yess that's a big question.. long long time ago scientist confused whether light is a particle or a wave.. soo by the end of the 18th centuries Thomas Young set out an experiment to proof that light is a wave..
the experiment is using a light as the source, the double slit and the screen for observe the fringes.
- the slits are millimeter in width
- the slits are about a millimeter apart
- the slits and the screen is about several a meter
this experiment was for conclude that light has waves properties. to proof the fact that light showed interferences effects to conclude light traveled as a wave. this is achieved in the experiment by passing a single beam of laser light through the two slits. a the light passes trough, it is diffracted so it spread to the screen.
- determining λ
λ=ax/D
where:
λ:wavelength
a:slit separation
x:fringe separation
D:slit to screen distance
λ:wavelength
a:slit separation
x:fringe separation
D:slit to screen distance
this equation derived from:
there are 2 ray made triangle (S1CX and S2CX). using phytagoras to write the expression:
S1C2 = S1X2 + CX2 = D2 + (x + a/2)2
S2C2 = S2Y2 + CY2 = D2 – (x – a/2)2
Subtracting it
S1C2 - S2C2 = (x + a/2)2 - (x – a/2)2
We can write it like this:
(S1C + S2C)(S1C – S2C) = (x + a/2 + x – a/2)(x + a/2 – x + a/2) = 2ax
Now both equation very similar in length, so we can write like this:
S1C + S2C = 2D
This differences equal to one wavelength, λ:
S1C - S2C = λ
Substituting gives:
2D λ = 2ax
Then à λ = ax/D
this is the video about two slits experiment
this is it about what is a light.. hope you understand it.
Thanks