the distance to the screen and d is the slit separation
Q.374Medium
For a stationary wave on a string fixed at both ends of length L = 1 m, the fundamental frequency is 100 Hz. What is the velocity of the transverse wave on the string?
Answer: C
For fundamental mode, L = λ/2, so λ = 2L = 2 m. v = fλ = 100 × 2 = 200 m/s
Q.375Medium
A source of frequency 500 Hz moves towards a stationary observer at 10 m/s. The speed of sound is 330 m/s. What is the apparent frequency heard by the observer?
Answer: A
Using Doppler effect, f' = f(v/(v-vs)) = 500(330/(330-10)) = 500(320330) = 515.6 Hz ≈ 515 Hz
Q.376Medium
A string of length 2 m vibrates in its third harmonic. The number of nodes (excluding ends) is:
Answer: B
In the nth harmonic, number of nodes excluding ends = n-1. For third harmonic, nodes = 3-1 = 2
Q.377Medium
A pipe open at both ends resonates at a frequency of 500 Hz for the fundamental mode. If it is closed at one end, the fundamental frequency becomes:
Answer: B
For open pipe: L = λ/2, For closed pipe: L = λ/4. Ratio of frequencies = 2:1. New frequency = 2500 = 250 Hz
Q.378Medium
Two identical sound sources emit waves in phase. An observer is at a point where the path difference is 1.5λ. What is the phase difference?
A stationary observer hears a frequency of 1100 Hz from a moving source. If the source is moving towards the observer with speed 10 m/s, and the actual frequency is 1000 Hz, what is the speed of sound?
Answer: C
Using f' = f(v/(v-vs)), 1100 = 1000(v/(v-10)). Solving: 1.1(v-10) = v, 0.1v = 11, v = 330 m/s
Q.380Medium
When a wave passes from a less dense to a more dense medium, the phenomenon that does NOT occur is:
Answer: D
Frequency remains constant when a wave passes between media. Velocity, wavelength, and direction (refraction) change, but frequency does not
