Questions related to the perception and measurement of light (primarily in the visible range), its mathematical description, the reproduction of colors by different means, color combinations, etc. Please use the tag [electromagnetic-radiation] if you want to refer to the general form of light.

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195 views

Can relativistic momentum (photons) be used as propulsion for 'free' after the initial generation?

In discussing this question about propelling a spacecraft with photons and their relativistic momentum, the author asked that I restate my comment as another question. If photons can really be used ...
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2answers
7k views

If nothing in the universe can travel faster than light, how come light can't escape a black hole? [duplicate]

Possible Duplicate: How does gravity escape a black hole? If nothing in the universe can travel faster than light, how come light can't escape a black hole? I mean, Einstein's relativity ...
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2answers
185 views

2 torches with the same power - More lumen?

I didn't find an answer for this. A friend and me argued about the brightness-impact of multiple bulbs/torches. I told him, that 2 torches with the same power (the same amount of lumen and ...
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1answer
178 views

The Movement of Light

I've asked several other questions like this, and I know it's a dumb question but I really can't understand. When light is emitted from a moving source, it still moves straight in that source's frame. ...
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2answers
2k views

What causes light to move through a vacuum

I was looking at rockets and stuff and thought about how they move through a vacuum using newtons 3rd law, and then I started thinking of any other ways you could move through a vacuum without using ...
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2answers
109 views

Could time dilation bend the path of a photon? Does time dilation have a refractive index?

The main question I am getting at is, does time dilation have a refractive index? What I mean is, if I were to shoot a laser past a black hole, would the laser's path "bend" strictly from time ...
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1answer
79 views

Determining the path of a light ray

Let's consider a ray in the plane $xy$. Let the refractive index be defined in any point of the plane with the function $n(x,y)$. In time $t=0$ the ray is located in coordinates $(x_0, y_0)$ and its ...
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3answers
85 views

Can we see a rainbow on moon or any other celestial body?

We see rainbows after rainfall on earth... According to my understanding a rainbow is formed as a result of scattering of light....Then is it possible that we can be able to see a rainbow on the ...
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2answers
1k views

Is there a substance that doesn't reflect OR absorb light from the visible light spectrum?

Is there a substance that doesn't reflect or absorb visible light but may reflect light from another spectrum? Is there a theoretical substance that would have these properties? EDIT: Sorry I wasn't ...
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2answers
1k views

What is the difference between light and visible light? [duplicate]

After watching a few videos on light and electromagnetic radiation, I am a little confused. The way things are explained, is that light is just the same as electromagnetic radiation I thought this ...
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1answer
1k views

Why does light travel at finite speed? [duplicate]

We have known for quite some time now that light travels at a finite speed of 3x10^8m/s (approx) through vacuum. But why is that? Why can't the speed be infinite? Or at least higher than the current ...
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2answers
231 views

Why photo cannot be captured while facing toward the Sun /

When the Camera is facing toward the Sun and a person is standing to and We're capturing his photo , all we get is a black shadow even when we capture with the flash light or anything. Why does it ...
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2answers
473 views

What is the color of Water?

What is the color of Water? I am not talking about large water bodies like Ocean or lake but a glass of water. If it has no color how can we see it?
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2answers
875 views

Colors in the secondary rainbow reverse of that in the primary rainbow

Why the colors of Secondary rainbow is reverse of that in the color in the Primary rainbow? What can be the possible reason among the following options Because it is formed by one internal ...
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2answers
795 views

Is a monochromatic, or non-monochromatic light visible at greater distance?

Just what the title states. Assuming identical conditions, excellent visibility - If a 1W monochromatic light source, and a 1W non-monochromatic light source were viewed at a location in deep ...
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3answers
277 views

Does the Lorentz transformation not apply to light?

Since you would know that light always travels at the constant velocity with respect to all frame of reference ....according to relativity whenever we are traveling at speed of light our time with ...
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2answers
3k views

What color a transparent object reflect?

We say an object's color is blue if the object is opaque, reflects blue color and absorbs waves of other color. What color a transparent object actually reflects?
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3answers
83 views

Path of light as it travels between two black holes

What would happen to light passing through a narrow space between the event horizons of two equal-mass black holes? Would it deviate or follow a straight path?
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1answer
65 views

Difference between speed of light and of bullet after passing through a barrier [duplicate]

I just read that, when a ray of light traveling in vacuum at $c$ strikes a glass slab, its speed decreases and then when it re-emerges it gets back to its original speed i.e $c$. If I draw a ...
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1answer
190 views

A yellow lemon is illuminated by green light. What color does the lemon appear?

I think it's blue, but I'm not sure. Is there a way to find out without simply testing it?
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2answers
168 views

Why does light travel as waves? [duplicate]

Why does light travel as waves instead of say just a straight line? What are the forces that make a light photon travel in a wavelike pattern?
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2answers
85 views

Black Holes and Light [duplicate]

It is said that photons have zero rest mass so how can gravitational force of a black hole affect light? And if photons do have some effective mass while traveling at speed of light then only can a ...
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1answer
72 views

How can monitors display a sharp image, if light travels in all direction?

The question title might be a misnomer, help me improve it. My question is, from my understanding, a light travels in all direction from it's origin point. Also I assume that in the example of a ...
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1answer
77 views

relation between photon number and energy

Suppose there are two light beams. One is red while the other is violet. The energy of both is the same. Which one of these beams has a larger number of photons, or is the number of photons relevant? ...
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2answers
82 views

Correlation between color of light visible to us and the intensity of transmitted light by the material

I wanted to know if there is any relation between the color of light observed by the eye and the intensity of light transmitted by a material. Let us assume that I have a table of intensities of light ...
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1answer
143 views

Why the crest of water absorb more light than the trough of water?

