26.The nuclides having the same number of neutrons but different number of protons or mass number are known as :

27.Phenomenon of formation of three dimensional image by interference of two light beams from laser or other coherent sources of light is called :

28.Water is neither acidic nor alkaline because :

29.A bomb is dropped at a point from a moving aeroplane. The pilot observes that :

30.Arrange the following discoverire in chronological order : 1. Radium 2. Oxygen3. Deuterium 4. Laser