Each line on the grating acts like the single slit, but the many slits interfere together and allow much more light to pass through. Now shine the laser on a single hair. You can use human hair and stretch it out and tape it between a hole in cardboard. Shine the laser on the hair, and you will see interference patterns similar to the double slit. You can also purchase simple spectroscopes that consist of a tube with a diffraction grating in one end and a slit in the other end.
Every student receives a diffraction grating.
Set up an incandescent bulb in the middle of the front of the room. Each student should observe it through the grating. A rainbow of colors should be seen. Place, one at a time, a color filter in front of the light bulb and observe. Now use the gas discharge tubes with the power supply and look at the light through the diffraction grating.