# Competitive Exams: Physics MCQs (Practice-Test 8 of 35)

1. A slab of thickness t and refractive index 1.5 is placed in between point A and B as shown in the above figure. The optical path length between A and B is

1. (x + t/2 + y)

2. (x + t + y)

3. (x + 3/2 t + y)

4. (x + 5/2 t + y)

2. Two thin convex lenses, having focal lengths ‘f1’ and ‘f2’ are kept coaxially in air separated by a distance ‘d’ If the space between these two lenses is filled by a liquid of refractive index n, then the equivalent focal length F of the combination will be

1. (f1f2)/{f1 + f2- (d/n) }

2. n[f1f2/(f1 + f2 − d) ]

3. f1f2/(f1 + f2 − d)

4. f1f2/(f1 + f2 − d/n)

3. The determinant of the translation matrix describing the transformation of an optical ray through an optical system has a

1. negative value

2. non-zero value, which is not unity

3. zero value

4. non-zero value, which is unity

4. The condition for achromatism for two thin lenses of focal lengths f1 and f 2 and made up of the same material (dispersive power w) placed coaxially and separated by a distance ‘d’ is

1. d = (f1 + f2)/2

2. d = (f1 − f2)/2

3. d = f1 + (f2/w)

4. d = f1- (f2/w)

5. The f-number of a camera lens represents

1. focal length x diameter of. The aperture

2. focal length/diameter of the aperture

3. power x diameter of the aperture

4. power/diameter of the aperture

6. Two sources of light are said to be coherent if they emit light

1. of equal amplitudes

2. having the same wavelength

3. having a constant phase relationship

4. having the same intensity

7. A waveform is divided into Fresnel's half-period zones. The total disturbance due to all the zones put together at any external point P is given by

1. the sum of the disturbance due to all the half-period zones

2. half the sum of the disturbance due to all the half-period zones

3. the disturbance due to the first half-period zone

4. half the disturbance due to the first half-period zone

8. Two stars emitting yellow light of wavelength l are a distance D apart along a line perpendicular to line of vision. They are at a distance R (R > > D) from, the point of observation. If these two stars are to be resolved by a telescope, then the minimum diameter of the lens should be

1. lD/R

2. 1.22 (lD/R)

3. 1.22 (lR/D)

4. 1.22 lD

9. A 20 mW laser source having 6 mm aperture sends abeam of wavelength 600 nm to the moon situated at a distance of 4 × 108m. The angular spread of the laser beam on the surface of the moon will be

1. 1.5 × 10 − 15 radian

2. 1.5 × 10 − 11 radian

3. 4.0 × 10 − 7radian

4. 1.0 × 10 − 4 radian

10. In a plane diffraction grating with N elements, the intensity of the diffracted beam is

1. proportional to? N

2. proportional to N

3. proportional to N2

4. independent of N.

11. A given calcite plate behaves as a half-wave plate for a particular wavelength? If the variation of refractive index with k is negligible, then for a light of wavelength 2? the given plate would behave as a

1. half-wave plate

2. quarter-wave plate

3. plane polaroid

4. non-polarising plate

12. The wave number of Stokes and anti-Stokes lines in a Raman spectrum are 18116 cm-1 and 18514 cm-1 respectively: The wave number of exciting line is

1. 18315

2. 18308

3. 17718

4. 17116

13. The diameter of nth Newton's ring is observed to be ‘d’ when light of wavelength 490 nm is used. If light of wavelength 640 nm is used, then the corresponding diameter will be

1. d

2. 0.766 d

3. 1.306 d

4. 1.143 d

14. In a biprism experiment, a glass plate of refractive index ‘n'and thickness't’ is introduced in the path of light from one of the coherent sources. This. Shifts the central fringe through a distance formerly occupied by mth bright fringe. If? is the wavelength of light from coherent sources, then the thickness of the glass plate introduced is given by

1. t = (n − 1) (?/m).

2. t = {1/(n − 1) } (?/m).

3. t = m?/n − 1

4. t = m?/(n + 1)