CONCEPT: Interference: The combination of two or more electromagnetic waveforms to form a resultant wave that may have greater, lower, or the same amplitude is called interference. YDSE (Young’s Double Slit Experiment): If a monochromatic light falls on two narrow slits which are very close to each other act as coherent source superpose each other and an interference pattern is observed on the screen. [ alt="F1 P.Y Madhu 13.04.20 D 3" src="//storage.googleapis.com/tb-img/production/20/04/F1_P.Y_Madhu_13.04.20_D%203.png" style="width: 212px; height: 147px;"> Condition for Obtaining Clear and Broad Interference Bands The screen should be far away from the source. The wavelength of light used must be larger. The two coherent sources should be very close. Two sources of light must be of the same frequency. The amplitude of interfering light waves must be the same. EXPLANATION:​​​ In order to obtain the interference pattern, the light source should be coherent. The coherent source can be produced in all three manners. An extended source of light placed behind the slit will produce light of different wavelengths. The slit will convert it to coherent light. A well-collimated beam of light has a constant frequency and wavelength and therefore, it is used as a coherent source. If the source is itself coherent, then no slit is required for the interference of light. So, option (1),(2)and(3) are correct. So the correct answer is option 1.