Huygens Principle

1 Mark Questions

1. When light travels from a rarer to a denser medium, the speed decreases. Does this

decrease in speed imply a decrease in the energy carried by the light wave? Justify your answer.[All India 2010]

Ans. speed decreases due to decrease of wavelength of wave but energy carried by the light wave depends on the amplitude of electric field vector

1. What type of wave front will emerge from a

(i)point source

(ii)distant light source? [Delhi 2009]

Ans.(i)When source of light is a point source, the wavefront is spherical.

1. At very large distances from the source, a portio of spherical or cylindrical waver at appears to be plane.

3.Draw a diagram to show refraction of a plane wave front incident on a convex lens and hence draw the refracted wave front. [Delhi 2009]

Ans. The refraction of a plane wavefront is shown in the figure below Incident Refracted wavefront wavefront В B (1)

4.Differentiate between a ray and a wave front.[Delhi 2009]

Ans. Ray A ray always remains perpendicular to the wave front and directed along the direction of propagation of wave.

Wave front The locus of all those particles which are vibrating in the same phase at any instant is called wave front.

5.Draw the wave front coming out from a convex lens, when a point source of light is placed at its focus. [Foreign 2009]

Ans. The wavelength in the given condition is shown in figure below

The wavelength in the given condition is shown in figure below Α S

Incident wavefront Refracted wavefront (1)

1. Sketch the shape of wave front emerging from a point source of light and also mark the rays. [Foreign 2009]

Ans.

When source of light is a point source, then wavefront is spherical.

Point Rays source

Spherical wavefront (1) 7.Define a wave front. [Foreign 2009]

Ans. When light is emitted from a source, then the particles present around it begins to vibrate.