5.5 More about interference

Question 1

Why are double slits described as coherent sources

Answer 1

Because they emit light waves of the same frequency with a constant phase difference

Question 2

Why do the double slits emit wavefronts with a constant phase difference

Answer 2

As each crest or wave trough from the single slit always passes through one of the double slits a fixed time after it passes through the other slit.

Question 3

Why can’t light from two nearby lamp bulbs form an interference pattern

Answer 3

Because the two light sources emit light waves at random

The points of cancellation and reinforcement would change at random so no interference pattern is possible

Question 4

What does fringe separation depend on in the double slit experiment

Answer 4

On the colour of light used

Question 5

The longer the wavelength of the light used, the ______ the fringe separation

Answer 5

Greater

Question 6

Vapour lamps and discharge tubes produce light with a ______ colour. Eg. a sodium vapour lamp produces a _______/______ glow A sodium vapour lamp is in effect a ______ light source because its spectrum is dominated by light of a certain colour

Answer 6

Dominant
Yellow/Orange
Monochromatic

Question 7

Light from a filament bulb or from the sun is composed of the colours of the ______ and therefore covers a ______ ______ of wavelengths

Answer 7

Spectrum

Continuous Range

Question 8

Light from a laser is highly ______ which means we can specify its wavelength to within a nanometre. The wavelength depends on the ______ of laser that produces it, Eg. a helium-neon laser produces ______ light. Because a laser beam is almost perfectly ______, a ______ lens can focus it to a very fine spot.

Answer 8

Monochromatic
Type
Red
Parallel
Convex

Question 9

Never look along a laser beam even after reflection as the eye lens would ______ the beam on a tiny spot on the ______ and the intense ______ of light at that spot would destroy the ______

Answer 9

Focus
Retina
Concentration
Retina

Question 10

A laser is a convenient source of _______ light. When we use a laser to demonstrate double slit interference, we can illuminate the double slits _______. We do not need to make the light pass through a ______ _______ slit first as we do with light from a non-laser source.

Answer 10

Coherent
Directly
Narrow Single