2. Young’s experiment provided strong evidence
supporting the wave model of light
He placed a screen with two slits cut out in
front of a monochromatic (single colour) light
and shined light through
3. Only rays at the same position of the slits will
pass through
Viewing screen will show a pattern of two
exact lines
4. Light ray diffract as they pass through the
slits
5. Crest to crest, constructive interference
occurs resulting in a bright spot on the
viewing screen
Crest to trough, destructive interference
occurs results in a black/dark spot on the
viewing screen
6. The pattern on the viewing screen will
resemble the following, proving light acts as a
wave due to its diffracting nature:
7. Mathematical relationship for angles at which
maximum light intensity from double slit
occur
• Upper θ in right-angled
triangle = deflection θ to point
P (measured from a normal to
slit plate)
• dsinθ = extra distance that
light from S2 travels to reach
point P
8. Constructive Interference
dsinθ = mλ
Where m is an integer called spectral order
Destructive Interference
dsinθ = (m+1/2)λ
When m = 0, θ = 0 for any value λ
▪ i.e. shine white light at pair of slits, beam in center of
interference pattern always white
9. When m = any other value, maxima for different λ
occur at different θ
▪ Longer λ = larger deflection θ
Direct central maximum = zero order
Spectral orders (m)
- - - - + + + +
10. IfYoung’s double-slit experiment was
performed underwater, how would the fringe
pattern change?
11. IfYoung’s double-slit experiment was
performed underwater, how would the fringe
pattern change?
Light wavelength in water smaller than air
d sin θ = mλ indicates θ is decreased as well
for m and d.
Therefore fringes on viewing screen appear
closer together when performed underwater
12. We can hear sounds around corners, but we
cannot see around corners; yet both sound
and light are waves. Explain the difference.
13. We can hear sounds around corners, but we
cannot see around corners; yet both sound
and light are waves. Explain the difference.
Sound waves have long wavelengths
compared to light waves, making effects of
diffraction effects more obvious.
Diffraction effects very noticeable when size
of the object wave is diffracting around
=wavelength
14. Two slits are 0.5mm apart. The viewing
screen is l= 1m away and the 5th bright fringe
is found to be h=9.5mm from the central
fringe.What is the wavelength of light used?
15. Given l, h, use trig to find the angle
tan θ = h/l
θ = tan-1(h/l) = tan-1(9.5/1000)
= 0.5440
16. Plug in value of θ and m = 5 to find the
wavelength
dsinθ = mλ
λ = dsinθ/m
= (5x10-4)(0.544)/5 = 5.44 x 10-5 m
17. In the double slit experience, green light of
wavelength 500nm gives a third-order
maximum at position x on the viewing
screen.What wavelength of visible light
would have a minimum at x?
18. Equate the constructive and destructive
interference equations since slit separation
and angle must be the same for the two
conditions to share the same position x.
dsinθ = mλ = 3(500) = 1500nm
dsinθ = (m’+1/2)λ where m’ = 0, 1, 2, 3…
19. (m’+1/2)λ = 1500nm
m’ = 0 3000nm
m’ = 1 1000nm
m’ = 2 600nm
m’ = 3 429nm
Only 429nm and 600nm are in the range of
visible light which is 390-700nm
20. Thank for watching!
Diagrams are courtesy of
http://studyphysics.ca
