# Homework Help: Optics: What is the color of the object as seen from under water?

1. Jul 7, 2017

### Jahnavi

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

2. Relevant equations

3. The attempt at a solution

Since wavelength of light reduces on entering
water , then the wavelength of light entering the eyes of diver is less than the wavelength of green light .

Out of the given options , option B) i.e blue color is the only one with wavelength less than green .

But this is not the correct option .

What is the mistake ?

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2. Jul 7, 2017

### Drakkith

Staff Emeritus
Does the light exit the water and immediately hit your retina?

3. Jul 7, 2017

### Jahnavi

Exit or enter ?

4. Jul 7, 2017

### Drakkith

Staff Emeritus
Exit. What's between your retina and the water?

5. Jul 7, 2017

### Jahnavi

Sorry . I am not understanding you .

Light from the green object enters the water , gets refracted and enters the eyes of diver . Isn't it ?

Why are you emphasizing on light exiting water ?

6. Jul 7, 2017

### cnh1995

I think the diver is inside the water and the green object is outside the water. The diver is looking at the green object.

I am no expert here, but dividing the wavelength of green light by the refractive index of water (1.33) gives a wavelength below 400 nm and the visible range is 400-700 nm.

7. Jul 7, 2017

### Drakkith

Staff Emeritus
Whether the diver and the object are immersed in the same medium makes no difference. Otherwise we would experience drastic color changes viewing things in a fish tank, in a pool, etc.

8. Jul 7, 2017

### Drakkith

Staff Emeritus
If the light is entering your eye, it has to exit the water, right? What is your eye composed of?

9. Jul 7, 2017

### Jahnavi

Why ?

We see an object when the light rays originated/reflected from that object enter our eyes .That's it .

Where is the point of light exiting the water ?

I might be missing something very fundamental .

10. Jul 7, 2017

### Jahnavi

Are you suggesting that light exiting the water enters air somewhere before entering the eyes ?

11. Jul 7, 2017

### Staff: Mentor

I'd say it would be green still.

The reasoning is that the frequency of the photons doesn't change as it traverses the water and into your eye hence you still see it as green.

12. Jul 7, 2017

### Jahnavi

But colour of an object is determined by the wavelength of light not the frequency ?

13. Jul 7, 2017

### Staff: Mentor

14. Jul 7, 2017

### Drakkith

Staff Emeritus
Prior to hitting your retina, the light has to proceed through the cornea, the lens, and finally the vitreous humour, which is what the retina is in. All of which are their own optical mediums. Does it matter whether the light originates in air, water, or some other medium if it all ends up in the same medium in the end?

15. Jul 7, 2017

### Jahnavi

This is what I was saying in post 10 .

But where is air just before the light reaches the retina ?

Light originated in air and just before it reaches retina , it traverses through vitreous humour .

It the object were to look green to the diver , it has to enter air before it reaches retina .

Does this make sense ?

16. Jul 7, 2017

### Jahnavi

17. Jul 7, 2017

### Drakkith

Staff Emeritus
There is none.

No, as there is no air inside your eye, nor is any required to see the correct color. The wavelength of green light, when it strikes the retina, is not 525 nm. It has been reduced because the refractive index of the vitreous humour is greater than 1. And this has always been the case for your whole life, regardless of whether you're underwater or not. When we talk about the wavelength of a color, we are talking about the wavelength of that light as if it was traveling in a vacuum, not when it is inside the eye. The real wavelength at your retina is always smaller.

18. Jul 7, 2017

### Staff: Mentor

19. Jul 7, 2017

### Jahnavi

To a person in the same medium as the object , the object looks green .

To a person under water the object looks green .

Does this mean , colour of an object as perceived by eyes does not depend on wavelength of light originated from object ?

20. Jul 7, 2017

### Drakkith

Staff Emeritus
The simple answer is yes, but as others have pointed out, we would also need to talk about photons, energy, and frequency, which would greatly complicate things.

21. Jul 7, 2017

### Jahnavi

If colours as perceived by eyes do not depend on wavelength , then how do we see the sparkling of a diamond or a rainbow ?

@haruspex

22. Jul 7, 2017

### Jahnavi

23. Jul 7, 2017

### Drakkith

Staff Emeritus
Frequency and wavelength are directly related to each other. The shorter the wavelength, the higher the frequency, just like any type of wave. Nothing anyone has said here is contradictory in any way. What Jedi was saying about frequency being important and not wavelength is part of the "complications" I mentioned earlier.

24. Jul 7, 2017

### Jahnavi

You are comparing lights of different wavelengths travelling in the same medium .

This is not the case in OP .

For a monochromatic light , the frequency of light doesn't change as it travels from one medium to other .

In the problem given , the frequency of light originated from the object doesn't change as it enters water . Speed and wavelength changes .

25. Jul 7, 2017