Announcements

Homework for tomorrow…
(Ch. 23, CQ 3, Probs. 3, 8, & 10)
CQ5: λ < a
22.16: 20 mm
22.18: 0.24 mm
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TR 9-10 am
F 1-2 pm
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Chapter 23
Ray Optics
(Reflection & Refraction)
Last time…

The Law of Reflection is…
i.e. 23.1:
Light reflecting from a mirror
A dressing mirror on a closet door is 1.50 m tall. The bottom is 0.50 m
above the floor. A bare light bulb hangs 1.00 m from the closet door, 2.50
m above the floor.
How long is the streak of reflected light across the floor?
Diffuse Reflection…
The Law of Reflection is obeyed at each point but the irregularities of the
surface cause the reflected rays to leave in many directions.
The Plane Mirror…

Consider a source of rays from point P, which reflect from a mirror…

The reflected rays appear to emanate from P’, which is the same
distance behind the mirror as P is in front of the mirror (s = s’)
Quiz Question 1
You are looking at the image of a pencil in a mirror.
What do you see in the mirror if the top half of the mirror is covered with a
piece of dark paper?
1.
2.
3.
4.
The full image of the pencil.
The top half only of the pencil.
The bottom half only of the pencil.
No pencil, only the paper.
i.e. 23.2:
How high is the mirror?
If your height is h, what is the shortest mirror on the wall in which you can
see your full image? Where must the top of the mirror be hung?
23.3:
Refraction
Two things happen when a light ray is incident on a smooth boundary
between two transparent materials:
1.
2.
Part of the light reflects from the boundary, obeying the law of
reflection.
Part of the light continues into the second medium. The transmission of
light from one medium to another, but with a change in direction, is
called refraction.
23.3:
Refraction
A ray refracts between medium 1 and medium 2, having indices of
refraction n1 and n2 and ray angles θ1 and θ2 in the two media..
What is Snell’s Law?
23.3:
Refraction
A ray refracts between medium 1 and medium 2, having indices of
refraction n1 and n2 and ray angles θ1 and θ2 in the two media..
What is Snell’s Law?
23.3:
Refraction
What if the ray direction is reversed?
23.3:
Refraction
What if the ray direction is reversed?
Snell’s Law is still obeyed!
23.3:
Refraction


When a ray is transmitted into a material with a higher index of
refraction, it bends toward the normal.
When a ray is transmitted into a material with a lower index of
refraction, it bends away from the normal.
Quiz Question 2
A laser beam passing from medium 1 to medium 2 is refracted
as shown.
Which is true?
1.
2.
3.
n1 < n2.
n1 > n2.
There’s not enough information to compare n1 and n2.