PHY101 ASSIGNMENT NO 5 SOLUTION FALL 2010

PHYSICS
(PHY101) TOTAL
MARKS: 20 Due
Date: 24/01/2011
Assignment 5: (Fall 2010)
Q # 1
A ray of light in air strikes a glass surface. Is there a range of angles for which total internal reflection occurs?

Explain.
The critical angle of glass is 42°. If this ray of light strikes at this surface perpendicular it enters the surface without chage of direction and will make an angle of 45°.the 45° is greater than 42°so ray will be totally internally reflected.

(b)all angles of incidence greater than critical angel of 45 will give the total internal reflections i-e 45 so on 
Q # 3Light traveling in air is incident on the surface of a block of plastic at an angle of 72.7û to the normal and is bent so that it makes a 57.1û angle with the normal in the plastic. Find the speed of light in the plastic. Marks = 6
Interference can be observed only if size of film is thin or nearly equal to wavelength of waves. because when thickness is greater diffraction is not significant and waves keep their initial motion even after passing through the slit and their shapes remain straight.however a little diffraction effect appear near the corners.
(b) dividing line should be placed such that the separation b/w thin and thick is equal to wavelength.other wise the spreading of thick and thin will not be observed 

Q # 3 Light traveling in air is incident on the surface of a block of plastic at an angle of 72.7û to the normal and is bent so that it makes a 57.1û angle with the normal in the plastic. Find the speed of light in the plastic. Marks = 6
n= sini/sin r
=sine (72.7)/sin (57.1)
=0.954/0.839
=1.137
Speed of light in plastic= speed of light in air/n
=3x108/1.137
=2.6x108 ms-1 is the speed of light in the plastic 

Q # 4 Can water waves be reflected and refracted? Give examples.
Does Huygens’s principle apply to water waves?
Explain. Mar
ks = 4
.Reflection: involves a change in direction of water waves when they bounce off a barrier;refraction of water waves involves a change in the direction ofwater waves as they pass from one medium to another
Refraction of the water waves example:
This phenomenon explains why waves on a shoreline tend to strike the shore close to a perpendicular angle. As the waves travel from deep water into shallower water near the shore, they are refracted from their original direction of travel to an angle more normal to the shoreline
Reflection of the water waves example:
When waves approach a hard vertical surface,e.g. a concrete wall or a dock, most of the energy is converted into waves moving in the opposite direction, a reflection. Of course the reflected waves are superimposed onto the original waves , and the two sets of wave moving in opposite directions can create a real mess.
(b)Yes huygen rule is applicable for water waves
Explaination: Huygens noticed that if water waves pass through a tiny hole smaller than their wavelength they again spread out from that point in spherical waves . He said that spherical secondary waves propagated from any point but are only seen clearly when a barrier shields the contributions from other points.