# Why don't opaque objects reflect light?

My sister was doing a quiz and I tried to point her in the right direction by giving her scenarios to imagine. One of the questions in the quiz was:

Which of the following objects do not reflect light:

1. Polished metal

2. Mirror

3. Undisturbed water

4. Book

She suggested that the answer was "undisturbed water" and that made sense to me too.

But the answer given was "book", which didn't make sense to me. How can you even see the book if it didn't reflect light in the first place?

Is this terrible framing by her teacher or am I having a conceptual misunderstanding?

• Oh, my! that quiz question is terrible! What if the book is bright glowing blue? :) Dec 17, 2020 at 15:39
• The book is most definitely wrong wrong wrong. Everything reflects a portion of incoming light (save a perfectly created 1/4-wave coating and infinitely narrowband wavelength) Dec 17, 2020 at 17:30
• The real lesson that one takes out of school is that there's plenty of incompetent teachers out there (and that you may sadly have to suck it up and play along, and that life is unfair, and the list goes on...) Dec 17, 2020 at 21:03
• None of the choices is right. You never seen your reflection in a pond?
– nasu
Dec 17, 2020 at 22:22
• @Bill Alsept Are you serious or just kidding? Undisturbed here means flat, without waves. Like water in a pond in a day withour wind. According to your meaning it should say that the light is reflected by disturbed mirrors?
– nasu
Dec 18, 2020 at 3:19

The question is asking "which of the following objects will you not see a reflection?".

A distinction (albeit poorly) is being made between specular reflection and diffuse reflection.

The objects in options 1-3 will exhibit specular reflection, while option 4 "a book" will exhibit diffuse reflection. So the correct option will be "4 Book" since this object will not exhibit specular reflection, whereas "1. Polished metal, 2. Mirror" and "3. Undisturbed water" all exhibit specular reflection.

You are correct and the question should probably have been worded similar to this:

"Which of the following objects would exhibit diffuse reflection, as oppose to specular reflection?"

Now with the understanding that the question posed by the teacher was probably at an elementary school level, it should be noted that a more technical answer (and more accurate answer) should explain these two forms of reflection in detail, so see more in the links below. But to briefly summarize:

Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered at many angles rather than at just one angle as in the case of specular reflection.

Specular reflection is described as:

Specular reflection, or regular reflection, is the mirror-like reflection of waves, such as light, from a surface.

• I don't think the correct answer to your last question would be "Book". Undisturbed water will reflect significantly less light than the pages of a book.
– noah
Dec 17, 2020 at 14:10
• Even then, "see[ing] your reflection" depends more on surface smoothness than on reflectivity Dec 17, 2020 at 17:31
• @noah Depends on the angle. Dec 17, 2020 at 21:54
• @Philip I disagree, although I freely admit it’s a question of semantics. When I say “reflection”, I’m talking about specular reflection (of which a book does very little). When I’m talking about the light seen from a book, I’d call it “scattered” light. I suspect the teacher uses similar nomenclature to conceptually distinguish scattered from specularly reflected light, and that’s the context within which this question makes sense. Dec 18, 2020 at 1:40
• This answer is wrong, and I’m shocked that it’s been upvoted so many times. “which objects reflect back most of the incident light, as oppose to objects that absorb most of the incident light” / “objects which reflect significantly more light than they absorb” These are orthogonal to whether you can see your own reflection in it (i.e. specular reflection). An object could (diffusely) reflect back a lot of light without showing your reflection, and conversely it could reflect back less light overall but show your reflection (a dark mirror). Dec 18, 2020 at 19:59

In my opinion, one more thing which is possible here is that may be the question is

Which among the following causes diffuse reflection ?

Undisturbed water can also give specular reflections but the surface of a book have many microscopic as well as macroscopic irregularities which causes diffusion of the reflected rays and thus you can't see your face on your book's surface like you do in mirrors.

Hope it helps 🙂.

• Did you mean to use a block quote rather than a spoiler tag? Dec 17, 2020 at 13:26
• Indeed, if "book" is the intended answer, then the proper question is what you have stated. Dec 17, 2020 at 15:35

There are two main types of reflection:

1. specular (polished metal, mirror, undisturbed water)

In the case of specular reflection, the surface is on a micro level so smooth, that it is able to reflect (elastically scatter) all photons at the same as the incident angle.

On the image you can see your example of undisturbed water.

Specular reflection reflects all light which arrives from a given direction at the same angle, whereas diffuse reflection reflects light in a broad range of directions.

https://en.wikipedia.org/wiki/Specular_reflection

1. Diffuse (book)

Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered at many angles rather than at just one angle as in the case of specular reflection.

https://en.wikipedia.org/wiki/Diffuse_reflection

In the case of diffuse reflection, in your example the book, the surface might be non-absorbing (plaster, marble) or absorbing (like a normal book cover).

