In describing the propagation of light as a wave we need to understand:
wavefronts: a surface passing through points of a wave that have the same phase.
rays: a ray describes the direction of wave propagation. A ray is a vector perpendicular to the wavefront.
Reflection and RefractionWhen a light ray travels from one medium to
another, part of the incident light is reflected and part of the light is transmitted at the boundary between the two media.
The transmitted part is said to be refracted in the second medium.
reflected rayincident ray
The Law of ReflectionFor specular reflection the incident angle qi
equals the reflected angle qr:
qi = qr
The angles are measured relative to the normal, shown here as a dotted line.
Types of Reflection
• specular reflection
• diffuse reflection
• Mirrors reflect light and allow us to see ourselves.• O is the object or its coordinate• i is the image or its coordinate• p is the distance of the object to a mirror,
refracting surface or lens• q (or i) is the distance of the image to a mirror ,
refracting surface or lens• h is the object height• h’ is the image height• lateral magnification is the ratio of image height
to object height• an image is real if the light converges to form the
image in space• an image is virtual if the light appears to come
from a place where it cannot
Plan Mirrors
• Illustrating formation of an image by a plane mirror. • Since QR is common to both triangle PQR and
triangle P’QR and q is is the same angle at vertex P and vertex P’ the right triangles are congruent, and p = - q, alsoh = h’ or the lateral magnification (M) is +1.
• The image is upright, the same size and left-right reversed
Images from Mirrors
Spherical Mirrors• Definitions for the following terms
– Center of curvature (C)– Radius of curvature (R or r)– Principle Axis (or symmetry axis)– Vertex (V)
Spherical Mirrors• When parallel rays (e.g. rays from a distance
source) are incident upon a spherical mirror, the reflected rays intersect at the focal point f, a distance R/2 from the mirror.
The image formeation
R2
q1
p1
pq
M
• Sign Convention
Spherical Mirrors
• Convex mirrors: virtual images only
Spherical Mirrors
Focus and focal length
R2
q1
p1
R2
q11
fq 2R
f f1
q1
p1
Spherical Mirrors
Light inside a medium
Law of Refraction (Snell’s Law)
n1 sin1 = n2 sin2
sinc = n2 / n1
Critical AngleRequired
:• n1 > n2
• q1 > qc
Spherical Refracting Surfaces
Flat refracting surfaces and apparent depth
Rnn
qn
pn 1221
1221 nnqn
pn
qn
pn 21
pnn
q1
2
Thin Lenses
• Two spherical refracting surfaces back to back
• Thickness of lens is small (negligible)
Thin Lenses
1 2
11 1 n
p q R R
21 R1
R1
1nq1
p1
21 R1
R1
1nf1
f1
q1
p1
Thin Lenses
Thin Lenses
Converging and Diverging Lenses
Thin Lenses
Converging and Diverging Lenses
Principle Rays
Thin Lenses
Multiple Lens Systems
How do you locate the final image?
Where is the final image?
A- An object is placed 20 cm in front of a converging lens of focal length 10 cm. Where is the image? Is it upright or inverted? Real or virtual? What is the magnification of the image?
• Solution:P=20 cm, f=10 cm
• the same size , real image
• An object is placed 5 cm in front of a converging lens of focal length 10 cm. Where is the image? Is it upright or inverted? Real or virtual? What is the magnification of the image?
• Solution:P=5cm, f=10cm
q=-10 cm
Virtual image, as viewed from the right, the light appears to be coming from the (virtual) image, and not the object.
Magnification = +2
• An object is placed 8 cm in front of a diverging lens of focal length 4 cm. Where is the image? Is it upright or inverted? Real or virtual? What is the magnification of the image?
• Solution:P=8 cm, f=-4cm(concave)
The image is upright, virtual, smaller
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