A convex lens of focal length 30 cm is used as a magnifying glass. For maximum magnification, the object should be placed:
Answer: C
For magnifying glass (virtual image), object is placed between f and lens. Maximum magnification occurs when final image is at least distance of distinct vision (25 cm).
Q.82Medium
Snell's law is violated when light travels from denser to rarer medium at angle of incidence greater than critical angle because:
Answer: B
Beyond critical angle, total internal reflection occurs - Snell's law cannot be applied as there is no refracted ray, only reflected ray.
Q.83Easy
Critical angle for a medium is 45°. Its refractive index is:
In Young's double slit experiment, if one slit is covered with a glass plate of thickness t and refractive index n, the central bright fringe shifts by:
An object is placed 15 cm in front of a concave mirror of radius of curvature 20 cm. The magnification is:
Answer: A
f = 10 cm, u = 15 cm. Using m = -v/u, first find v from mirror formula: v = uf/(u-f) = 5150 = 30 cm. m = -1530 = -2
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Q.86Medium
Diffraction fringes are observed when light passes through a single slit. The first minimum occurs at angle θ where:
Answer: A
For single slit diffraction, first minimum occurs when path difference = λ, giving b sin θ = λ, or sin θ = λ/b where b is slit width.
Q.87Easy
When white light passes through a prism, violet light deviates more than red light because:
Answer: D
Violet has smaller λ, higher frequency, and the medium has higher n for violet (dispersion). All factors lead to greater deviation.
Q.88Easy
A lens of power -5 diopters is used to correct vision. This lens is:
Answer: B
Negative power indicates concave lens (f = -20 cm). Concave lens is used to correct myopia (short-sightedness).
Q.89Hard
In a diffraction grating with 5000 lines/cm, the second order spectrum is observed at angle 30°. The wavelength of light is:
Answer: B
d = 1/(5000×10^2) = 2×10^-6 m. Using d sin θ = mλ: 2×10^-6 × sin(30°) = 2 × λ. λ = 10^-56 = 200 nm
Q.90Hard
In a compound microscope, the objective has focal length 0.5 cm and eyepiece has focal length 5 cm. The distance between them is 20 cm. The magnifying power is approximately:
Answer: B
M = -(v₀/u₀) × (D/f_e) where v₀ ≈ L = 20 cm, u₀ ≈ f₀ for final image at infinity. M ≈ (020.5) × (525) = 40 × 5 = 200. For near point at lens: ~400.
Q.91Medium
Light is incident on a transparent glass slab at Brewster's angle. At this angle:
Answer: C
At Brewster's angle, tan θ_B = n, and reflected ray is perpendicular to refracted ray. This is the polarization angle.
Q.92Hard
A concave mirror forms a real image 4 times larger than the object. The object is placed at distance u from the mirror. The focal length is:
Answer: A
Magnification m = -4 (real, inverted). m = -v/u, so v = 4u. Using mirror formula: 1/f = 1/u + 41u = 45u, therefore f = 4u/5. Rechecking: f = u/3 when m=-3.
Q.93Easy
The resolving power of a microscope increases when:
