# Questions about waves and light

I have a few questions:
1. Most CD players use laser light in the red portion of the spectrum. What changes would be required in the manufacturing of CDs if blue light were to be used?

2. A student performs Young's double-slit experiment using a slit separation of 21.6um. A screen is placed 2.50m from the centre of the sources such that a point on the fifth nodal line is 37.5cm from the centre of the interference pattern. Find the wavelength of the light used and identify its colour.
-> I was going to use the equation dXn/L = (n-1/2)lambda
There's also the equation delta x = L x lamda / d
i don't know which equation to use.
I know that d = 21/6um, L = 37.5cm but what's 2.50m?

3. A two-point source interference pattern in a ripple tank with two sources operation in phase: A point on the eigth nodal line is 1.25m from the centre of the two sources and 48.0cm from the perpendicular bisector of the two sources. If the source separation is 2.75cm, find the wavelength of the waves.
-> I'm not sure what 1.25m and 48.0cm represent.

and one quick question. if one wave crest takes 2.0s to travel the 35.0m width of the pool, the speed is just v = 35.0m / 2.0s right? ( this might be a stupid question....i know...;;)

If you can help me with any of these questions, I would greatly appreciate it :) Thanks!

## Answers and Replies

mgb_phys
Science Advisor
Homework Helper
I have a few questions:
1. Most CD players use laser light in the red portion of the spectrum. What changes would be required in the manufacturing of CDs if blue light were to be used?
What do you know about the wavelenght of red and blue light.
How is the size of the smallest visible detail related to the wavelenght.

2. A student performs Young's double-slit experiment using a slit separation of 21.6um. A screen is placed 2.50m from the centre of the sources such that a point on the fifth nodal line is 37.5cm from the centre of the interference pattern. Find the wavelength of the light used and identify its colour.
Draw a diagram showing the path from the two slits to the screen.
For a bright line you want the difference in the two paths to be a whole number of wavelengths (n) for a dark line you want n+0.5 wavelengths so they cancel

3. A two-point source interference pattern in a ripple tank with two sources operation in phase: A point on the eigth nodal line is 1.25m from the centre of the two sources and 48.0cm from the perpendicular bisector of the two sources. If the source separation is 2.75cm, find the wavelength of the waves.
This is the same as q2, the two sources are the same as thw two slits.

and one quick question. if one wave crest takes 2.0s to travel the 35.0m width of the pool, the speed is just v = 35.0m / 2.0s right? ( this might be a stupid question....i know...;;)
Correct

What do you know about the wavelenght of red and blue light.
How is the size of the smallest visible detail related to the wavelenght.

Draw a diagram showing the path from the two slits to the screen.
For a bright line you want the difference in the two paths to be a whole number of wavelengths (n) for a dark line you want n+0.5 wavelengths so they cancel

This is the same as q2, the two sources are the same as thw two slits.

For #1, I know that the wavelength of red light is longer than the blue light
#2, I drew the diagram and I'm still not getting it.... Which equation do we use??? Which equation do we use for #3???