1. Optics: Reflection, Refraction Mirrors and Lenses
05/25/2006
Optics
Mirrors and Lenses
Lecture 16

2. Light Light can be a wave or a particle.
UCSD: Physics 8; 2006
Light
Light can be a wave or a particle.
Individual particles of light are called photons.
Photons have no mass, they are a bundle of energy.
The higher the frequency the greater the wave has.

3. UCSD: Physics 8; 2006
Light Intensity
Intensity is the the amount of light covering a given surface area.
Intensity is a measure of power, so it is the amount of power over a certain amount of space.
As light gets further away it spreads out more, which means the amount of power on a given area is less and there fore the intensity is less.
(don’t worry I am not making you do the math here)
Spring 2006

4. Light Intensity
Spring 2006

5. Optics: Reflection, Refraction
05/25/2006
Reflection
We describe the path of light as straight-line rays
Reflection off a flat surface follows a simple rule:
angle in (incidence) equals angle out (reflection)
angles measured from surface “normal” (perpendicular)
surface normal
same
angle
exit ray
reflected ray
incident ray
Lecture 16

6. Reflection Vocabulary
Real Image –
Image is made from “real” light rays that converge at a real focal point so the image is REAL
Can be projected onto a screen because light actually passes through the point where the image appears
Always inverted

7. Reflection Vocabulary
Virtual Image–
“Not Real” because it cannot be projected
Image only seems to be there!

8. Optics: Reflection, Refraction
05/25/2006
Hall Mirror
Useful to think in terms of images
“real” you
“image” you
mirror only
needs to be half as
high as you are tall. Your
image will be twice as far from you
as the mirror.
Lecture 16

9. LEFT- RIGHT REVERSAL
AMBULANCE

10. Optics: Reflection, Refraction
05/25/2006
Curved mirrors
What if the mirror isn’t flat?
light still follows the same rules, with local surface normal
Parabolic mirrors have exact focus
used in telescopes, backyard satellite dishes, etc.
also forms virtual image
Lecture 16

11. Two types of curved mirrors
UCSD: Physics 8; 2006
Two types of curved mirrors
Convex: Means the mirrors surface bulges outward.
Rays are reflected away from the center.
Spring 2006

12. Two types of curved mirrors
UCSD: Physics 8; 2006
Two types of curved mirrors
Concave: Means the mirrors surface curves inwards.
Rays are focused towards a middle point.
Spring 2006

13. Parts of the mirror image
UCSD: Physics 8; 2006
Parts of the mirror image
We can see an image where the rays meet.
This is called a focal point.
The distance from the mirror surface to the focal point is called the focal length.
For convex mirrors the image is virtual.
For concave mirrors the image is real.
Spring 2006

14. UCSD: Physics 8; 2006
Curved Mirrors
The line running perpendicular through the middle of the mirror is called the principal axis.
If we were to make a complete circle for the mirror the very center of the circle would be at “C” (center of curvature)
Spring 2006

15. UCSD: Physics 8; 2006
Objects in mirrors
Where you place an object relative to a mirror determines the following:
Is the image real or virtual
Is the image larger or smaller than the true object.
Is the image inverted (up side down) or up right.
Spring 2006

16. Optics: Reflection, Refraction
For a real object between f and the mirror, a virtual image is formed behind the mirror. The position of the image is found by tracing the reflected rays back behind the mirror to where they meet. The image is upright and larger than the object.
Optics: Reflection, Refraction
05/25/2006
For a real object between f and the mirror, a virtual image is formed behind the mirror. The image is upright and larger than the object.
Lecture 16

17. Optics: Reflection, Refraction
For a real object between f and the mirror, a virtual image is formed behind the mirror. The position of the image is found by tracing the reflected rays back behind the mirror to where they meet. The image is upright and larger than the object.
For a real object between C and f, a real image is formed outside of C. The image is inverted and larger than the object.
For a real object close to the mirror but outside of the center of curvature, the real image is formed between C and f. The image is inverted and smaller than the object.
Optics: Reflection, Refraction
05/25/2006
For a real object between C and f, a real image is formed outside of C. The image is inverted and larger than the object.
Lecture 16

18. Optics: Reflection, Refraction
For a real object between f and the mirror, a virtual image is formed behind the mirror. The position of the image is found by tracing the reflected rays back behind the mirror to where they meet. The image is upright and larger than the object.
For a real object between C and f, a real image is formed outside of C. The image is inverted and larger than the object.
For a real object at C, the real image is formed at C. The image is inverted and the same size as the object.
For a real object close to the mirror but outside of the center of curvature, the real image is formed between C and f. The image is inverted and smaller than the object.
Optics: Reflection, Refraction
05/25/2006
For a real object at C, the real image is formed at C. The image is inverted and the same size as the object.
Lecture 16

19. Optics: Reflection, Refraction
For a real object between f and the mirror, a virtual image is formed behind the mirror. The position of the image is found by tracing the reflected rays back behind the mirror to where they meet. The image is upright and larger than the object.
For a real object close to the mirror but outside of the center of curvature, the real image is formed between C and f. The image is inverted and smaller than the object.
Optics: Reflection, Refraction
05/25/2006
For a real object close to the mirror but outside of the center of curvature, the real image is formed between C and f. The image is inverted and smaller than the object.
Lecture 16

20. Optics: Reflection, Refraction
For a real object at f, no image is formed. The reflected rays are parallel and never converge.
Optics: Reflection, Refraction
05/25/2006
For a real object at f, no image is formed. The reflected rays are parallel and never converge.
Lecture 16

21. UCSD: Physics 8; 2006
Convex Mirrors
No matter where I place an object in front of a convex mirror
The image will always be inside (behind) the mirror.

22. UCSD: Physics 8; 2006
Mirror Equation
We can use math to determine focal length and magnification.
Mirror Equation: 1/f = 1/di + 1/do
f = focal length,
di = distance to image
do = ditsance to object
Spring 2006

23. Magnification You can find out by how much the image was enlarged or
UCSD: Physics 8; 2006
Magnification
You can find out by how much the image was enlarged or
Shrunk by simply comparing the two heights.
Magnification = hi / ho
It also happens to be the same as –di / do
Spring 2006

24. UCSD: Physics 8; 2006
Practice Problem
An object is placed 12 cm form a mirror and the image appears
20 cm away. What is the focal length?
An image appears at a distance of 14m and the mirror has a
Focal length of 10m. How far is the object from the mirror?
Spring 2006

25. Optics: Reflection, Refraction
05/25/2006
Refraction
Light also goes through some things
glass, water, eyeball, air
The presence of material slows light’s progress
interactions with electrical properties of atoms.
The “light slowing factor” is called the index of refraction
Light bends at interface between refractive indices
bends more the larger the difference in refractive index
n2 = 1.5
n1 = 1.0 A B
Lecture 16

26. Convex Lenses Thicker in the center than edges.
Lens that converges (brings together) light rays.
Forms real images and virtual images depending on position of the object.
A real image is inverted and smaller.
Virtual is larger and upright.
The Magnifier

27. Concave Lenses
Lenses that are thicker at the edges and thinner in the center.
Diverges light rays
All images are upright and smaller.
The De-Magnifier

28. Lens Equations
The lens equation is the exact same as the mirror equation.
1/f = 1/di + 1/do
The magnification equation is the same too.
m = hi / h0