Spherical Mirrors – Learning Outcomes Recognise and use key words relating to mirrors.

Centre of curvature

Focus / focal point, focal length

Pole

Principal axis

Use ray tracing to demonstrate reflection.

Find images in spherical mirrors using ray tracing.

Describe the images formed in spherical mirrors.

Differentiate between real and virtual images.

Spherical Mirrors – Learning Outcomes Use formulas to solve problems about spherical mirrors:

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Give uses of concave and convex mirrors.

Measure the focal length of a concave mirror.

Spherical Mirrors

Spherical Mirrors – Ray Tracing A ray striking the pole is reflected at an equal angle with

the principal axis.

Spherical Mirrors – Ray Tracing A ray passing through the centre of curvature will be

reflected back through the centre of curvature.

Spherical Mirrors – Ray Tracing A ray incident parallel to the principal axis will reflect

back through the focus

Spherical Mirrors – Ray Tracing A ray passing through the focus will reflect parallel to the

principal axis.

Spherical Mirrors - Images Unlike in a plane mirror, spherical mirrors can form both

real and virtual images.

Real images are formed by the actual intersection of rays

They can be formed on a screen or found by using no parallax

Virtual images are formed by the apparent intersection of rays.

They cannot be formed on a screen, but may be found using no parallax.

Spherical Mirrors - Images To focus an image of a distant object.

1. Use a bright distant object (e.g. a window in a dark room).

2. Face a concave mirror towards the object.

3. Hold a piece of paper or cardboard in front of the mirror, and move it back and forth to focus the image.

4. If the object was very far away, the image will form at the focus of the mirror.

Concave Mirrors - Images1. An object outside the centre of curvature.

Image is:

real

inverted

diminished

between C and f

Concave Mirrors - Images2. An object at the centre of curvature.

Image is:

real

inverted

same size

at C

Concave Mirrors - Images3. An object between the centre of curvature and the

focus. Image is:

real

inverted

magnified

outside C

Concave Mirrors - Images4. An object at the focus.

Image is:

nonexistant

at infinity

Concave Mirrors - Images5. An object inside the focus.

Image is:

virtual

upright

magnified

behind mirror

Convex Mirrors – Ray Tracing1. A ray which strikes the pole is reflected at an equal

angle to the principal axis.

Convex Mirrors – Ray Tracing2. A ray heading for the centre of curvature will be

reflected back along its path.

Convex Mirrors – Ray Tracing3. A ray incident parallel to the principal axis is reflected

back as if it came from the focus.

Convex Mirrors – Ray Tracing4. A ray travelling towards the focus is reflected parallel to

the principal axis.

Convex Mirrors - Images An object anywhere in front of a convex mirror will yield

the same result – image is virtual, diminished, upright, and behind the mirror.

Formula for Spherical Mirrors

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Note that v is positive for real images, negative for virtual images. (RIP – real is positive)

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m = magnification, u = object distance/height, v = image distance/height

Calculations e.g. Chad holds a concave mirror 30cm in front of a

bulb. How far from the mirror does the image form if the focal length of the mirror is:

20cm?

40cm?

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Calculations e.g. An object is placed 30cm in front of a concave

mirror. A real image of the object is formed 50cm from the mirror.

What is the focal length of the mirror?

If the object is 5cm high, what is the height of the image?

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Calculations e.g. An image is formed in a concave mirror of focal

length 20cm. The image is three times the size of the object. Where must the object be placed if:

the image is real?

the image is virtual?

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Uses of Mirrors Concave – magnify when object inside C

Dentist mirrors

Cosmetic mirrors

Searchlights / floodlights / car headlights

Convex – wide field of view

Door mirror in a car

At concealed entrances

At ATMs and in banks