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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