Reflection from Curved Mirrors

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Curved mirrors• The centre of the mirror is called the pole.• A line at right angles to this is called the

principal axis. • The focal length of a mirror is half the

radius of curvature.• The radius of curvature is the radius of the

ball that the mirror would have been cut from.

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Curved Mirrors• C = centre of curvature• r = radius of curvature • F = Focal point or focus f = focal length• pa = principal axis P = pole

C F P

r

pa

f

f = r/2

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Concave Mirrors

• Concave (or converging) mirrors focus light at the focal point.

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Convex Mirrors • Convex mirrors have a focal point behind

the mirror.• Convex (or diverging) mirrors spread the

light rays apart so that they appear to have come from the focal point

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Ray Diagrams• Used to find the size, nature and

position of images.• The nature of an image formed by a

mirror or lens can be described according to 3 characteristics: Is it

• a) upright or inverted• b) magnified, diminished or the same

size• c) Real or virtual

Concave Mirrors

1. Look at your reflection in a concave mirror. The image is virtual and ……………

2. Move the mirror away from you. Why do you think the image has disappeared?

3. What happens to the image as the mirror is moved further away from the object (demo)

4. Think of a use for a concave mirror.

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Ray Diagrams• Rule One: An incident ray parallel to the

pa is reflected back through the focal point.

Remember that pa = principal axis

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Ray Diagrams• Rule Two: An incident ray that passes

through the focal point on the way to the mirror is reflected back parallel to the pa.

Drawing diagrams to solve problems

• An object of height 5cm is placed 10 cm away from a concave mirror of focal length 6cm. Draw a ray diagram to show the location of the image. Describe the image and calculate its magnification using the formula:

Magnification = height of image height of object

What about drawing virtual images

Page 64 in your text book. Look at example G

The image is virtual because the rays of light do not meet. They seem to be coming from a point which is behind the mirror (just like a plane mirror) but because they are diverging, they give the illusion that the object is larger.

Formula for Spherical Mirrors

• Descartes’ Formula:

• Or:• m=magnification factor• h=height of image or object• d=distance from mirror to image or

object• Distances behind the mirror are

negative

oi ddf

111

o

i

o

i

d

d

h

hm

• An object of height 12cm is placed in front of a concave mirror of focal length 10cm. Complete the table

Focal length (f)

Position of object (do)

Position of image (di)

Height of image (hi)

Magnification

10cm 2cm

10cm 5cm

10cm 10cm

10cm 15cm

Complete activity 5B (pg 67)

• Complete questions 1-4

Summarise images produced by concave mirrors

Convex Mirrors

• Images are virtual (the rays do not meet after reflection), diminished and upright.

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Ray Diagrams• The same 2 rules for drawing light rays can

be applied to convex mirrors with a few small changes

F

•All convex mirror images are virtual, diminished and upright.

Name a use for convex mirrors

• Car mirrors• Mirrors positions on sharp corners