# NCERT Physics Class 12 Exemplar Ch 10 Wave Optics Part 1

Q 10 Is Huygen’s principle valid for longitudunal sound waves?

Ans:

Yes.

Q 11 Consider a point at the focal point of a convergent lens. Another convergent lens of short focal length is placed on the other side. What is the nature of the wave fronts emerging from the final image?

Ans:

Spherical.

Q 12 What is the shape of the wave front on earth for sunlight?

Ans:

Spherical with huge radius as compared to the earth’s radius so that it is almost a plane.

Q 13 Why is the diffraction of sound waves more evident in daily experience than that of light wave?

Ans:

• Sound wave has frequencies 20 Hz to 20 kHz. The corresponding wavelengths are 15m and 15mm, respectively.

• Diffraction effects are seen if there are slits of width a such that.

For light waves, wavelengths. Thus diffraction effects will show when

Whereas for sound they will show for .

Q 14 The human eye has an approximate angular resolution ofrad and a typical photoprinter prints a minimum of 300 dpi (dots per inch, 1 inch). At what minimal distance z should a printed page be held so that one does not see the individual dots?

Ans:

• The linear distance between two dots is ; .

• At a distance of Z cm this subtends an angle.

.

Q 15 A polariod (I) is placed in front of a monochromatic source. Another polatiod (II) is placed in front of this Polaroid (I) and rotated till no light passes. A third Polaroid (III) is now placed in between (I) and (II). In this case, will light emerge from (II). Explain.

Ans:

• Only in the special cases when the pass axis of (III) is parollel to (I) or (II) there shall be no light emerging.

• In all other cases there shall be light emerging because the pass axis of (II) is no longer perpendicular to the pass axis of (III).

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