Ray Model

A useful model under certain circumstances to explain image formation.

Ray Model: Light travels in straight-line paths, called rays, in ALL directions from the source.

Concave Mirror

Focal point

Rays parallel to the axis of the mirror pass through the focal point of the mirror.

Object distance > f

Object distance < f

Sketching ray diagrams1. Select a single source point.

2. Sketch a ray parallel to the axis of the mirror that reflects through the focal point.

3. Sketch a ray that reflects off the center of the mirror at the mirror’s axis at an angle equal to the angle of incidence.

4. Sketch a ray that is incident normal to the mirror and reflects back through itself.

5. Sketch a ray that passes through the focal point and reflects parallel to the axis of the mirror.

6. At the intersection of these rays is the image point.

Relate object distance, image distance, and focal length

sign convention for mirrors

Example

An object is 10 cm from a concave mirror of focal length 20 cm. (a) What is the image distance? (b) What is the magnification? (c) Is the image real or virtual? (d) Is it inverted or upright?

.C

V

12

3

A ketchup bottle is positioned in front of a concave mirror. Three of the incident rays are shown. a. _________ get(s) reflected through point C.A. Ray 1

B. Ray 2C. Ray 3D. All three raysE. None of the rays

.C

V

12

3

A ketchup bottle is positioned in front of a concave mirror. Three of the incident rays are shown. b. _________ get(s) reflected horizontally to the left.

A. Ray 1B. Ray 2C. Ray 3D. All three raysE. None of the rays

.C

V

12

3

A ketchup bottle is positioned in front of a concave mirror. Three of the incident rays are shown. c. _________ make(s) the same angle with the normal as its reflected ray does.

A. Ray 1B. Ray 2C. Ray 3D. All three raysE. None of the rays

.C

V

12

3

A ketchup bottle is positioned in front of a concave mirror. Three of the incident rays are shown. d. _________ make(s) the same angle with the horizontal as its reflected ray does.A. Only ray 1

B. Only ray 2C. Only ray 3D. More than one of the raysE. None of the rays

.C

V

A concave mirror forms an image of an object as shown.

object

image

Show in Slide Show view to have question appear in stages.

front viewof card

edge view of card

?

Suppose the top half of the mirror is blocked by an opaque card.

A. no imageB. an image of the entire objectC. an image of just the top half of the objectD. an image of just the bottom half of the object

With the card in place, we see__________. Note that rays reflect off of all parts of

the mirror and intersect at the image.

C

V

The concave mirror has a radius of curvature R. At which of the numbered points could you place an object perpendicular to the central axis (like this: )and see a. a reduced real image?

R

R/2

4 123

A. 1 B. 2 C. 3 D. 4 E. none of these

C

V

The concave mirror has a radius of curvature R. At which of the numbered points could you place an object perpendicular to the central axis (like this: )and see b. an enlarged virtual image?

R

R/2

4 123

A. 1 B. 2 C. 3 D. 4 E. none of these

C

V

The concave mirror has a radius of curvature R. At which of the numbered points could you place an object perpendicular to the central axis (like this: )and see c. a reduced virtual image?

R

R/2

4 123

A. 1 B. 2 C. 3 D. 4 E. none of these

C

V

The concave mirror has a radius of curvature R. At which of the numbered points could you place an object perpendicular to the central axis (like this: )and see d. an enlarged real image?

R

R/2

4 123

A. 1 B. 2 C. 3 D. 4 E. none of these

C

V

The concave mirror has a radius of curvature R. At which of the numbered points could you place an object perpendicular to the central axis (like this: )and see e. no image?

R

R/2

4 123

A. 1 B. 2 C. 3 D. 4 E. none of these

Note: the image distance is infinity since the object distance is equal to the focal length.

Convex Mirror

so>f; image is virtual and upright

Convex Mirror

so< f ; image is virtual and upright

Poll

Is the side mirror on a car a concave or convex mirror?

1. concave

2. convex

3. neither, because it is a plane mirror

Activity

Use the Optics Workbench to create simulations of the of the following situations. Compare your own calculation to the result from the simulation.

1. Concave mirror; f = 1 m; so = 3 m; h=0.5 m;What is the image distance and image height?

2. Concave mirror; f = 1 m; so = 0.5 m; h=0.5 m;What is the image distance and image height?

3. Convex mirror: f = -1 m; so=0.5 m; h=0.5 m;What is the image distance and image height?