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It's a movie trope that snipers are spotted because someone catches a glimpse of a reflection of the scope. It seems to me that this is a serious design flaw.

I also know that a good number of scopes have "lids" to protect against dirt, etc., when not in use.

My thought is this: Why is the front-most (nearest the target) lens/glass so close to the end of the pipe (as in ╞═════╪╡)? Why isn't that piece of glass hidden way inside the pipe (as in ╞═════╪════╡)? That extra bit of pipe could then block out any light/reflections apart from what is in the view of the shooter. It would also make the scope longer, but that can't be that much of a downside, can it?

If there are such scopes, how common are they? Or, put the other way around, given that reflections are undesired, how is this actually dealt with outside of the movies?

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This is not just something that happens in the movies. Look up some history on the famous sniper Carlos Hathcock and his encounter with the enemy sniper "Cobra". Each knew the other was in the area. Hathcock saw a flash of light and fired at it. This turned out to be a reflection from Cobra's scope. The bullet went right through the scope into Cobra's eye.

To answer your question, I don't know for real, but certainly there is a tradeoff between length and clunkiness and the possibility of greater reflection from the front element of the lens. Modern lens coating should be able to mitigate reflections a good amount without requiring a extra long barrel. Keep in mind that the barrel would have to be flared to match the outer angle of the field of view through the scope. Something that large has its own drawbacks in a sniping situation.

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The lenses which make up the scope will always reflect some light (~0.5%).

All optical materials will reflect some amount of light. This is due to the fact that light travels slower inside of the medium than it does in the surrounding air. The slowing down of a wave always results in some reflection; in optics it is governed by the Fresnel equations, but there are analogous reflections in electronic circuits and waves traveling along a string.

Partial Transmission

There are techniques which use destructive interference between multiple reflections at successive boundaries to give optical devices very, very low reflectivities at a specific wavelength. However, for a rifle scope it is important that the reflectivity be low for the entire visible range. Browsing the coatings offered by Edmunds shows that a good coating can reduce the reflectivity down to about 0.5% across the entire visible range.

The lens tube cannot simply be extended without degrading the image quality.

There is a lot of information on hoods on photography.SE (here and here for instance), but I'll briefly summarize. The length of the hood is limited by the field of view. Essentially, light from every point in the scene needs to be able to reach every part of the front lens. I.E. it is not enough for the light to reach only the center portion of the lens.

For this reason, simply extending the tube beyond the front of the objective lens is not an option because light from the edge of the scene would not reach the edge of the lens. The hood must have a radius which is larger than the objective lens itself before it can be extended; otherwise you are throwing away light from part of the scene. Pay attention next time you see a professional photographer (or enthused amateur) with a lens hood and you will immediately notice this. The most effective hood is actually a cone with an angle set by the field of view which will start to make the scope very large and bulky.

Lens Hood from Flikr

The reflection is less noticeable than you might imagine.

Since the objective lens of the scope is a focusing lens, the outer surface is curved like a diverging mirror. This means that the light redirected toward the enemy sniper is significantly less intense than it would be for a flat mirror. For a flat mirror the entire surface of the mirror redirects light from the sun to the enemy's eye, while a curved mirror has a much smaller area which gives the correct angle between the enemy's eye and the sun.

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It is possible to buy anti reflection devices for your rifle scope, such as the one shown below (found here).

Anti-reflective device

Why they don't come with this to begin with I can only guess, but you will get less light in the scope when using devices like this, so that may be a reason.

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    $\begingroup$ Hi initram, welcome to Engineering SE. It's always a good idea when linking to a product as a new user to add some context about your relationship to the product. For example, how you found it, or if you have any relationship with the manufacturer, and so on. We do allow linking to products but at the same time we are on the lookout for spam and unsolicited advertisements. You can find some more information in this Meta discussion. $\endgroup$
    – Air
    Mar 23 '15 at 16:05
  • $\begingroup$ Ok, I was not aware of this. I have no affiliation with this product it was merely the one of the first results when searching for "anti reflection device" on google. $\endgroup$
    – initram
    Mar 25 '15 at 8:16
  • $\begingroup$ Interesting. Photographers attach these grids to their strobes to limit the spill of light. When used on a rifle scope I'd think that off-axis light into the grid would kill contrast due to flare. I've seen pictures of snipers using something like camouflage panty hose material on the front of their scopes, but perhaps this is more to disguise the circular shape of the scope objective than cut down on reflections. $\endgroup$
    – BobT
    Jun 23 '17 at 1:06
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Lenses always reflect part of the light, so lense optics do too. The reflection is strongest for light shining into the telescope along the line of sight. As a rule of the thumb, if a lightsource can be seen through the telescope, someone standing at the lightsource has a chance to see the reflection of this lightsource. This effect is common to all lense optices and is exploited in products to find hidden cameras.

I'd still need to re-research the reasons lenses always in this specific way, it's been a decade since the relevant physics class.

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  • $\begingroup$ Lenses reflect due to the discontinuity in the index of refraction of the air and the lens material. $\endgroup$ Mar 23 '15 at 14:55

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