Some conceptual questions about light confused >_<

michaelw

Hello
Im trying to understand light, I did it briefly inhigh school and im writing the mcat this august... and im quite confused

Is the wavelength of light at a specific frequency in water (n = 1.3) equal to the wavelength in a vacuum / 1.3?

Why is violet light not bright? Intensity is proportional to frequency ^2, so shouldnt violet be much more intense than red or yellow light?

Why can we see total internal reflection? Say in a diamond, light within the diamond hits an edge at the critical angle, causing the light to be reflected along the edges of the face.. why can we see this (the sparkle?).. no light is reaching our eyes, so why can we see the sparkle?

In my book it says "plane polarized light loses half its intensity.. plane polarized light is light having an electric field in just one direction". but why is it only reduced by half, if the electric fields of photons are in random directions? wouldnt you be blocking much more than 50% of the photons?

What exactly is intensity? is it just the brightness of the light? is it energy itself or proportional to it?

Please help me.. ive been reading this book for hours and im still so confused with light

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Doc Al

Mentor
michaelw said:
Is the wavelength of light at a specific frequency in water (n = 1.3) equal to the wavelength in a vacuum / 1.3?
Right. When light passes from air to water the frequency stays the same but the speed and wavelength are decreased by a factor of $1/n$.
Why is violet light not bright? Intensity is proportional to frequency ^2, so shouldnt violet be much more intense than red or yellow light?
Intensity is proportional to amplitude squared, not frequency.

Why can we see total internal reflection? Say in a diamond, light within the diamond hits an edge at the critical angle, causing the light to be reflected along the edges of the face.. why can we see this (the sparkle?).. no light is reaching our eyes, so why can we see the sparkle?
The light, of course, does reach your eyes. The internal reflection causes the facets to act as perfect mirrors; without such internal reflection, most of the light would "leak out" and be dispersed and not be reflected back so brightly.

In my book it says "plane polarized light loses half its intensity.. plane polarized light is light having an electric field in just one direction". but why is it only reduced by half, if the electric fields of photons are in random directions? wouldnt you be blocking much more than 50% of the photons?
If you think in terms of classical light, then the polarizer admits only the component parallel to its optic axis. (Look up the law of Malus.) For a randomly polarized incident beam, the transmitted beam ends up being half as intense. If you wish to think in terms of photons, you must view them quantum mechanically. A photon with polarization at an angle with respect to the polarizer still has a probability of being transmitted.

What exactly is intensity? is it just the brightness of the light? is it energy itself or proportional to it?
Think of the intensity as the power per unit area.

michaelw

Doc Al said:
Right. When light passes from air to water the frequency stays the same but the speed and wavelength are decreased by a factor of $1/n$.

Intensity is proportional to amplitude squared, not frequency.
The equation in the book reads
I = (1/2)p(w^2)(A^2)v
where w = 2pi*frequency

Doc Al

Mentor
michaelw said:
The equation in the book reads
I = (1/2)p(w^2)(A^2)v
where w = 2pi*frequency
Can you tell me what this equation is describing? What are p, w, A, & v?

michaelw

I is intensity of a wave
p is density of medium
w is angular frequency
A is amplitude
v is wave velocity

Doc Al

Mentor
That seems more a description of sound intensity in a medium, not light.

michaelw

Doc Al said:
That seems more a description of sound intensity in a medium, not light.
ah youre right

would you happen to know the equation of intensity for light?

mezarashi

Homework Helper
Because the intensity of light is a non-fundamental quantity. We may choose to definite it however we want. For example, in high-school you can learn that:

I = k/d^2

which is useful if you want to calculate the intensity of light at a certain distance relative to another. In planar wave form, you may use the Poyting vector and the average power of the electromagnetic waves, kind of similar to how we get the average power in electrical AC systems.

Lastly, if you really want it simple, the intensity of light is the amount of photons per second per unit area.

E = hf
Etotal = nhf (where n is the number of photons)
Power = nhf/second
Intensity = Power/area = (nhf/second)/area

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