Dave is attending an outdoor concert. As he moves along the back of the crowd to get some chips, he notices that there are areas where the music from the speakers seems to get louder, and areas where it gets quieter.

Use the diagram on the next slide to explain why.

Light bulbs Light bulbs

Interference, slits and fringes

LO – What happens when light passes through very thin slits?

- Complete the following diagrams and questions

P

w D / s

s

w

d

d

Lines per meter = 1/d

• What are fringes?• What are coherent sources?• What does monochromatic mean?• Why are lasers good for showing the double

slit interference pattern?• What would the fringes formed by white light

look like and why?

Visible, monochromatic light is transmitted through a diffraction grating of 3.7x105 lines per meter.

The first order maximum is at an angle of 14.2 degrees to the normal.

Find the wavelength.