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 are slits, rather than 2 seperate light sources are used in the Young’s double slit experiment?

Answer 2

» They could not form an interference pattern
» As the 2 light sources emit light waves at random
» The points of cancellation and reinforcement would change at random

Question 3

What is white light composed of?

Answer 3

» A continuous spectrum of colors - corresponding to a continuous range of wavelengths

Question 4

Why does red light have a larger the fringe seperation then blue light?

Answer 4

» Red light has a longer wavelength
» According to : w = (lambda x D)/s
» Therefore the longer the wavelnegth of the light used, the greater the fringe seperation

Question 5

What do vapour lamps and discharge tubes produce?

Answer 5

» Light with a dominant color
» Emit other wavelengths of light - but not as dominant
» Effectively are monochromatic sources of light

Question 6

What type of light is produced from a filament lamp or from the sun?

Answer 6

» Composed of the colors of the spectrum and therefore covers a continous range of wavelengths

Question 7

What type of light is produced from a laser?

Answer 7

» Highly monochromatic - a single wavelength is produced
» Almost perfectly parallel

Question 8

Why is a laser used to demonstrate the double slit interference?

Answer 8

» Coherent source of light
» As the photons in the laser are in phase with each other when de-excitation happens

Question 9

What are 4 precautions to take when working with lasers?

Answer 9

» Wear laser saftey goggles
» Display a warning sign
» Dont shine the laser at reflective surfaces

Question 10

What are some things you may observe when using white light instead of monochromatic light?

Answer 10

» A wider maxima
» Less intense diffraction pattern
» A central white fringe
» Alternating bright fringes
» Red furthest, blue closest

Question 11

How does the knowledge and understanding of any scientfic concept change over time?

Answer 11

» According to the experimental evidence gathered by the scientfic community

Question 12

Why are the blue fringes closer then the red fringes in the fringe pattern of white light?

Answer 12

» As red fringes are more spaced out than the blue fringes
» Due to larger wavelengths

Question 13

Describe the appearances of fringes from a monchromatic source of light?

Answer 13

» Monochromatic pattern - dark fringe, light fringe
» Equal sized fringes
» Equally spaced - fringe seperation is equal
» Can be either vertical or parallel - pay attention to the question

Question 14

Explain why more fringes will be seen if each of the slits is made narrower, assuming that no other changes are made?

Answer 14

» More diffraction occurs
» More overlap
» More fringes - with the same width