# Homework Help: Waves/light Help!

1. Feb 1, 2013

### chipM

1. The problem statement, all variables and given/known data

(1)in which of these cases will a ripe banana appear black
red yellow green or blue light ?

red light

(2) at the beach , you can get a sunburn while under th shade of an umbrella, why ?
some UV can pass through the umbrella or reflect from the sand or water.

3) what exactly are you seeing when you observe a water on the road mirage?
Air has its own refractive index which depends on the temperature of the air. So, if there is hot air, it has different refractive index to having cold air. On a hot day, the sun will come down, it will heat the surface of the road and make the air close to the road very hot in comparison to the air above it. That means that light rays are bent away from the road, and so, when looked down at the road, the rays are actually bending away from the road and back up into the sky, and we are seeing a patch of the sky in the road. Water also looks like that because water is reflecting the sky, making the sky appear in the road. So the mirage looks exactly like water.
4) when does a photon behave like a wave , when does it behave like a particle ?
during difraction acts like a wave.
when donating energy to electrons acts like a particle.

5) polarised light is part of the nature but not sound , why ?

polarised light often makes up a large proportion of reflected light so is common on nature.

light is polarised as its has transverse waves, but sound waves are longitudinal, with compression and rarefaction in the same direction of travel.

6) will light from 2 very close stars produce an interference pattern?

No. the light is not cherent so they wont make an interference pattern. also as they are not point sourses and the light they emit is broadly directed and dispersed.

are the answear right for ach questions . if they are not please healp me solve them . thanks !

2. Relevant equations

3. The attempt at a solution

2. Feb 1, 2013

### haruspex

I hate questions like this. They reinforce an inaccurate understanding of colour perception.
Depending on exactly why an object appears a certain colour, it might appear black in just about any pure colour of light.
What do you know about colour perception in the eye? Do you know about cones?
Probably the second more than the first, but yes.
Almost right, but you reversed the direction of the light rays in there, having them go from your eye to the sky.
Should that read "5) polarisation is part of the nature of light but not sound "?
You could go a little more into why transverse waves can exhibit polarisation whereas longitudinal waves cannot.

3. Feb 1, 2013

### Basic_Physics

(1) yellow is produced by combining red and green, so in which color should the banana appear black then? Explain your answer.

4. Feb 1, 2013

### haruspex

Sorry, Basic_Physics, but that's the sort of misrepresentation of colour perception I had in mind. Yes, we perceive red+green as yellow, but that doesn't mean all yellow is produced by red and green. The yellow in a rainbow isn't.
The trouble is that 'yellow' really means two different things: there's the band of wavelengths which we perceive as yellow, but the sensation of yellow can also be produced by all sorts of other combinations of wavelengths. If an object only reflected wavelengths in that narrow rainbow band and you lit it with a pure red light it would appear black. But of course, a banana isn't like that.

5. Feb 1, 2013

### Basic_Physics

Get your point. Banana most likely absorbs the blue part of the white light falling on it and re-emits/reflects predominantly the lower parts of the spectrum - red and green. Physiologically the eye has 3 different types of cones those responsive to low frequencies (red) mid range frequencies (green) and high frequencies (blue). The yellow part of the spectrum is mid between the red and green so it should reflect no blue light. I think he/she wants the short answer though.

6. Feb 1, 2013

### chipM

so instead it will be light from sky to the eye ?

regarding question 1 is it green ?

and umm what about questiong 6?

Last edited: Feb 1, 2013
7. Feb 1, 2013

### haruspex

Yes.
No. In your eye there are three types of colour receptors ('cones'). They are commonly referred to as red, green and blue, but in truth each responds to a range of wavelengths. The ranges overlap, but the peak responses are in the red, green and blue parts of the spectrum.
For some reason, if the brain gets a strong response from both the red and green cones but little or nothing from the blue, we perceive it as a separate colour entirely, which we call yellow. Other mixes, like red and blue to produce mauve, or blue and green to produce cyan we may give specific names to, but it remains obvious that they correspond to those mixes of response.
Note that I was referring to mixes of response there. 'Yellow' light can consist of a narrow band of wavelengths from the yellow part of the spectrum, as is the case when you see yellow in the rainbow. In this case, both red and green cones are responding to the same wavelength about equally. But a mix of pure red and pure green light can produce the same response in the eye, so is also seen as yellow. (Similarly cyan can be pure or a mix of blue and green light, but mauve can only be a mix of wavelengths, so does not turn up in the rainbow.)
So, back to the question. A ripe banana in daylight doesn't just appear yellow, it appears bright yellow. From this you can deduce that it is reflecting a broad range of wavelengths. This mixture you interpret as yellow, from which you can deduce that the wavelengths being reflected do not include much of ... what colour?

8. Feb 1, 2013

### chipM

So its yellow ?

regard question 5 about polarised light

also i am just going to assume question 4 is right as well ?

Sound waves cannot be polarized because the direction of the movement of the waves and the direction of vibration of the particles within the wave are the same

in light waves the direction of the movement of the waves and the direction of vibration is 90 degrees to each other and in different direction hence light can be polarised.

is this correct for 5 ?

9. Feb 1, 2013

### haruspex

Er.. no. According to what I told you, when you see yellow, which cones are not picking up much?
Maybe mention refraction too.
Yes, that's better. You don't need to add "and in different direction"; that follows from the 90 degrees bit. You could add that this leaves (infinitely) many possibilities for the direction of vibration.

10. Feb 1, 2013

### chipM

so photon acts like a particle when donating energy to electron and like a wave during difraction and refraction

Sound waves cannot be polarized because the direction of the movement of the waves and the direction of vibration of the particles within the wave are the same

in light waves the direction of the movement of the waves and the direction of vibration is 90 degrees to each other hence light can be polarised as this leaves many possibilities for the direction of vibration .

both correct yes ?

regarding question one it would be cool if you just gave me the straight answear.

11. Feb 1, 2013

### haruspex

Yes
Easy, but not cool.
You see yellow if your red and green cones are detecting plenty of light, but your blue ones are not. What does that tell you about the wavelengths that a banana tends to reflect?

12. Feb 2, 2013

### chipM

so its blue !

13. Feb 2, 2013

Bingo.