Seeing Things Viewing Images in Plane Mirrors

When we look in the Mirror we see an Image of the Object.

In the previous slide show you saw that the Image appears to be the same distance behind the Mirror as the Object is in front of it. You also saw that a line connecting the Object and Image intersects the Mirror at 90 °

The Image appears to be behind the Mirror. But the light rays can not come through the Mirror. Mirrors are Opaque.

Question 2) How can the light rays appear to go through an opaque Mirror?

Get this graphic from your notes.

Place a MIRA or Reflect View on it as shown.

Notice you can see an Image in the “Mirror” If you look over the top, behind the Mirror there is no Image. The light rays must come from the Object and reflect off the Mirror. How do they do this?

On the graphic in your notes, the back of the Mirror is identified by diagonal lines. This is where light is reflected on glass mirrors

First, find the location of the Image by drawing a line from the top of the Object through the Mirror so that it intersects at 90°.

Measure the distance from the top of the Object to the Mirror (d O ).

Copy this distance (d Otop ) to the other side of the Mirror. It becomes (d Itop ).

Repeat this process for the bottom of the Object.

These two points give the location of the top and bottom of the Image. For any point d O = d I

To make the diagram less cluttered I’ve removed the unneeded part of the dotted line

Rays of light must be entering the eye as if they had come straight from the Image. That is the way the eye sees things.

The line is dotted behind the mirror because light rays can not through an opaque Mirror. They travel in this direction but could not come from the Images location.

The light rays that appear to come from the top of the Image really came from the top of the Object and reflect off the Mirror into the eye.

The light rays that appear to come from the bottom of the Image really came from the bottom of the Object and reflect off the Mirror into the eye.

The light rays reflect most strongly off the back of glass mirrors. A metallic coating applied to the back of the glass causes the reflection.

Reality Check Use a MIRA or Reflect View to see if the Predicted Image you drew and the real Image, overlap. If they do the light rays must reflect as predicted

Your Turn Answer the questions in your notes,

Your Turn Answer the questions in your notes,

Question Answers follow

Your Turn In your notes, 3)Show how light is reflected from the mirror so that the eye sees the Image behind it.

A) should look like. Try B)

B) should look like. Try C)

C) should look like. Try D)

D) should look like. Try the next questions

3)Which Eye/ Brains can see the Object?

C) and D) only. Light travels in a straight line from the object to the Eye/Brain. Rectilinear Propagation is the term for this

4)Which Eye/ Brains can see the Image in the Mirror?

A) C) and D) only. Light appears to travel in a straight line from the Image to the Eye/Brain. Rectilinear Propagation is the term for this

Full Height Mirror. 4)How big does a Mirror have to be in order to see yourself “Head to Toe”? (Hint You see your image)

As usual find the location of the Image, by measuring the perpendicular distance from the Object to the Mirror.

Copy this distance to the other side of the Mirror.

This gives the location of the Image.

Draw a ray of light, entering the eye as if they had come straight from the top of the Image.

This light ray really came from the top of the Object.

Draw a ray of light, entering the eye as if they had come straight from the bottom of the Image.

This light ray really came from the bottom of the Object and reflected off the Mirror.

The large orange triangle has a height which is the distance from the Object to the Image (d I +d O = 2d O ) Its base is the height of the person (two arrows)

The smaller light orange triangle has a height which is the distance from the Object to the Mirror (1 d O ) or half the height of the larger triangle. From similar triangles the size of the mirror is half the height of the person

8)Does how far away you stand affect the size of the mirror? You still want to see yourself “Head to Toe”.

As usual find the location of the Image, by measuring the perpendicular distance from the Object to the Mirror.

Copy this distance to the other side of the Mirror.

This gives the location of the Image.

Draw a ray of light, entering the eye as if they had come straight from the top of the Image.

This light ray really came from the top of the Object.

Draw a ray of light, entering the eye as if they had come straight from the bottom of the Image.

This light ray really came from the bottom of the Object and reflected off the Mirror.

The large orange triangle has a height which is the distance from the Object to the Image (d I +d O = 2d O ) Its base is the height of the person (two arrows)

As before the smaller light orange triangle has a height equal to the distance from the Object to the Mirror (1 d O ) or half the height of the larger triangle. From similar triangles the size of the mirror is half the height of the person

So where is the Image - Really Below are two eye charts, one is inverted so that it is easily read in a mirror. The other is normal. Each line is exactly half the size of the line above it

A Inverted Eye Chart is held. Its Reflection is viewed in a Mirror. 9)How difficult will it be to read the Image of an Eye Chart held by the student? Explain your Choice

a)Would it be the as difficult as reading the an Eye Chart held half the distance to the Mirror?

b)Would it be the as difficult as reading the an Eye Chart held at the distance of the Mirror?

c)Would it be the as difficult as reading the an Eye Chart held twice the distance to the Mirror?

Answer and Explanation follows Before looking at the theoretical (academic) solution do the group lab.

The Image of an Eye Chart held by a student would be as far behind the Mirror as the student is in front.

Therefore it would be as difficult as reading the an Eye Chart held twice the distance to the Mirror

10)If you can just read the Middle line of an Eye Chart taped onto a Mirror Which line of the same sized Eye Chart could you be able to read when viewed in a Mirror? Would it be, a) one Line up (2X’s Larger) b) the same line c) one line down (Smaller by half)

The size of the Image viewed in a Mirror is given by the angle between the rays coming from the top and bottom of the Eye Chart Image. This angle is the same as that of an Eye Chart One half as high at the Mirror. The lines would all be shrunken by one half. Therefore you would could only read the larger lines (one up or 2X’s Larger, on the Image)