Description of Interactive  

 
Locating an Image Using a Concave Mirror

There are three rules for drawing light rays using concave mirrors:

1. A light ray directed parallel to the principal axis will reflect off the mirror through the focal point on the same side of the mirror.
2. A light ray directed through the focal point will reflect off the mirror parallel to the principal axis.
3. A light ray directed towards the centre of the mirror (the vertex) will reflect at the same angle made with the principal axis.

NOTE:

The third rule simply follows the Law of Reflection: the angle of reflection of the reflected ray is equal to the angle of incidence for the incident ray.

Using these three rules, you will be able to find the image of the arrow that is focused in the mirror.

Question 1: Object starts at C.
Question 2: Object starts between C and F.
Question 3: Object starts at F.

Use the LOST acronym to characterize the images while answering the questions.

1. Object starts at C.

Questions

1. Where is the image located?
   Answer

   The image is located at C

2. Is the image upright or inverted (upside down)?
   Answer

   Compared to the object, the image is inverted

3. What is the size of the image compared to the object?
   Answer

   The image is the same size as the object.

4. Is the image real or virtual?
   Answer

   The image is real because the light rays intersect on the same side of the mirror.

2. Object starts between C and F.

Questions

1. Where is the image located?
   Answer

   The image is located beyond C.

2. Is the image upright or inverted (upside down)?
   Answer

   Compared to the object, the image is inverted

3. What is the size of the image compared to the object?
   Answer

   The image is larger than the object.

4. Is the image real or virtual?
   Answer

   The image is real because the light rays intersect on the same side of the mirror.

3. Object starts at F.

Questions

1. Where is the image located?
   Answer

   No image exists because the light rays never intersect.

2. Is the image upright or inverted (upside down)?
   Answer

   No image exists because the light rays never intersect.

3. What is the size of the image compared to the object?
   Answer

   No image exists because the light rays never intersect.

4. Is the image real or virtual?
   Answer

   No image exists because the light rays never intersect

Technically, only two light rays need to be drawn in order to see where they intersect. A third light ray is used to make sure that all three intersect at the same point. If they do not, a mistake has been made. 
This means that in order to predict the location of an image, you need to follow only two of the three rules.

NOTE:
A special case exists if the object is between the focal point F and the mirror.

Using Rule #1 and Rule #3, the light rays seem to be spreading apart, not coming together, so they will never intersect on the left side of the mirror.

In this case, we extend the reflected rays backwards with dotted lines onto the right side (behind the mirror), from where it appears they could be coming if they originated from a point in the top right corner of the screen.Here, these virtual rays meet up behind the mirror, producing a virtual image that is much larger and above the principal axis.

Try it out for yourself!

4. Object starts between F and the mirror.

Questions

1. Where is the image located?
   Answer

   The image is located behind on the right side of the mirror.

2. Is the image upright or inverted (upside down)?
   Answer

   Compared to the object, the image is upright

3. What is the size of the image compared to the object?
   Answer

   The image is larger compared to the object.

4. Is the image real or virtual?
   Answer

   The image is virtual. The light rays do not intersect on the left side of the mirror, but they do behind on the right side of the mirror if you extend them virtually.