8.2 Diffraction & Interference

A student is conducting a series of investigations on diffraction and interference with two slits.

In the first investigation, monochromatic light passes through a double-slit arrangement. The intensity of the fringes varies with distance from the central fringe. This is observed on a screen, as shown in Fig. 1.1.

Fig. 1.1

The intensity of the monochromatic light passing through one of the slits is reduced.

Explain the effect of this change on the appearance of

In an investigation, a white light source is incident on an orange filter, a single slit, and then a double-slit, as shown in Fig. 1.2.

An interference pattern of light and dark fringes is observed on the screen.

Fig. 1.2 

In a third experiment, the white light is replaced by orange light of wavelength 600 nm. The double-slit has a separation of 0.350 mm and the screen is 6.35 m away. 

Calculate the distance between the central and first maximum as seen on the screen.

The light source is now changed to a blue LED of wavelength 450 nm.

Explain the features of the interference pattern that will now be observed on the screen.

A teacher explains a diffraction investigation to a group of physics students.

'In this investigation a laser will be used to shine monochromatic light of wavelength 581.9 nm onto a diffraction grating. The light passing through the grating will therefore be coherent.'

Define the following terms used by the teacher

During the investigation, a second-order maximum is produced at an angle of 35° measured from the normal to the grating.

Calculate the angular separation between the highest order and second order maxima.

The investigation is repeated using the same grating but a different monochromatic source.

In this experiment, the second-order maximum is observed at an angle of 25.7°.

Calculate the wavelength of this second source.