Young double slit experiment

**Ukitake Jyuushirou** · Nov17-05, 08:33 AM

hello,

i am doing a work sheet on the title of this thread and i do not have the answer sheet for the questions so can some one please go through my answers and see if i am correct

Young used a monochromatic light source
a) wat does this mean?
my ans: light of a single wavelength

b) how did he achieve this?
my ans: by using sunlight

c)wat is the purpose of the single slit?
my ans: so that only light from the single slit will reach the double slit

d)when does destructive interference occur?
when the wavelength is 1/2 of the wavelength of the bright fringes

**Doc Al** · Nov17-05, 01:18 PM

Originally Posted by Ukitake Jyuushirou

Young used a monochromatic light source
a) wat does this mean?
my ans: light of a single wavelength

That's the correct meaning of monochromatic.

b) how did he achieve this?
my ans: by using sunlight

Sunlight is not monochromatic.

c)wat is the purpose of the single slit?
my ans: so that only light from the single slit will reach the double slit

Well, yes, but why is that important? (The key is that it keeps the path length the same for light hitting the double slits.)

d)when does destructive interference occur?
when the wavelength is 1/2 of the wavelength of the bright fringes

Not sure what that means. Hint: What must be the path length difference between light from each slit arriving at a point on the screen so that there is destructive interference?

**Ukitake Jyuushirou** · Nov18-05, 03:08 AM

thank u doc

for the qn 2 , wat then qualifies as a monochromatic light?

for the last qn, i mean to say the path difference between the 2 sources must be 1/2 wavelength

**Doc Al** · Nov18-05, 02:14 PM

Originally Posted by Ukitake Jyuushirou

for the qn 2 , wat then qualifies as a monochromatic light?

I don't know what Young did in his orginal experiments to get a monochromatic source, but he could have used a filter or a prism to select a single wavelength. (If you don't use a monochromatic source, then you'll get overlapping fringes from each wavelength.) Nowadays, getting a monochromatic source is easy using lasers.

for the last qn, i mean to say the path difference between the 2 sources must be 1/2 wavelength

Good. The path difference of the light from each slit must be 1/2 wavelength for destructive interference.

**Just some guy** · Nov18-05, 05:40 PM

Originally Posted by Doc Al

I don't know what Young did in his orginal experiments to get a monochromatic source, but he could have used a filter or a prism to select a single wavelength. (If you don't use a monochromatic source, then you'll get overlapping fringes from each wavelength.) Nowadays, getting a monochromatic source is easy using lasers.
Good. The path difference of the light from each slit must be 1/2 wavelength for destructive interference.

I assume he used a sodium lamp - it's monochromatic enough (not coherent though - that doesn't mean it's impossible to do but it makes it bl--dy hard (speaking from experience here

)

**brewnog** · Nov18-05, 06:54 PM

I believe Young did use sunlight in his original experiment.

**Doc Al** · Nov18-05, 07:01 PM

Originally Posted by brewnog

I believe Young did use sunlight in his original experiment.

Absolutely. I just don't know what pains, if any, he took to select a particular wavelength.