Physics is where JEE ranks are usually won or lost, since a single conceptual slip changes the whole answer. Questions here span mechanics, rotational motion, thermodynamics, waves and oscillations, electrostatics, current electricity, magnetism, optics, and modern physics. Numerical problems carry full derivations so you can see which step you skipped, not just which option was right.
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
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
