32. The principal axis of a lens is the straight line passing through the optic centre that enters the lens at right angles to the face of the lens.
33. The principal focus or simply the focus is the point through which light travelling parallel to the principal axis converges to for a convex lens or appears to diverge from for a concave lens.
34. The focal length is distance from the focus to the optic centre. 35. The power of a lens is the inverse of the focal length.
36. Compound lenses are combinations of two or more lenses in contact with each other.
37. Accommodation is the process of adjusting the focal length of the lens to focus distant or near objects clearly on the retina.
38. Short-sightedness means that distant objects cannot be seen clearly. 39. Long-sightedness means that near objects cannot be seen clearly.
40. The spectrum is the range of wavelengths present in the electromagnetic radiation.
41. Dispersion is the separating out of the different wavelengths of light.
Electricity 1. Electric charge indicates if a body has an excess or a deficiency of electrons. 2. A conductor is any substance through which electric charge can flow. 3. An insulator is any substance through which electric charge cannot flow.
4. Point discharge is the phenomenon where ions in the air are either strongly attracted or repelled from the tip of a charged conductor and move towards or away from it.
5. An electric field is the region of space in which electric charges at rest experience an electrostatic force.
6. An electric field line is the line along which a positive charge would move if placed in the electric field.
7. The electric field strength at a point in an electric field is the force per unit positive charge at that point.
8. The potential difference (voltage) between two points is the work done in moving a charge of one coulomb from one point to the other.
9. An electromotive force (emf) is a voltage that is applied to a circuit.
10. Capacitance is the ratio of the charge on a capacitor to the potential difference applied across it.
11. A capacitor is an electrical device capable of storing electric charge. 12. Electric current is the flow of electric charge. 13. The resistance of an object is the ratio of voltage across it to the current through it.
14. A resistor is a device that converts electrical potential energy to some other form.
