1. Lecture 36: FRI 17 APR 34.1–4: Geometrical optics Physics 2102
Jonathan
Dowling
Lecture 36: FRI 17 APR
34.1–4: Geometrical optics

2. Geometrical Optics
“Geometrical” optics (rough approximation): light rays (“particles”) that travel in straight lines.
“Physical” Classical optics (good approximation): electromagnetic waves which have amplitude and phase that can change. • Quantum Optics (exact): Light is BOTH a particle (photon) and a wave: wave-particle duality.

3. Plane Mirrors object image i= -p for a plane mirror
Light rays reflect on a plane mirror, and produce a virtual image behind the mirror.
What’s a virtual image? It means the light rays are NOT coming from a real point, there is no light where the image appears.
object
image
i= -p for a plane mirror

5. Ray Tracing Plane (Flat) Mirror
• Use i=r To Draw Reflected Rays to Eye.
• Extend Reflected Rays Behind Mirror to Find Virtual Image
• Draw Two Rays From Object to Point on Mirror.
• Draw Perpendicular Line to Mirror From Each Point.

6. Ray Tracing Plane (Flat) Mirror
m = ho/hi = +1
hi=+ho ho
 = –do
Image distance is minus Object distance, do = –di , means image is behind (minus) the mirror and | do |= |di| means image is behind mirror same distance object is in front.
Virtual image means no light actually is at the image location (optical illusion).
Magnification m = +1 means image has same height as object (|m|=1) and image is right-side up (m is plus).

7. Spherical mirrors
Focal point is at half the curvatuire radius: f= - r/2 .
Rays parallel to the axis, reflect through the focal point.
Rays hitting the mirror after going to the focal point, emerge parallel.
Rays going through the center of curvature, reflect back on themselves.
Concave mirrors: r > 0
Convex mirrors: r < 0

8. Images from spherical mirrors
Consider an object placed between the focal point and the mirror. It will produce a virtual image behind the mirror.
When the object is at the focal point the image is produced at infinity.
If the object is beyond the focal point, a real image forms at a distance i from the mirror.
lateral
magnification
Check the signs!!

9. Ray Tracing Concave Mirror: Image Outside Focal Point
• Horizontal Ray Bounces Through Focal Point
• Ray Through Focal Point Bounces into Horizontal
• Ray Through Center Bounces Straight Back At’cha!
• Rays Cross at Image Point
• Image is Real (Light At Image), Inverted, and Shrunk (m=ho/hi is minus and |m|< 1) and do and di are both positive (to left)

10. Ray Tracing Concave Mirror: Image Inside Focal Point
• Horizontal Ray Bounces Through Focal Point
• Ray Through Focal Point Bounces into Horizontal
• Ray Through Center Bounces Straight Back At’cha!
• Project Behind Mirror to Cross at Virtual Image
• Image is Virtual (no light), Upright, and Enlarged (m=ho/hi is plus and > 1)
And do = + and di = – are on opposite sides of mirror.

11. Ray Tracing Convex Mirror
• Horizontal Ray Bounces Into Line of Focal Point
• Ray Towards Focal Point Bounces into Horizontal
• Ray Towards Center Bounces Straight Back At’cha!
• Project Rays Behind Mirror Cross at Virtual Image
• Image is Virtual (no light), Upright, and Shrunk (m=ho/hi is plus and < 1)
And do = + and di = – are on opposite sides of mirror.

12. Example
An object 2cm high is located 10cm from a convex mirror with a radius of curvature of 10cm. Locate the image, and find its height.
Focal length: f= r/2= -10cm/2= -5cm.
Image position: 1/i=1/f-1/p= -1/5cm –1/10cm= -3/10cm
i= - 10/3cm = cm: the image is virtual.
Magnification: m= -i/p= - (-3.33cm)/(10cm)=+0.33 (upright, smaller).
If the object image is 2cm, the image height is 0.33 x 2cm=0.67 cm.

13. Newton’s Reflecting Telescope

14. Hubble, Hubble, Toil and Trouble
Hubble mirror screw up: The central region of the mirror was flatter than it should be - by just one-fiftieth of the width of a human hair. This is equivalent to only four wavelengths of visible light, but it was enough. One insider said that the Hubble mirror was "very accurate, very accurately the wrong shape".
A star seen with a ground telescope and with “old” Hubble
The same star seen with the “new” Hubble
Before and after...