Physics for Scientists and Engineers II , Summer Semester 2009

1

Lecture 26: July 29th 2009

Physics for Scientists and Engineers II

Physics for Scientists and Engineers II , Summer Semester 2009

2

Ray Diagrams for Thin Lenses

1F 2F

Incoming Ray Outgoing Ray

Parallel to principal axis As if coming from focal point in front of the lens (F1)

Towards focal point behind the lens (F2)

Parallel to principal axis

Through center of lens Goes straight through

O I

Physics for Scientists and Engineers II , Summer Semester 2009

3

Combination of Thin Lenses

Similar to the procedure we used when calculating the effect of two refracting surfaces:

Step 1: Ignore lens 2 and calculate the image position due to lens 1.

Step 2: Determine whether image position from Step 1 is in front or behind lens 2. In front of lens 2: Object position for lens 2 positive (real object) Behind lens 2: Object position for lens 2 is negative (virtual object)

….and determine the distance to lens 2, of course.

Step 3: Determine the image position (of the object determined in step 2) due to lens 2.

21MMM Overall magnification:

Physics for Scientists and Engineers II , Summer Semester 2009

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Example of Combination of Thin Lenses

O

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cmf 252

cmd 15cmp 351

Physics for Scientists and Engineers II , Summer Semester 2009

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Example – Step 1 (ignore lens 2)

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Physics for Scientists and Engineers II , Summer Semester 2009

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Example – Step 2 (ignore lens 2)

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Physics for Scientists and Engineers II , Summer Semester 2009

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Example – Step 3 (ignore lens 1)

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Physics for Scientists and Engineers II , Summer Semester 2009

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Example of Combination of Thin Lenses

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Physics for Scientists and Engineers II , Summer Semester 2009

9

The Simple Magnifier – Angle Subtended

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Objects appear larger if the angle subtended at the eye is larger.

Get closer to the object and becomes larger object appears larger.

Eye

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Problem: At some point the human eye can no longer focus on the object. Typically, the closest distance is 25cm, the “near point of the eye” (varies individually, especially with age).

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cm25

Physics for Scientists and Engineers II , Summer Semester 2009

10

The Simple Magnifier

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Adding a lens and putting eye close to the lens such that the image appears at 25cm(able to focus on the image).

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Physics for Scientists and Engineers II , Summer Semester 2009

11

The Simple Magnifier

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Definition

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Angular Magnification:

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Physics for Scientists and Engineers II , Summer Semester 2009

12

The Compound Microscope – Two converging lenses

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Physics for Scientists and Engineers II , Summer Semester 2009

13

The Compound Microscope – Magnification

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Magnification Using Compound Microscope:

Physics for Scientists and Engineers II , Summer Semester 2009

14

The Refracting Telescope

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Objective Eyepiece

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Physics for Scientists and Engineers II , Summer Semester 2009

15

Chapter 37: Condition for Interference of Light Waves

Interference = Superposition (adding) of waves

S1

S2

S1 S2

Example of Constructive Interference

Physics for Scientists and Engineers II , Summer Semester 2009

16

Condition for Interference of Light Waves

S1

S2

S1 S2

Example of Destructive Interference

Conditions for Interference: 1) Light from two sources must be “coherent”. That means, the relative phase

between S1 and S2 must remain constant.2) The light should be monochromatic (single wavelength).

Physics for Scientists and Engineers II , Summer Semester 2009

17

Young’s Double-Slit Experiment

Barrier with two small openings.Monochromaticlight.

Diffraction of light occurs.

Physics for Scientists and Engineers II , Summer Semester 2009

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Young’s Double-Slit Experiment

Screen

Maximum

Maximum

Maximum

Minimum

Minimum

Constructive and destructive interference depending on direction.

Physics for Scientists and Engineers II , Summer Semester 2009

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Center: Bright Spot

Constructive interference(bright fringe on screen)

Screen

Physics for Scientists and Engineers II , Summer Semester 2009

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First Minimum (Destructive Interference)

Destructive interference(Dark fringe on screen)

Screen

Physics for Scientists and Engineers II , Summer Semester 2009

21

The next Maximum

Constructive interference(Bright fringe on screen)

Screen

Physics for Scientists and Engineers II , Summer Semester 2009

22

Light Waves in Interference

Od

S2

S1

y

P

Q

2r

1r

sin12 drr