Real or Virtual?

• Real imagesA real image is one which can be produced on a

screen and is formed by rays that converge.

• Imaginary imagesA virtual image cannot be formed on a screen and is

produced by rays which only seem to come from the image.

Lateral Inversion

• Image where left and right are swapped.

Upright Inverted

Reflection off a plain mirror

Reflection off a plain mirror

• Image is:– Virtual.– Upright.– The same size as object.– Laterally inverted.

• Apparent distance from mirror to image is the same as the distance from the object to the mirror.

Reflection off a convex mirror

F = virtual focal point

Reflection off a convex mirror

• Image is: – Virtual– Smaller than object– Upright

Reflection off a concave mirror

F = real focal point

Reflection off a concave mirror

• Type of image formed depends on the distance of the object from the mirror.

Reflection off a concave mirror

• If object is between mirror and F image is:– Virtual– Upright– Larger

Reflection off a concave mirror

• If object is further than F from the mirror image is:– Real– Inverted– Laterally inverted– As the distance is increased the image size

decreases.http://www.phys.ufl.edu/~phy3054/light/mirror/applets/cavemir/Welcome.html

Drawing a ray diagram for a concave mirror

F F

1

2

Draw a ray that travels parallel to the axis towards the mirror and reflects through the focal point in front of the mirror.

Draw a ray that travels through the focal point in front of the mirror and reflects parallel to the axis.

Example 1

Drawing a ray diagram for a concave mirror

F F

1

2Draw a ray that travels parallel to the axis towards the mirror and reflects through the focal point in front of the mirror.

Draw a ray that reflects off the centre of the mirror at the same angle it was incident.

Example 2

Drawing a ray diagram for a convex mirror

F F

1

2

Example 1

Draw a ray that travels parallel to the axis towards the mirror and reflects in a direction that makes it look like it originated at the focal point behind the mirror.

Draw a ray that travels towards the focal point behind the mirror and reflects parallel to the axis.

Drawing a ray diagram for a convex mirror

F F

1

2

Example 2

Draw a ray that travels parallel to the axis towards the mirror and reflects in a direction that makes it look like it originated at the focal point behind the mirror.

Draw a ray that travels towards the focal point behind the mirror and reflects parallel to the axis.

Light through a concave lens

Light through a concave lens

• Light rays diverge.• They appear to

originate from a focal point in front of the lens.

• The image is:– Virtual– Upright

Light through a convex lens

f = focal lengthF = focal point C = optical centre

Light through a convex lens

• Light rays converge to focal point behind lens.

• Type of image formed depends on the distance of the object from the lens.

• http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=48

Light through a convex lens

• If an object is further than 2F image is:– Real– Inverted– Smaller

Light through a convex lens

• If an object is at 2F image is:– Real– Inverted– Same size

Light through a convex lens

• If an object is between 2F and F image is:– Real– Inverted– Larger

Light through a convex lens

• If an object is between F and C image is:– Virtual– Erect– Larger

Drawing a ray diagram for a convex lens

• Draw:1. A ray straight through the centre of the lens.2. A ray that travels parallel to the axis towards the

lens and refracts through the focal point behind the lens.

3. A ray through the focal point in front of the lens that refracts parallel to the axis.

Drawing a ray diagram for a convex lens

F F

1

2

3

Example 1

Drawing a ray diagram for a convex lens

F F

1

2

Example 2

Drawing a ray diagram for a concave lens

• Draw:1. A ray straight through the centre of the lens.2. A ray that travels parallel to the axis which

refracts as if it came from the focal point in front of the lens.

3. A ray that travels towards the focal point behind the lens which refracts parallel to the axis.

Drawing a ray diagram for a concave lens

F F

1

2

3

Example 1

Drawing a ray diagram for a concave lens

F F1

2

3

Example 2