In YDSE, 10th bright fringe is observed at angle \(\theta\). Path difference at this fringe:
A\(5\lambda\)
B\(10\lambda\)
C\(9.5\lambda\)
D\(\lambda/10\)
✅ Correct: B
For nth bright fringe: path difference \(= n\lambda\). For 10th bright fringe: path difference \(= 10\lambda = \mathbf{10\lambda}\).
Q11
Resolving power of microscope depends on:
AWavelength of light and numerical aperture
BMagnification only
CAperture only
DEyepiece focal length
✅ Correct: A
Resolving power \(= 2\mu\sin\theta/\lambda\) (Abbe's criterion). Depends on wavelength \(\lambda\) and numerical aperture \(\mu\sin\theta\). Smaller \(\lambda\) and larger NA give better resolution.
Q12
Magnifying power of simple microscope (short-sighted person with near point \(D\)):
A\(D/f\)
B\(1+D/f\)
C\(f/D\)
D\(D/f-1\)
✅ Correct: B
For image at infinity: \(M = D/f\). For image at near point: \(M = 1 + D/f\). Standard for relaxed eye (image at \(\infty\)): \(M = D/f\). For maximum magnification: \(M = \mathbf{1+D/f}\).
Q13
Polarisation of light proves its:
AParticle nature
BTransverse wave nature
CLongitudinal wave nature
DDual nature
✅ Correct: B
Polarisation occurs only for transverse waves. Since light can be polarised, it proves light is a transverse wave. Sound cannot be polarised (longitudinal wave). This was crucial evidence for the wave theory of light.
Q14
Thin film of thickness \(t\) and refractive index \(\mu\) in reflected light. Condition for constructive interference:
A\(2\mu t = m\lambda\)
B\(2\mu t = (2m+1)\lambda/2\)
C\(\mu t = m\lambda\)
D\(2t = m\lambda\)
✅ Correct: B
For thin film in reflected light (one reflection from denser medium → phase change of \(\pi\)): constructive when \(2\mu t = (2m+1)\lambda/2\), i.e., odd multiples of \(\lambda/2\). Destructive when \(2\mu t = m\lambda\). \(\mathbf{2\mu t = (2m+1)\lambda/2}\).
Q15
In total internal reflection, which is true?
AAngle of incidence must be less than critical angle
BLight must go from rarer to denser medium
CAngle of incidence must exceed critical angle in denser medium
DWavelength must change
✅ Correct: C
TIR occurs when: (1) light travels from denser to rarer medium, and (2) angle of incidence exceeds the critical angle. Both conditions must be satisfied. Applications: optical fibres, prism binoculars, diamonds.
Q16
Diffraction is most pronounced when:
AWavelength >> slit width
BWavelength << slit width
CWavelength = slit width
DWavelength = 0
✅ Correct: C
Diffraction is most pronounced when wavelength ≈ slit width. When \(\lambda >> a\): wide spreading but less intensity pattern. When \(\lambda << a\): almost no diffraction, ray optics applies. \(\lambda \approx a\) gives maximum diffraction effect.
Q17
Power of a concave lens of focal length −25 cm:
A+4 D
B−4 D
C+0.25 D
D−0.25 D
✅ Correct: B
\(P = 1/f(\text{in metres}) = 1/(-0.25) = \mathbf{-4\text{ D}}\). Negative power confirms diverging (concave) lens. Dioptre = inverse of focal length in metres.
Q18
Two thin lenses in contact. Equivalent focal length:
Wavelength of light in medium of refractive index \(\mu\):
A\(\mu\lambda_0\)
B\(\lambda_0/\mu\)
C\(\lambda_0\)
D\(\mu^2\lambda_0\)
✅ Correct: B
Speed in medium \(v = c/\mu\). Frequency unchanged. \(\lambda = v/f = c/(\mu f) = \lambda_0/\mu\). Wavelength decreases in denser medium. \(\mathbf{\lambda_0/\mu}\).
Q20
Rainbow is formed due to:
ARefraction only
BReflection only
CDispersion and total internal reflection
DDiffraction
✅ Correct: C
Rainbow formation involves: dispersion (splitting white light into colours due to different refractive indices) and total internal reflection inside water droplets, followed by refraction on exit. This combination separates colours at different angles.
R
Roshan
Expert · 5 Years Experience
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