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We did a physics practical in which I couldn't see the image formed by the convex lens without the screen. But I can see the virtual image formed by the plane mirror without a screen.

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    $\begingroup$ Duplicate You do not need a screen to see a real image. $\endgroup$ – Farcher Dec 30 '18 at 14:05
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You can see real images without a screen. Just look through a converging lens. If what you see is upside-down, then you are seeing a real image.

You are only able to use a screen to show where a real image is because light rays physically converge to a point in space.

When you use a screen, the screen "sees" the real image just as if your eye was where the screen was$^*$. Then we see what is reflected from the screen.

For the plane mirror you are just seeing a reflection, just as if the "mirror image" of your world was placed behind where the mirror if located without the mirror actually being there. Seeing things in a mirror is essentially just like seeing things normally.


$^*$ This is neglecting the fact that your eye is also a lens, so in reality you don't want your eye to be exactly at the image.

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  • $\begingroup$ I had practical class in which I could not see the real image until I put a screen there $\endgroup$ – Monkyman Dec 30 '18 at 13:18
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    $\begingroup$ @Monkyman If you had put your eye where the screen was you would have seen the real image. Screens are used to reflect that real image to your eyes. $\endgroup$ – BioPhysicist Dec 30 '18 at 13:20
  • $\begingroup$ but then for a virtual image on a plane mirror you need not put your eyes anywhere but you can still comfortably see the image . why is that so ? thanks for helping out $\endgroup$ – Monkyman Dec 30 '18 at 13:22
  • $\begingroup$ @Monkyman I have added more about the plane mirror $\endgroup$ – BioPhysicist Dec 30 '18 at 13:25
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    $\begingroup$ @AaronStevens With practice or the help of a translucent screen you can see a real image in mid air. The difficulty in seeing the image is that you have to focus on a region without any material reference points. Have a look at the link that I posted in my comment above. $\endgroup$ – Farcher Dec 30 '18 at 14:14
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This might help. Try focussing the sun with a convex mirror on your hand. You can not merely see it. You can feel it. That is a real image. You cannot catch a virtual image on a screen. You can catch a real image on a screen. (Just as you could do it on your hand.) It is a more convenient way to see a real image. I my school physics lab, we would use pointed needles to determine the distances. We have to adjust the position of the image pin to avoid parallax. When the room was a bit dusty we could actually see the real inverted image of the object pin from the side.

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    $\begingroup$ Your first experience is poorly described and could be dangerous depending on how it is understood. Please clarify, prove that it's safe or remove it. $\endgroup$ – Eric Duminil Dec 30 '18 at 19:07
  • $\begingroup$ I fail to see what is so dangerous about using a convex lense to focus the sun into the skin. You would feel the heat and move the lense fast enough. $\endgroup$ – Dr S T Lakshmikumar Dec 31 '18 at 13:05
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The mirror directs the reflected rays towards you so you see.

If you direct the rays from a lens towards you you will see the image.

So to see the image formed by an optical system the rays need to reach your eyes in some way.

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