Plane mirror form images of the same size as of the object.

Also if we need to see ourselves completely in mirror, we would require a mirror of at least half out height.

Assume I am 6 feet tall then if I use a mirror 3 feet tall then how come me and my image have that same size, should not my Image by 3 feet tall and if yes then why we say that the Plane mirror form images of the same size as of the object?

  • $\begingroup$ If you stand straight. And put down a 3 meter mirror in front of you(the mirror is parallel to your view). Can you see your head? Someone else can see but they have to bend themselves. (That's simple way of understanding) $\endgroup$ Commented Feb 27, 2022 at 13:06
  • $\begingroup$ @BillyIstiak Do you mean that we can see ourselves fully if we put a mirror of half our height parallel to us is false? $\endgroup$
    – Mohd Saad
    Commented Feb 27, 2022 at 13:08
  • 1
    $\begingroup$ It's an image that appears to be the same size as the object but twice as far away as the mirror is. $\endgroup$ Commented Feb 27, 2022 at 13:32
  • $\begingroup$ @GrapefruitIsAwesome Why the distance would affect the height? $\endgroup$
    – Mohd Saad
    Commented Feb 27, 2022 at 13:37
  • $\begingroup$ @MohdSaad our mirror-image will appear same as your height. But if you take 3 meter mirror, then you can see 3 meter of your body part...... But you can see full of your image by looking at bottom from upside(assumed mirror is directly infront of your head). $\endgroup$ Commented Feb 27, 2022 at 14:47

3 Answers 3


The optical ray diagram of a plane mirror may help

plane mirror image

Also here:


Let’s say you have a toy car, and it’s sitting in front of a regular bathroom mirror. The distance between the car and mirror is called the object distance, and it’s always positive. If you look at the image of the toy car in the mirror, it will appear to be the same distance behind the mirror as the real car is in front of the mirror, at the same height. It will also appear to be the same size as the real car. The image of the car looks like it’s behind the mirror (and the light we see does not directly emerge from the image), we say that the image is upright and virtual, and that the image distance is negative.

Because of the geometry of optical rays, plotting them, and measuring the sizes , plane mirror images have the same size as the original.

  • $\begingroup$ I find that diagrams always help in cases like this! And those ones are nice and clear. Nitpick: I was taught that distance was unsigned (i.e. always +ve, whichever way you measure it), but that displacement was signed (+ve in one direction, -ve in the other) — is that standard terminology? $\endgroup$
    – gidds
    Commented Feb 27, 2022 at 22:01
  • $\begingroup$ @gidds physicsclassroom.com/class/1DKin/Lesson-1/… $\endgroup$
    – anna v
    Commented Feb 28, 2022 at 4:57

Given great enough distance, I see a tiny mirror eventually acting like a pin-hole camera.

While in elementary school, I had a broken piece of mirror (maybe 3-in by 1-in) and it reflected the light from the sun in the shape of the broken piece onto a nearby wall.

However, the further I projected the reflection the more circular it became and the larger the image (so even in this distance it was acting as a pin-hole reflection)! (This also works for a solar eclipse, cast the sun's reflection on a house (which is of course shaded) maybe 200-ft away, go and examine the image (inverted as through polar inversion), and you'll see a large and good resolution image of the eclipse.) Therefore, if you reflect the image of the sun back to the sun, even with the broken piece, the light arriving at where the sun was before it had moved (due to the Earth's rotation and the time it takes to arrive to us) by travelling the same distance would be the same size as the sun.


To the second part of the question, consider this ray diagram:


Clearly, you can't see the head and the feet simultaneously together. If you bend your head downwards, you'll see the feet, but not the head. If you keep your head parallel to the ground, you'll see the head but not the feet.

All of this suggests that the image is still the same size as the object. When we say that a mirror of half your height is required, it actually means that the mirror is capable to display all your body features given you can move your head freely.


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