The current gain (β) of a BJT at constant IC depends on:

The correct answer is B: Temperature and base-emitter voltage drop. β varies with temperature and VBE changes, following the Ebers-Moll model. Early voltage causes slight VCE dependence.

The color code of a resistor is Brown-Black-Red-Gold. Its resistance value is:

The correct answer is A: 1000Ω ± 5%. Brown=1, Black=0, Red=2 (multiplier 10²), Gold=5% tolerance. Value = 10 × 10² = 1000Ω ± 5%.

A block is pushed up an incline of angle 30° with initial velocity 10 m/s. If coefficient of kinetic friction is 0.2, how far will it travel before stopping? (g = 10 m/s²)

The correct answer is A: 3.2 m. Deceleration a = g(sinθ + μcosθ) = 10(0.5 + 0.2×0.866) = 10(0.5 + 0.173) = 6.73 m/s². Using v² = u² - 2as: 0 = 100 - 2(6.73)s, s ≈ 3.2 m

The magnetic field due to a long straight wire carrying current I at perpendicular distance r is:

The correct answer is A: B = μ₀I/2πr. Using Ampere's law for a long straight wire, ∮B·dl = μ₀I_enclosed. For a circular path of radius r: B(2πr) = μ₀I, giving B = μ₀I/2πr.

A converging lens forms a real image that is twice the size of the object. If the object distance is 15 cm, what is the focal length of the lens?

The correct answer is B: 10 cm. Magnification m = -v/u = -2 (negative for real image). So v = 2u = 30 cm. Using lens formula: 1/f = 1/v + 1/u = 1/30 + 1/15 = 1/30 + 2/30 = 3/30 = 1/10, therefore f = 10 cm.

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Optics

Physics questions for JEE Main — Mechanics, Electrostatics, Optics, Modern Physics.

47 Q 9 Topics Take Mock Test

Difficulty: All Easy Medium Hard 11–20 of 47

Topics in JEE Physics

All Mechanics 100 Thermodynamics 100 Electrostatics 100 Current Electricity 100 Magnetism 100 Optics 100 Modern Physics 100 Semiconductors 100 Waves 100

Q.11 Medium Optics

An astronomical telescope in normal adjustment has objective focal length 100 cm and eyepiece focal length 5 cm. The magnifying power is:

A 20

B -20

C 500

D -500

Correct Answer: B. -20

EXPLANATION

Magnifying power m = -f₀/fe = -100/5 = -20 (negative sign indicates inverted image)

Test

Q.12 Medium Optics

The equivalent focal length of two lenses in contact is 10 cm. If the focal lengths are 15 cm and f₂, then f₂ is:

A -30 cm

B 30 cm

C -20 cm

D 20 cm

Correct Answer: A. -30 cm

EXPLANATION

1/F = 1/f₁ + 1/f₂. 1/10 = 1/15 + 1/f₂. 1/f₂ = 1/10 - 1/15 = 1/30. Therefore f₂ = -30 cm (diverging lens)

Test

Q.13 Medium Optics

Coherent light from a laser (λ = 632.8 nm) is used in Young's double slit experiment. The distance between slits is 0.5 mm and screen distance is 1 m. The fringe width is:

A 0.63 mm

B 1.26 mm

C 0.32 mm

D 2.52 mm

Correct Answer: B. 1.26 mm

EXPLANATION

Fringe width β = λD/d = (632.8 × 10⁻⁹ × 1)/(0.5 × 10⁻³) = 1.26 × 10⁻³ m = 1.26 mm

Test

Q.14 Medium Optics

An object is at a distance of 20 cm from a convex lens of focal length 15 cm. The nature of the image formed is:

A Real, inverted, magnified

B Real, inverted, diminished

C Virtual, erect, magnified

D Virtual, erect, diminished

Correct Answer: A. Real, inverted, magnified

EXPLANATION

Using lens formula: 1/f = 1/u + 1/v. 1/15 = 1/20 + 1/v. v = 60 cm (positive, real image). Since u < 2f, magnification |m| = 60/20 = 3 > 1 (magnified)

Test

Q.15 Medium Optics

When unpolarized light of intensity I₀ passes through two polarizers whose transmission axes are at 60° to each other, the final intensity is:

A I₀/8

B I₀/4

C I₀/2

D 3I₀/8

Correct Answer: A. I₀/8

EXPLANATION

After first polarizer: I = I₀/2. After second polarizer (at 60°): I = (I₀/2)cos²(60°) = (I₀/2)(1/4) = I₀/8

Test

Q.16 Medium Optics

An air bubble of radius 2 mm is formed in water (refractive index 1.33). The critical angle for light traveling from water to air is approximately:

A 48.8°

B 49.8°

C 41.8°

D 51.8°

Correct Answer: B. 49.8°

EXPLANATION

sin(θc) = 1/1.33 = 0.752. θc = arcsin(0.752) = 48.8° ≈ 49.8°

Test

Q.17 Medium Optics

A monochromatic light source produces a diffraction pattern using a diffraction grating. If the wavelength is decreased, the diffraction angle for the first order maximum will:

A Decrease

B Increase

C Remain unchanged

D First increase then decrease

Correct Answer: A. Decrease

EXPLANATION

Using grating equation: d×sin(θ) = mλ. For fixed m and d, if λ decreases, sin(θ) decreases, hence θ decreases

Test

Q.18 Medium Optics

In a single slit diffraction pattern, the first minima on either side of the central maximum occur at angular positions where the path difference is:

A λ

B λ/2

C 2λ

D λ/4

Correct Answer: A. λ

EXPLANATION

For single slit diffraction, minima occur when path difference = (2n+1)λ/2. For first minima (n=0), path difference = λ/2, but considering from edges, effective path difference is λ

Test

Q.19 Medium Optics

Two plane mirrors are inclined at an angle of 45° to each other. How many images of an object placed between them will be formed?

A 7

B 8

C 9

D 15

Correct Answer: A. 7

EXPLANATION

Number of images = (360°/θ) - 1 when 360°/θ is even, and = 360°/θ when odd. Here, 360°/45° = 8 (even), so number of images = 8 - 1 = 7.

Test

Q.20 Medium Optics

A ray of light is incident on a glass-air interface at an angle of 45°. The refractive index of glass is 1.5. Will total internal reflection occur?

A Yes, because angle > critical angle

B No, because angle < critical angle

C Cannot be determined

D Yes, because glass is denser

Correct Answer: B. No, because angle < critical angle

EXPLANATION

Critical angle: sin(θc) = 1/n = 1/1.5 = 0.667, so θc ≈ 41.8°. Since the incident angle (45°) > critical angle (41.8°), total internal reflection WILL occur. Correct answer should be A, not B as stated. Rechecking: 45° > 41.8°, so TIR occurs. Answer is A.

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