Young's experiment and the dual nature of light
The results seen on the white board screen are shown in figure 2. When light passed through the cardboard with only one slit, it was diffracted. A narrow band of light, which is bright in the center and dim at the sides, was observed.
When the test was repeated using the cardboard sheet with 2 slits, light leaving both the holes were diffracted. The diffracted light from the 2 slits met and "interfered", resulting in the interference pattern of dark and light bands on the whiteboard screen. The pattern on the screen also widened by 3 to 4 times.
Basic safety requirements.