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1. The diagram below shows the light intensity produced in Young's slit experiment for red light:

kdKeller - wikimedia commons

What will be the effect on the pattern if the red laser is replaced with a less intense blue laser?

• A. The peaks will be higher and more widely spaced.
• B. The peaks will be higher and closer together.
• C. The peaks will be lower and more widely spaced.
• D. The peaks will be lower and closer together.

Q2&3. The diagram below shows a full 'Young's slit' light intensity distribution, with many more fringes shown:

kdKeller - wikimedia commons

2. What causes the overall envelope shown by the black line?

• A. Interference between 2 slits.
• B. Constructive interference between the laser and reflected light.
• C. Single slit interference.
• D. Interference from 3 or more slits.

3. What would happen to the envelope shown by the black line if the individual slits were made wider, but the spacing between the slits remains the same?

• A. The black line peaks will be higher and more widely spaced.
• B. The black line peaks will be higher and closer together.
• C. The black line peaks will be the same height, but more widely spaced.
• D. The black line peaks will be the same height, but closer together.

4.A diffraction grating has 100 lines per millimetre. This means the spacing between the lines is:

• A. 10^-1 m
• B. 10^-3 m
• C. 10^-5 m
• D. 10^-7 m

5. A different grating has a line spacing of 2 x10^-6 m. If the wavelength of light used is 400 nm, the first order fringe (where n=1) will occur when:

• A. sin θ = 0.2
• B. sin θ = 0.02
• C. sin θ = 0.5
• D. sin θ = 0.05

6. What is the maximum possible value for θ in the diffraction grating formula?

• A. < 45⁰
• B. < 90⁰
• C. < 180⁰
• D. < 180⁰

7-10. A soap bubble consists of thin film of water surrounded on both sides by air.

When light hits the soap bubble from above, some light is reflected from the top surface of the film as well as some reflected from the bottom.

7. At which surface - if any - is the phase of the light wave inverted?

8. The two reflected waves can cause an interference pattern. The refractive index of the water is 1.33, and the thickness of the film is d.

Destructive interference occurs when

• A. 1.33d = ¼λ
• B. ¼d = 1.33n
• C. 1.33d = ½λ
• D. ½d = 1.33n

9. Thin films of transparent substances can look coloured. Why does the bubble look blue in the image shown above, when illuminated with white light?

• A. Only blue light is reflected, other wavelengths are absorbed.
• B. Only blue light is reflected at top and bottom surfaces.
• C. White light is reflected from both surfaces, but constructive interference occurs with wavelengths of visible light other than blue.
• D. White light is reflected from both surfaces, but destructive interference occurs with wavelengths of visible light other than blue.

10. A new soap bubble is produced that initially looks green. However the bubble is dripping as water is lost, making the film gradually thinner. What will an observer see?

• A. The bubble turning from green to blue.
• B. The bubble turning from green to yellow.
• C. The bubble turning from green to white.
• D. The bubble remains green.