Light

1
Light

• Whether seen as a source or reflected from an
  object, light is the only way we can see.

2
Ray Model of light

• Our brain understands the assumption that light
  travels in straight lines.
• Based on this assumption, we gain understanding
  of the physical world.
• The ray model of light assumes light travels in
  straight line paths called rays.

3
Reflection

• When light strikes an object, the energy can be
  absorbed, reflected, transmitted, or a
  combination of these depending on the material
  struck by the wave.
• For very shiny objects like a silvered mirror, as
  much as 95 of incident light can be reflected.
• When a narrow beam of light strikes an object, we
  define an angle of incidence, ?i, to be the angle
  between the incident ray and a line perpendicular
  to the surface called the normal.
• The angle of reflection, ?r, is the angle between
  the normal and the reflected ray.

4
Law of Reflection

• All types of waves follow the Law of Reflection
  as found by ancient Greeks
• The angle of incidence equals the angle of
  reflection.
• We can confirm this with a flashlight beam and a
  mirror.

5
Diffuse vs Specular Reflection

• When light strikes a rough surface, it is
  reflected in many different directions. This is
  called diffuse reflection.
• When light strikes a smooth surface, it is
  reflected more as one beam.
• Each individual ray still follows the Law of
  reflection.

6
Image formation

• Objects seen in a mirror appear as images that
  represent the object. Images are formed in plane
  mirrors by a combination between our brain and
  eyes.
• Light reflected off objects striking a mirror is
  sent to our eyes producing an image but our
  brain thinks light travels in straight lines, so
  we project the image to a location behind or
  inside the mirror.

7
Plane Mirror Images

• The image produced by a plane mirror appears very
  much like the object. Due to the Law of
  Reflection, the image will appear the same
  distance, di, behind the mirror as the object
  is, do, in front of the mirror.

8
Is it live or is it Memorex?

• Because light rays do not actually cross or pass
  through the image location itself, the image
  seems as if the rays appear there because our
  brain tells us light travels in straight lines.
  However we cannot project the image we see onto a
  screen or paper. This is a virtual image.
• When light rays actually DO cross at the image,
  we form a REAL image which CAN be projected onto
  a screen or paper.
• Lenses and mirrors can form real and virtual
  images.

9
Mirror, Mirror on the wall

• How tall must a full length mirror be (minimum)
  in order to see yourself from head to toe?
• (Lets try it out!)

10
Spherical Mirrors

• Spherical mirrors, unlike plane mirrors, are
  curved to form part a section of a sphere.
• Convex mirrors reflect light on the outer surface
  and are thicker in the middle. These types are
  used for providing a wide field of view and make
  small images. They are found in stores,
  passenger car mirrors, hospitals.
• Concave mirrors reflect light on the inner
  surface and are thinner in the middle (shaped
  like a cave). These mirrors are used to magnify
  images like for shaving or makeup application.
• Look in a spoon.

11
Curved Mirror Images

• Curved mirrors can bring parallel or nearly
  parallel rays of light to a central point called
  a focus, F. (If light waves travel very far
  distances, they get closer to parallel than when
  they travel short distances).
• The focal point lies along a principal axis,
  straight line through the center of the mirror
  and perpendicular to the mirrors center.

The focal length is ½ the radius of curvature of
the spherical mirror. fr/2
12
Image Location- 3 important rays

• In order to locate an image in a ray diagram, a
  minimum of 2 rays must cross. We use 3 rays
  traditionally to verify size, position, and image
  type.
• Ray 1-Leaves the object parallel to the principal
  axis, reflects off the mirror and emerges through
  the focal point. (Only for curved mirrors or
  lenses ? )
• Ray 2- Leaves the object heading through F,
  reflects off the mirror and emerges parallel to
  the principal axis.
• Ray 3- heads out perpendicular to the mirror,
  then reflects back on itself and goes through C
  (center of curvature).
• Image is formed where rays cross.

13
Describing images

• In a curved mirror, image distance and object
  distance are measured from the center of the
  mirror.
• The height of the object, ho, and the height of
  the image, hi, are related by where the object is
  located with regard to the focal point of the
  mirror.
• and with geometry
• The mirror equation relates image distance,
  object distance, and focal length, f where fr/2.

14
Magnification

• Comparing the height of the image formed in a
  mirror to the height of the object gives us the
  magnification, m.
• Sign conventions hi is positive if the image is
  upright, and negative if inverted relative to the
  object (ho is always positive). do and di are
  positive if image and object are on the
  reflecting side of the mirror. But if either
  image or object is behind mirror, corresponding
  distance is negative.
• Analysis and equations for concave mirrors can be
  applied to convex mirrors also, but quantities
  must be carefully defined. Read pg 695 carefully.

15
Index of Refraction

• The speed of light in a vacuum is rounded off to
• c 3.00 x 108 m/s.
• This speed applies to all electromagnetic waves
  including visible light.
• In air, the speed of light is only slightly less.
  In other materials like glass and water, speed
  is always less than in a vacuum.
• The ratio of the speed of light in a vacuum to
  its speed in another material, v, is called the
  Index of Refraction n, of that material. The
  Index of Refraction is never less than 1.
• (n1)

16
Refraction of Light

• When a light ray passes from one medium into
  another, the ray bends due to a change in speed.
  This bending is called Refraction.
• In a ray diagram when a wave enters a medium
  where the speed of light is less (slower), the
  ray will bend TOWARD the normal. (Like moving
  from air to water) If the speed of light is
  greater in the new medium, the ray will bend AWAY
  from the normal.( Like moving from water to air).

17
Snells Law

• The angle of refraction depends on the speed of
  light in the two media and the incident angle of
  the light ray.
• This was discovered analytically by Willebrord
  Snell (1591-1626) and is known as Snells Law or
  the Law of Refraction
• T1 is the angle of incidence and ?2 is the angle
  of refraction with respect to the normal
  perpendicular to the surface between the two
  media. And n1 and n2 are the respective indices
  of refraction.

18
Refraction Applied

• Why does a pencil appear bent in water even
  when it isnt?
• Why do a persons legs look shorter when standing
  in waist-deep water than on land?
• Why does the bottom of a pool appear to be
  shallower than it really is? (why do we think
  streams of water are not as shallow as they
  appear and why do fish seem closer to the
  surface?
• How would you spear a fish in stream? Where
  would you aim?

19
Your turn to Practice

• Please do Chapter 23 Review pg 716 QUESTIONS 1,
  3, 7.
• Please do ch 23 Review pg 717 Problems 1, 3, 9,
  12, 13, 14, 27, 30, 35.