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January 25, 2020, 10:55:46 am

### AuthorTopic: diffraction of light question  (Read 233 times) Tweet Share

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#### dream chaser

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##### diffraction of light question
« on: August 24, 2019, 07:24:37 pm »
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Hi Guys,

Comparing violet and red light, violet light refracts the most away from the normal as it enters the prism and refracts the most towards the normal as it leaves the prism. How does it reflect the reflective index’s of violet and red in the two scenarios. Does violet have the greater refractive index as it enters the prism but red have the greatest refractive index as it leaves the prism?

All replies will be much appreciated. Thanks

#### ^^^111^^^

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##### Re: diffraction of light question
« Reply #1 on: August 24, 2019, 07:40:48 pm »
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Hi Guys,

Comparing violet and red light, violet light refracts the most away from the normal as it enters the prism and refracts the most towards the normal as it leaves the prism. How does it reflect the reflective index’s of violet and red in the two scenarios. Does violet have the greater refractive index as it enters the prism but red have the greatest refractive index as it leaves the prism?

All replies will be much appreciated. Thanks
Violet light has a higher refractive index than red.

Basically the amount of bending depends on two things:
a) change in speed
b) the angle of the incident ray

Violet light has a refractive index of 1.532 while red has a refractive index of 1.513. Also red light has a longer wavelength that violet light and iirc, this could also affect the refraction of the light on the prism. Hope that helps

P.S. Just a quick fact, if you didn't realise already, the scenario that you have given me is one that is similar to newton's experiment using a triangular block of glass called a prism. He used it to model the spectrum of colours.

#### DrDusk

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##### Re: diffraction of light question
« Reply #2 on: August 24, 2019, 07:53:01 pm »
+1
Violet light has a higher refractive index than red.

This is incorrect. The light itself does NOT have a refractive index.

@Dreamchaser, the reason the angle of refraction is different is because each different color of light has an associated different wavelength. So if we look at Snell's Law
$sin(\theta_i) = nsin(\theta_r), n = \frac{c}{c_n} \Rightarrow sin(\theta_i) = \frac{c}{c_n}sin(\theta_r)$
$\therefore sin(\theta_r) = \frac{sin(\theta_i)}{c}c_n$

As we can see, the equation above is a linear equation of the form y = mx. This means as c_n gets higher, sin(theta_r) also increases. For different colors of light, they each will have a different wavelength. However the speed they travel in the prism will be different as this will depend directly on it's wavelength. Hence this leads to the fact that the angle of refraction will be different. Using some data you can calculate if it's more towards the normal or away..
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#### 830cracker

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##### Re: diffraction of light question
« Reply #3 on: August 24, 2019, 08:09:49 pm »
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Hi Guys,

Comparing violet and red light, violet light refracts the most away from the normal as it enters the prism and refracts the most towards the normal as it leaves the prism. How does it reflect the reflective index’s of violet and red in the two scenarios. Does violet have the greater refractive index as it enters the prism but red have the greatest refractive index as it leaves the prism?

All replies will be much appreciated. Thanks

Hey there, I had done a similar question today but sorry if this answer is explained too basic compares to the other ones:

As the violet light refracts more and has a greater frequency(shorter wavelength), it leaves the prism with a lower refraction compares to the red light, and the rays are bending down. Therefore, as the light from the air enters the prism, it refracts normal; however when light from glass enters the air, it refracts away from the normals.

Hops that would contribute a bit of help
welp I’m procrastinating rn

#### ^^^111^^^

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##### Re: diffraction of light question
« Reply #4 on: August 24, 2019, 08:23:06 pm »
+1
This is incorrect. The light itself does NOT have a refractive index.

@Dreamchaser, the reason the angle of refraction is different is because each different color of light has an associated different wavelength. So if we look at Snell's Law
$sin(\theta_i) = nsin(\theta_r), n = \frac{c}{c_n} \Rightarrow sin(\theta_i) = \frac{c}{c_n}sin(\theta_r)$
$\therefore sin(\theta_r) = \frac{sin(\theta_i)}{c}c_n$

As we can see, the equation above is a linear equation of the form y = mx. This means as c_n gets higher, sin(theta_r) also increases. For different colors of light, they each will have a different wavelength. However the speed they travel in the prism will be different as this will depend directly on it's wavelength. Hence this leads to the fact that the angle of refraction will be different. Using some data you can calculate if it's more towards the normal or away..
I realised my mistake. What I had meant to mean was the angle of  refraction and incidence. And yes, we can use Snell's law, but without sufficient data, we will not be able to tell if which angle of refraction is greater than the other.
To add on to 830cracker's post, if light enters any substance with a higher refractive index (such as from air into glass) it slows down. The light bends towards the normal line.

If light travels enters into a substance with a lower refractive index (such as from water into air) it speeds up. The light bends away from the normal line.
Hope that helped