## December 3, 2015

### Senior Inter Physics Important Questions - Wave Optics

Q: Explain Doppler effect in light. Distinguish between Red Shift and
Blue Shift. (4 marks)
A: Doppler effect in light:
To an observer on the earth light from a star moving towards him
appears with high frequency or less wavelength. Similarly the apparent
wavelength of light received by the observer is high in the case of
star moving away from him.

This change in apparent frequency or wavelength of light due to
relative motion between light source and observer is called Doppler
effect in light.

Red Shift: When light source is moving away from an observer, the
frequency of light seen by the observer is less than the frequency of
light emitted by the source (or) the apparent wavelength is more than
the wavelength of light emitted by the source i.e., apparent
wavelength shifts towards the red end of the visible spectrum.

Blue Shift: When a light source is approaching an observer the
apparent frequency of light seen by the observer is more than the
frequency of light emitted from the source (or) the apparent
wavelength decreases i.e. shifts towards the blue end of the visible
spectrum.

Q: Explain Brewster's law. (2 marks)
A: When unpolarised light is incident on transparent surface, at a
particular angle of incidence called Brewster angle (or) polarizing
angle (ip), the reflected light wave is totally polarised and is
perpendicular to the refracted
light wave. The tangent of the angle of polarization is numerically
equal to the refractive index (ì) of the refracting medium.
ì = tan (ip)

Q: What is the shape of wavefront is each of the following cases
(a) Light diverging from point source.
(b) Light emerging out of a convex lens when a point source is placed
at its focus.
(c) The portion of wavefront of light from a distance star intercepted
by the earth. (2 marks)

A: (a) Shape of wavefront is spherical
(b) Shape of wavefront is plane.
(c) Shape of wavefront is plane.