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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.

Peter Mortensen
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meteor
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3 Answers3

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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.

BioPhysicist
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  • I had practical class in which I could not see the real image until I put a screen there – meteor Dec 30 '18 at 13:18
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    @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. – BioPhysicist Dec 30 '18 at 13:20
  • 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 – meteor Dec 30 '18 at 13:22
  • @Monkyman I have added more about the plane mirror – BioPhysicist Dec 30 '18 at 13:25
  • but I still dont get how you have to place your eyes at the point of image formation for a real image to see the image and for a virtual image you can just stand anywhere and still see the image – meteor Dec 30 '18 at 13:29
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    @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. – Farcher Dec 30 '18 at 14:14
  • @Monkeyman The issue isn't that a converging lens only has one location for an image. There is still an image at every point in space for a lens just like there is for a mirror. I think what your are wondering instead is how everything is in focus for a mirror and not a lens. And it's because the plane mirror relflects all light rays "in the same way", where a lens "bends" rays to focus them in a particular way. – BioPhysicist Dec 30 '18 at 17:11
  • @AaronStevens If you had put your eye where the screen was you would have seen the real image. Surely not. You hadn't seen anything but a diffused glow. And in case of a Sun's image produced by a lens, your retina had been at severe risk. To see a real image with eyes, your eyes must be placed farther than image from lens, at least some 25 cm for a young eye. In any case, a thing never to be done with Sun. The link Farcher gave is an excellent explanation. – Elio Fabri Dec 31 '18 at 14:09
  • @ElioFabri Yeah I was neglecting the fact that your eye is also a lens as to not confused the main issue. I'm also not advocating looking at the sun with or without lenses. – BioPhysicist Dec 31 '18 at 14:34
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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.

Dr S T Lakshmikumar
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    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. – Eric Duminil Dec 30 '18 at 19:07
  • 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. – 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.

user591849
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