Unit 16: Waves in 2-D and Light

Reflection of Waves in two dimensions

Refraction of Waves

Diffraction

Interference

Optics

What is light???

• Reflection of Light

• Refraction of Light

• Snell's Law

• Lenses

• Thin Lens Equation

• Ray Tracing on a Scale Drawing

• Interference of Light as a Wave and WHY this evidence for a wave

• Maxwell's prediction of light being an electro-magnetic wave

• Light as a particle and the photo electric effect and WHY this is evidence for a particle

• Electron and its interference and why this is so strange

Reflection

Refraction

Interference

Interference, Maxwell, and Potoelectric Effect HW

Read Ch22 and Section 28.2 Photoelectric effect explains the particle properties

1. Explain why interference is evidence for light being a WAVE and not a particle.

2. Light having a wavelength of 500 nm passes through a double slit separated b .010 mm and strikes a screen 2 m away. Determine the distance on the axis the 1st, 2nd, and 3rd maxima will be from the central maxima. Draw a picture labeling m, s, a, and y.

3. James Clerk Maxwell mathematically analyzed the relationship between electricity and magnetism to make two bold conjectures. What are they?

4. In class we observed that only certain colors of light had enough energy to leave marks on the photo paper. In fact the red high intensity laser did nothing, while the low intensity blue light did a lot! This phenomenon is known as the photoelectric effect in which Einstein won the Noble Prize for his explanation. Why is the photoelectric effect evidence for light being a PARTICLE and not a wave. Recall the analogy of tidal waves and ripples.

5. Electrons are particles. They have mass. In AP Physics we do a lab to measure their mass. A force can be exerted on them by a magnetic field and can be deflected. Electrons are particles. However...what specific observations suggest a serious discrepancy.

Review Sheet Solutions

Extra Problems

1. The fifth maxima of a 600 nm laser is located 50 cm from the central maxima and the screen is 86.6 cm from the double slit.

a. How far apart are the two slits?

b. Determine the angle for the fifth maxima?

c. At what angle is the first maxima?

d. Determine the distance from the central maxima for the first maxima.

Answers/Hints

1a.

Given:

lambda = 600 nm or 600 x 10^-9 m

m = 5

y = 50 cm or 0.50 m

s = 86.6 cm

a = ?

(m*lambda)/a = y/s ONLY if theta is small. If not then (m*lambda)/a = y/L.

Hmph...y looks pretty big compared to s. Probably not a small angle. Better do part b. first.

b. Based on the geometry, s is the adjacent and y is the opposite. TOA!!! tan (q) = 50 cm/86.6 cm. Therefore theta = 30 degrees, which is NOT small so can't use s, must use L!

Back to part a.

Need to know what L is. Can use trig or pythag to determine but L comes out to 100 cm or 1 m.

So

(m*lambda)/a = y/L Therefore a =(m*lambda)*L/ y

a = 6 micrometers.

c.

Given:

lambda = 600 nm or 600 x 10^-9 mm = 5

s = 86.6 cm

a = 6 micrometers or 6 *10^-6 m

m = 1

theta = ?

sin theta =(m*lambda)/a

Doing the math gets theta = 5.74 degrees.

d.

In this case the angle is relatively small and L is approximately equal to s. If you do the trig you can figure this out (L = 87.0 cm), so you can use either

(m*lambda)/a = y/s OR (m*lambda)/a = y/L where is L = s = 86.6 cm and solve for y.

OR

You can use trig to find y. s is the adj, and y is the opp. TOA!!! tan (5.76) = y/86.6 cm. Therefore y = 8.74 cm or 0.0874 m

Ideas to distinguish wave and particle characteristics. Interference involves waves and crests canceling. Recall that for waves amplitude matters more than frequency for energy, however for a vibrating particle the frequency matters more than the amplitude for energy. Think about the observations we made in class.

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