38 
ment which shall enable you to determine the number of vibra- 
tions per second due to a note of any given pitch. 
6. What are nodes ? What are ventral segments ? How does a 
square plate divide when it gives its deepest note ? How does a 
bell divide itself? What other modes of division are possible to a 
bell ? Describe Chladni’s mode of investigating the vibration of 
plates. 
7. Give a full and clear explanation of the beats which are 
heard when two organ pipes of nearly the same length are sounded 
together. 
[Four of these nine .] 
1 . How has it been strictly proved that the angle of incidence 
is equal to the angle of reflection? prove from this law that the 
image of an object in a plane mirror is as far behind as object is 
in front. 
2. If a plane mirror from which a luminous beam is reflected 
be caused to rotate, the reflected ray will move with twice the 
angular velocity of the mirror ; prove this. 
3. In the case of a concave spherical mirror an object is placed 
first at an infinite distance, then at a place beyond the centre, 
then at the centre, then between centre and principal focus, 
then at the principal focus, and finally between the principal 
focus and surface of mirror ; state the character and position of 
image formed in each of these cases. 
4. What is meant by index of refraction ? Under what cir- 
cumstances does total reflection take place ? 
5. What is meant by the dispersion of light ? Is it possible to 
neutralize the dispersion without at the same time neutralizing 
the refraction ? 
G. Describe an experiment which shall demonstrate the 
spherical aberration of a lens, and another that shall exhibit 
its chromatic aberration. 
7. Give a sketch of the undulatory theory of light. What do 
you understand by plane polarized light ? What is the relation 
between the reflected and the transmitted beam at the polarizing 
angle ? 
8. Describe an experiment which shall prove that the two 
beams into which a single beam is divided by passing through 
Iceland Spar are polarized at right angles to each other. 
9. What is meant by the interference of light ? Apply, as far 
as you can, the principal of interference to explain the colours of 