In your case, the book has a surface that on the micro level is diffusely reflecting most of the incident photons, but it does absorb (and re-emit) a considerable portion of the incident photons too.

So the reason the book is the answer is that that is the only one that besides reflecting (diffusely) most photons, it does absorb and re-emit a considerable portion of photons too.

• Pictures sometimes explain more then words :-) Dec 18, 2020 at 19:26
• This should be the top answer. The current top answer is incorrect. Dec 18, 2020 at 20:12

Yeah, clearly this teacher regards the following two as synonymous: 'mirror reflection' and 'reflection'.

Presumably this teacher is unaware of the possibility/existence of diffuse reflection.

This means that in order to answer quiz questions by this teacher you need to try and reconstruct the thought process of the teacher, who may be dropping the ball left and right.

More generally: the choice of options illustrates how this teacher was struggling. The first three are general concepts:

1. polished metal
2. mirror
3. undisturbed water

The fourth one is oddly specific:

1. book

This teacher had the following problem: is there something that is guaranteed to have a dull surface?

This teacher took the easy way out, and took 'book'; a book is almost never glossy.

It would be interesting to try and think of something that is as general as the first three options, but has properties such that mirror reflection is inherently impossible.

(Of course, 'matte finished metal' is intentionally matte, hence guaranteed not to have mirror reflection, but then the property that the student is supposed to figure out is stated explicitly, defeating the purpose of the question.)

• sand would be a "general" example of something that won't do specular reflection, along the same lines as undisturbed water. Or plants. (Polished / varnished wood is arguably not still a plant.) Dec 18, 2020 at 2:46
• @PeterCordes is a very shiny piece of fruit still a plant? 😉
– Tim
Dec 18, 2020 at 16:33
• @Tim: Are there any fruits that a truly naturally shiny, even after being washed by rain? Our experience with shiny fruit usually involves them having been waxed after picking. theatlantic.com/technology/archive/2017/04/… says That grocery-store gloss is artificial fruit wax (the whole article is about the history of that practice.) But yeah, I was more sure about sand than plants; easy to imagine that there might be a few somewhat shiny plants I wasn't thinking of at the time. Dec 18, 2020 at 16:46
• @PeterCordes Pollia Condesta berries are quite reflective, but like most organic reflectivity it’s iridescent like a scarab beetle, rather than mirror like
– Tim
Dec 18, 2020 at 16:51
• @PeterCordes I'm leaning towards the manmade material felt. It's not quite a solid, but it's solid enough. (That is, unlike a pile of grains of sand it's a single object.) The surface is inherently coarse. Not that it can repair that question; that question is unsalvagable. Still, we will agree the underlying idea is good: to get the student to think about how the properties of a surface affect how light interacts with it. Dec 18, 2020 at 17:21

This is a case where there's a word that technically refers to a general category, but is often used to refer to a particular subset of that category. The term "reflection" can refer to any redirection of light, but is often understood to refer to specular reflection. If the teacher had asked "Which of the following objects do not specularly reflect light?" it would have been more precise, but then there would be students wondering what "specularly" means.

It is known that if we can see anything in world, it is because that object reflects the light incident on it.

And I can see all objects in the list.

So, I think the answer is none of them.

• So, You're saying, if somebody set a perfectly black object on a table in front of you, you would not be able to see it? (I agree, that you wouldn't be able to see the texture of its surface, but not see it at all???) Dec 18, 2020 at 15:29
• One rather needs to define "see" better before being able to answer that. Give me a definition of "see" and I well tell you whether it applies to a black body. Our sense of shape and space is created in part by the way that re-reflections between objects create interactions between them. A truly black body might somehow create a weird absence-ness in the way that we perceive bulk and shape though shadow, specular highights and difuse colour-bleeds. I can't prove it but suspect we would "see" a truly black object in a rather unsettling way. Dec 18, 2020 at 17:25
• You can see things that emit light too, even if they perfectly absorb all light that hits them. Dec 19, 2020 at 16:25

All surfaces reflect light except for light sources. However, not all surfaces have specular reflection like a mirror as they reflect light well. Now, let’s take a closer look at your question.

1.polished metal

2.mirror

3.undisturbed water

4.book

All of the give reflect light. However, options 1, 2 and 3 have specular reflection leaving 4 as the only one that has diffuse reflection. This question should have been phrased wrongly. The teacher might have thought of “Which of the following objects gives off diffuse reflection”.