I have done my experiment using water ripple to study the behavior of waves. I know that the crest will absorb more light than the trough so that the image created on the white paper sheet appeared to ...
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1answer
140 views

What physical phenomena underly the shape of this graph of diurnal UV index?

When I dig up a graphs of UV index versus time-of-day, most graphs are characteristically shaped like one of the plots shown here: That graph is from this site. What physical phenomena underly ...
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4answers
83 views

Wall visible after light source removed

Let's say you have a lamp pointed at a wall, in a dark room with no other light source, if you turned off that lamp Would the wall still be visible for any amount of time?
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2answers
204 views

Do any other particles get excited(or absorb energy) by photons like electrons?

Electrons get excited to different energy levels when photons of specific frequencies fall on them.But, is there other particles which absorb the energy of the photons?
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2answers
1k views

Is there 100% reflective mirror…i just want one

I have heard there are none right now...but i saw something that said something about 100% reflection. Forgotten completely. sorry. I want it, i think it'd be very amazing to save sunlight in it from ...
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1answer
80 views

Some sort of light reflection

One morning I woke up to a text message and picked up my phone but I was not yet fully awake and my eyes had not really gotten its focus. When I stared at the phone I saw two phones. Which is ...
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2answers
611 views

If we are travelling with the speed of light, can we see what's behind us? [closed]

If we are travelling with the speed of light, can we see whats behind us(like if we are moving away from earth can we able see the earth)? And how we see the things that we are approaching with speed ...
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2answers
188 views

Can I see the light from 10 million km away?

If light is switched ON, only for a second, and the distance between the observer and the light source is 10 million kilometers, can I still see the light spark? For example, let's assume that the ...
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1answer
1k views

Huygens Principle and principal of rectilinear propagation of light

Suppose I have an wave source and light waves are radiating from it. If I have a point source, then after a time t, with a radius of ct I will have a circular wave front.By Huygens principle each ...
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3answers
2k views

Deriving the Poynting Theorem

I am trying to derive the Poynting theorem. So far, I've only been able to narrow down which equations I think I'll need to do so. These are the equations: Maxwell's Equations: $$ \nabla\times{\bf E} ...
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1answer
130 views

Light in the absence of colloids?

If Earth had no colloids (Don't take LIFE into account) or any kind of scattering medium (like atmosphere) that promote to Tyndall effect, how would we able to receive the sun's rays? In the other ...
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1answer
175 views

How was Dorsey able to measure speed of light with 3 significant digits?

How was Dorsey able to measure speed of light with 3 significant digits in the year of 1907?
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2answers
230 views

Identifying hue, brightness and chroma of color and reaction time

If someone knows how identify hue, brightness, and chroma of color, please let me know. I am a PhD student at Educational Linguistics UNM.
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2answers
383 views

Do and can phone signals come inside AC car which is glass-packed

Can phone signals penetrate glass, so can I expect phone signals to come if I am sitting inside a closed AC car. Thanks,
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2answers
40 views

How do you convert RGB lux into a singular value? [closed]

I'm trying to understand the output from an RGBW optical sensor. According to the specification for the sensor this outputs lux. I was hoping someone could explain how the 3 RGB lux values relate to ...
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1answer
64 views

Why do night sky appear red when it is cloudy?

Since childhood, I have seen that whenever at night, the sky is cloudy, the sky becomes red. But, clouds are grayish, so it is expected that the sky becomes more darker. But no, it is not! Why does ...
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1answer
58 views

Why mirrors form image and others don't?

In the case of mirror, I understand how a image is formed, If I replace plane mirror by wood or some other opaque material in the above image and position of object(bottle) and eye remain the same , ...
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2answers
58 views

Frames of reference and why are they different

Here is a quote from "The Elegant Universe" by Brian Greene. Imagine two countries that have been at war are sitting down to sign a treaty ending hostilities while traveling aboard a train that is ...
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2answers
107 views

Does a photon travel in all directions?

For example i am standing and a beam of light is passing in front of me. I am able to see that beam of light so does it mean that photons are travelling in all directions other than the photons which ...
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1answer
123 views

How many foci does a converging or diverging lens have?

I thought this was a simple question but can't seem to find a direct answer. In ray diagrams with a converging or diverging lens, we always draw F (the focal length) on both sides of the lens. But ...
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2answers
62 views

When does a monochromatic beam of light on a reflective surface get completely transmitted?

My book says when when a monochromatic beam of light is normally incident on reflective surface it gets completely transmitted. I am bit confused could you explain me?
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1answer
59 views

An accelerated charge produces an electromagnetic wave. Is light an electromagnetic wave despite not having charged particles? [duplicate]

Light is an electromagnetic radiation contrary to the law of electromagnetism that states that: an accelerated charge produce electromagnetic wave. Please explain.
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2answers
163 views

Plane polarized light?

I have heard about plane polarized light: light wave which has vibration in one plane. My curiosity forces me to ask a doubt, is there any way to produce polarized light wave which has vibrations in ...
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1answer
44 views

Beam of electromagnetic wave, with radius of 3cm or less

I need to create a beam of electromagnetic wave, with radius of 3cm or less, for distance about few meters. What is the minimum frequency (maximum wavelength) that it is really possible?
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5answers
87 views

Is anything coloured “infra-red”?

All our day-to-day objects are things we can see, and thus we can conclude that their colour lies in the visual spectrum. Is there matter that does not have a colour within the visual spectrum? What ...